Sound and Light. 50 min. 50 min. PS 2e, PS 6a, UCP 1, UCP 5, SAI 1, ST min. PS 6a, PS 6b, UCP 1, UCP 2, UCP 3, SAI 2, ST 2, HNS 1, HNS 2, HNS 3

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1 CHAPTER PLANNER 16 Sound and Light CHAPTER OPENER, pp min. Standards Teach Key Ideas SECTION 1 Sound, pp V Properties of Sound V Musical Instruments V Hearing and the Ear V Ultrasound and Sonar 50 min. PS 2e, PS 6a, UCP 1, UCP 5, SAI 1, ST 2 Bellringer Transparency Teaching Transparency P15 The Ear Visual Concepts Speed of Sound Sound Intensity and Decibel Level Pitch SECTION 2 The Nature of Light, pp V Waves and Particles V The Electromagnetic Spectrum 50 min. PS 6a, PS 6b, UCP 1, UCP 2, UCP 3, SAI 2, ST 2, HNS 1, HNS 2, HNS 3 Bellringer Transparency Teaching Transparencies TM44 Wave Frequency and Photon Energy E18 Electromagnetic Spectrum Visual Concepts The Dual Nature of Light Energy of a Photon Electromagnetic Spectrum SECTION 3 Reflection and Color, pp V Reflection of Light V Mirrors V Seeing Colors 50 min. UCP 1, UCP 2, SAI 1 Bellringer Transparency Teaching Transparencies TM45 Law of Reflection TM46 Flat Mirror Visual Concepts Reflection Law of Reflection Comparing Real and Virtual Images Additive Color Mixing SECTION 4 Refraction, Lenses, and Prisms, pp V Refraction of Light V Lenses V Dispersion and Prisms 50 min. UCP 1, UCP 5, SAI 1, ST 2 Bellringer Transparency Teaching Transparencies P16 Refraction P17 The Eye Visual Concepts Refraction Dispersion of Light Converging and Diverging Lenses Parts of the Human Eye See also PowerPoint Resources Chapter Review and Assessment Resources SE Science Skills: Using Fractions, p. 576 SE Chapter Summary, p. 577 SE Chapter Review, pp SE Standardized Test Prep, pp Concept Review Worksheets Chapter Tests A and B Holt Online Assessment CHAPTER Fast Track To shorten instruction because of time limitations, omit Section 3 and the chapter lab. Basic Learners TE Speeds of Sound, p. 544 TE Sound Levels, p. 545 TE Law of Reflection, p. 563 Science Skills Worksheets Differentiated Datasheets A for Labs and Activities Study Guide A Advanced Learners TE Music and Math, p. 547 TE Benefits and Drawbacks of Cell Phones, p. 559 TE Mirror Height, p. 562 TE Index of Refraction, p. 567 TE Vision Problems, p. 569 Cross-Disciplinary Worksheets Differentiated Datasheets C for Labs and Activities 540A CHAPTER 16 Sound and Light

2 Key SE Student Edition TE Teacher s Edition Chapter Resource File Workbook Transparency CD or CD-ROM * Datasheet or blackline master available Also available in Spanish All resources listed below are also available on the Teacher s One-Stop Planner. Why It Matters Build student motivation with resources about high-interest applications. Hands-On Skills Development Assessment SE Inquiry Lab Colors in White Light, p. 541* TE Reading Toolbox Assessing Prior Knowledge, p. 540 SE Reading Toolbox p. 542 Pretest TE Elephant Sounds, p. 546 TE Cochlear Implants, p. 549 SE What Are Sonograms? p. 550 TE Demonstration Loudness and Pitch, p. 543 SE Quick Lab Sound in Different Mediums, p. 544* SE Quick Lab Frequency and Pitch, p. 547* SE Inquiry Lab How Can You Amplify the Sound of a Tuning Fork? p. 548* TE Demonstration Showing Waves with a Stroboscope, p. 548 TE Reading Toolbox Visual Literacy, p. 545 TE Reading Toolbox Visual Literacy, p. 546 SE Reading Toolbox Mnemonics, p. 549 TE Reading Toolbox Visual Literacy, p. 550 TE Reteaching Key Ideas Retracing the Process of Sound and Hearing, p. 551 TE Formative Assessment, p. 551 Spanish Assessment * Section Quiz TE The Radio Spectrum, p. 556 SE How Do Cell Phones Work? p. 559 Cross-Disciplinary Worksheet Real World Applications How Does Sunscreen Work? TE Demonstration Two- Slit Experiment, p. 552 TE Reading Toolbox Visual Literacy, p. 553 SE Reading Toolbox Booklet, p. 554 TE Reading Toolbox Visual Literacy, p. 554 TE Reteaching Key Ideas Comparing Models, p. 558 TE Formative Assessment, p. 558 Spanish Assessment * Section Quiz Cross-Disciplinary Worksheet Integrating Space Science The Refracting Telescope at Yerkes TE Demonstration Adding Colors, p. 560 SE Quick Lab Curved Mirror, p. 563* SE Quick Lab Filtering Light, p. 564* TE Reading Toolbox Visual Literacy, p. 561 SE Reading Toolbox Prefixes, p. 562 TE Reteaching Key Ideas What Happens to Light? p. 565 TE Formative Assessment, p. 565 Spanish Assessment * Section Quiz TE Eyeglass Lenses, p. 568 SE Detecting Counterfeit Money, pp TE Demonstration Refraction, p. 566 SE Quick Lab Water Prism, p. 570* TE Demonstration Dispersion, p. 570 SE Application Lab Lenses and Images, pp * TE Reading Toolbox Visual Literacy, p. 567 TE Reading Toolbox Visual Literacy, p. 569 SE Reading Toolbox Booklet, p. 571 TE Reading Toolbox Visual Literacy, p. 573 TE Reteaching Key Ideas Reviewing Refraction, p. 571 TE Formative Assessment, p. 571 Spanish Assessment * Section Quiz See also Lab Generator See also Holt Online Assessment Resources Resources for Differentiated Instruction English Learners TE Pitch Versus Volume, p. 546 TE Learning Prefixes, p. 555 TE Vocabulary, p. 568 Differentiated Datasheets A, B, and C for Labs and Activities Study Guide A Struggling Readers TE Summarizing, p. 553 TE Distinguishing Words, p. 563 Interactive Reader Special Education Students TE Feeling Vibrations, p. 544 TE Sunscreen and SPF, p. 557 Alternative Assessment TE Instruments of Other Cultures, p. 547 TE Comparing Frequency and Intensity, p. 548 TE The Newton-Huygens Debate, p. 554 TE Electromagnetic Wave Applications, p. 556 TE Making a Periscope, p. 561 TE Brochure, p. 572 TE Comparing Concepts, p. 577 Chapter Planning Guide 540B

3 CHAPTER16 16 Sound and Light Overview This chapter covers sound, including the properties of sound, musical instruments, the human ear, and ultrasound and sonar. Next, it explores the wave and particle properties of light and the electromagnetic spectrum. It also covers the reflection of light, including mirrors, and explains how we see colors. Finally, the chapter covers the refraction of light, including the use of lenses in microscopes, telescopes, and the human eye. The chapter concludes with a discussion of dispersion and prisms. Assessing Prior Knowledge Students should understand the following concepts: electrons structure of atoms energy transformation types of waves relationship of vibrations and waves reflection diffraction interference standing waves waves and energy Science education research has identified the following misconception about sound. Students believe that sound cannot travel through solids and liquids, but can travel through a vacuum and space. (To demonstrate that sound travels through solids, ask students to place an ear against their desktop and then tap the desk with their finger. To demonstrate that sound cannot travel through a vacuum, place a wind-up alarm clock inside a sealed jar. Use a vacuum pump to remove the air from the bell jar as the alarm clock rings. Students should hear the sound of the alarm clock fade as a vacuum is formed inside the jar.) Chapter Outline 1 Sound Properties of Sound Musical Instruments Hearing and the Ear Ultrasound and Sonar 2 The Nature of Light Waves and Particles The Electromagnetic Spectrum 3 Reflection and Color Reflection of Light Mirrors Seeing Colors 4 Refraction, Lenses, and Prisms Refraction of Light Lenses Dispersion and Prisms The following correlations show the National Science Standards that relate to this chapter. For the full text of the standards, see the National Science Education Standards at the front of the book. PS 2e In solids the structure is nearly rigid; in liquids molecules or atoms move around each other but do not move apart; and in gases molecules or atoms move almost independently of each other and are mostly far apart. (Section 1) PS 6a Waves, including sound and seismic waves, waves on water, and light waves, have energy and can transfer energy when they interact with matter. (Sections 1, 2) PS 6b Electromagnetic waves include radio waves (the longest wavelength), microwaves, infrared radiation (radiant heat), visible light, ultraviolet radiation, x-rays, and gamma rays. The energy of electromagnetic waves is carried in packets whose magnitude is inversely proportional to the wavelength. (Section 2) UCP 1 Systems, order, and organization (Sections 1 4) UCP 2 Evidence, models, and explanation (Sections 2, 3) Why It Matters The properties of sound and light waves affect how we experience our surroundings. The properties of light can explain both the colors of the car lights and the colors of the road signs on this busy highway system. National Science Education Standards UCP 3 Constancy, change, and measurement (Section 2) UCP 5 Form and function (Sections 1, 4) SAI 1 Abilities necessary to do scientific inquiry (Sections 1, 3, 4) SAI 2 Understandings about scientific inquiry (Section 2) ST 2 Understandings about science and technology (Sections 1, 2, 4) HNS 1 Science as a human endeavor (Section 2) HNS 2 Nature of scientific knowledge (Section 2) HNS 3 Historical perspectives (Section 2) 540 CHAPTER 16 Sound and Light

4 20 min Colors in White Light Your teacher will give you a spectroscope or instructions for making one. Turn on an incandescent light bulb. Look at the light bulb through your spectroscope. Write a description of what you see. Then, repeat the procedure with a fluorescent light bulb. Again, describe what you see. Questions to Get You Started 1. Compare your observations of the incandescent light bulb and your observations of the fluorescent light bulb. 2. Both kinds of bulbs produce white light. What did you learn about white light by using the spectroscope? 3. Light from a flame is yellowish but is similar to white light. What do you think you would see if you used a spectroscope to look at light from a flame? Teacher s Notes The instructions for making a spectroscope are as follows. 1. Cut a narrow slit in the center of a piece of black construction paper. Tape the paper to one end of a paper-towel tube. The paper should cover the opening of the tube. 2. Look through the open end of the tube at an incandescent light bulb. If no light passes through the slit in the paper, make the slit a little wider. 3. Hold a diffraction grating against the open end of the tube. Look at the light bulb through the grating. Make sure the slit in the paper is vertical. 4. Rotate the diffraction grating until you see colors inside the tube to the left and right sides of the slit. Tape the diffraction grating to the tube in this position. Materials per Group light bulb, incandescent light bulb, fluorescent spectroscope Answers 1. When I looked at the incandescent light I saw a continuous spectrum of colors on the sides of the spectroscope tube. When I looked at the fluorescent light I saw a spectrum of colors, but some bands were bright and others were faint. 2. White light is made up of many colors of light. 3. I would expect to see more colors Key Resources Datasheet Colors in White Light CHAPTER 16 Sound and Light 541

5 Word Parts Sample table: WORD ROOT DEFINITION ultrasound sound beyond the range of human hearing (beyond 20 khz) ultraviolet violet beyond the violet end of the visible spectrum infrared red below the red end of the visible spectrum Mnemonics Sample answer: Red ostriches yodel for great big vittles. FoldNotes Answers may vary. Students Booklets should look similar to the example shown. The front of the Booklet should be labeled Sound. The booklet should contain notes about Section 1. Word Parts Prefixes Analyzing the parts of a science term can help you figure out the meaning of the word. Some word parts and their meanings are listed in Appendix A. Prefixes are used in front of word roots to modify the meaning of the word root. Two prefixes that are used in this chapter are listed below. ultra- means beyond or extremely infra- means below Your Turn As you read this chapter, make a table like the one started below that lists each word that contains the prefix ultra- or infra-, the root of the word, and the word s definition. WORDS WITH THE PREFIX ULTRA- OR INFRA- WORD ROOT DEFINITION infrasound sound sound below the range of human hearing (below 20 Hz) ultrasound These reading tools can help you learn the material in this chapter. For more information on how to use these and other tools, see Appendix A. Mnemonics Colors of the Spectrum A mnemonic device can help you remember related words. Although the electromagnetic spectrum is continuous, the colors in the visible part can be divided into six main colors red, orange, yellow, green, blue, and violet. One way to remember these colors and their order is by using a made-up name, such as the one below, as a mnemonic device. ROY G BiV Note that the letter i is added so that you can pronounce the last name. Your Turn Practice saying the name above to help yourself learn and remember the colors of the visible spectrum and the order of the colors. Make up a sentence with words that start with these letters to create another type of mnemonic device. FoldNotes Booklet FoldNotes can help you remember ideas that you learn as you read. You can use a booklet to organize the details of a main topic in the chapter. Your Turn Make a booklet by following the instructions in Appendix A. 1 Label the front of the booklet Sound. 2 Label each of the pages of the booklet with one of the red headings from Section 1. 3 Take notes from Section 1 on the appropriate page of the booklet. SOUND Colors of the Spectrum Some students may have learned that the spectrum contains seven main colors. Discuss that indigo is no longer considered one of the main colors in the visible spectrum. The seven colors were defined by Sir Isaac Newton, who used a prism to separate the colors in visible light. He defined seven colors to parallel the seven main notes on a musical scale. 542 CHAPTER 16 Sound and Light

6 1 Sound Key Ideas Key Terms Why It Matters V What are the characteristics of sound waves? V How do musical instruments make sound? V How do ears help humans hear sound waves? V How are the reflections of sound waves used? sound wave pitch infrasound ultrasound resonance sonar When you listen to your favorite musical group, you hear a variety of sounds. Although these sounds come from different sources, they are all longitudinal waves that are produced by vibrating objects. Musical instruments and stereo speakers make sound waves in the air. Properties of Sound The head of a drum vibrates up and down when a drummer hits it. Each time the drumhead moves upward, it compresses the air above it. When the head moves back down, it leaves a small region of air that has a lower pressure. A series of compressions and rarefactions is created in the air as the drumhead moves up and down repeatedly, as Figure 1 shows. Sound waves, such as those created by a drum, are longitudinal waves caused by vibrations. The particles of air in these waves vibrate in the same direction the waves travel. V Sound waves are caused by vibrations and carry energy through a medium. All sound waves are made by vibrating objects that cause the surrounding medium to move. In air, the waves spread out in all directions away from the source. When sound waves from the drum reach your ears, the waves cause your eardrums to vibrate. Sound waves are responsible for much more than the sounds we hear. They are used to create images of the human body that are used in medicine. sound wave (SOWND WAYV) a longitudinal wave that is caused by vibrations and that travels through a material medium Figure 1 Vibrations create sound waves. 1 The head of a drum vibrates up and down when it is struck by the drummer s hand. 2 The vibrations of the drumhead create sound waves in the air. SECTION 1 In this section, students relate properties of sound to wave characteristics. The section also covers musical instruments, the human ear, and applications of ultrasound and sonar. Bellringer Use the Bellringer transparency to prepare students for this section. Loudness and Pitch Borrow a practice piano that you can take to your classroom on rollers. If a piano is not available, use an acoustic guitar or other musical instruments. First, demonstrate differences in loudness using notes of the same pitch. Use a sound level meter to reinforce the judgment of students. Then, demonstrate difference in pitch. Students may be able to see that the lowest string of a guitar vibrates with noticeably lower frequency than the highest string. Having students touch the strings lightly as they vibrate will also help convey this idea. Musical Key Resources Teaching Transparency P15 The Ear Visual Concepts Speed of Sound Sound Intensity and Decibel Level Pitch Datasheets Sound in Different Mediums Frequency and Pitch How Can You Amplify the Sound of a Tuning Fork? Echoes and Distance Science Skills Worksheet Equations Involving a Constant SECTION 1 Sound 543

7 Sound in Different Mediums 10 min Teacher s Notes The utensil should not be one with parts that rattle. Use hard kite string. Students should wrap the string near their fingertips and try to arrange the string so that the last loop before the downward part passes over the ends of their fingers ensuring that the string is pushed against their ears. Heavier objects, such as wrenches and oven racks make the loudest sounds. Try striking with a soft object such as a rubber stopper. Materials per Group spoon or other utensil string, 1 2 m Answers to Analysis 1. The sound that I heard through the string was louder. It was a ringing, bell-like sound. 2. Sound travels better through the string. The sound was louder. 3. The sound waves that reached my ear through the string were more intense than the sound waves that traveled through air. The intensity was due to sound waves traveling faster through solids than gases. Teaching Key Ideas Sources of Sound Some students believe that sound can be produced without any materials. Ask students to name examples of sounds, and write them in a column on the board. Have students determine the source of the sound. Write these next to each example. Use these examples to illustrate that all sounds can be traced back to a vibrating source. Logical Academic Vocabulary transmit (trans MIT) to send from one place to another Topic: Properties of Sound Code: HK81233 Procedure 1 Tie a spoon or other utensil to the middle of a 1 to 2 m length of string. 2 Wrap the loose ends of the string around your index fingers, and place your fingers against your ears. 3 Swing the spoon so that it strikes a tabletop. The speed of sound depends on the medium. If you stand near a drummer, you may think that you hear the sound from the drum at the same time that the drummer s hand strikes the drum head. Sound waves travel very fast. The speed of sound in air at room temperature is about 346 m/s. The speed of a sound wave depends on the temperature and the material through which the wave travels, as Figure 2 shows. The speed of sound in a medium depends on how quickly the particles transmit the motion of the sound waves. The molecules in a gaseous medium, such as air, are farther apart than the particles in a solid or liquid are, so sound waves travel slower in air. Gas molecules move faster and collide more frequently at high temperatures than at low temperatures, so sound waves travel faster at high temperatures. In a liquid or solid, the particles are much closer together than in a gas, so the vibrations are transferred more rapidly from one particle to the next. However, some solids, such as rubber, dampen vibrations so that sound does not travel well. Materials like rubber can be used for soundproofing. Figure 2 Speed of Sound in Various Mediums Medium Speed of sound (m/s) Analysis Medium Gases Liquids at 25 C Speed of sound (m/s) Air (0 C) 331 Water 1,490 Air (25 C) 346 Sea water 1,530 Air (100 C) 386 Solids 1. Compare the volume and quality of the sound received through the string with the volume and quality of the sound received through the air. 2. Does sound travel better through the string or through the air? Explain. 3. Explain your results. Helium (0 C) 972 Copper 3,813 Hydrogen (0 C) 1,290 Iron 5,000 Oxygen (0 C) 317 Rubber 54 Special Education Students Feeling Vibrations Make sure students understand the word vibration. Borrow a drum from the band room. Place the drum on its side. Have two or three students hold their hands on the bottom of the drum. Firmly hit the top of the drum several times. Ask students if they can feel the vibrations. Repeat with students hands at the edge. Ask students if they can feel the hit any more loudly when their hands are on the top of the drum. Kinesthetic Basic Learners Speeds of Sound Using Figure 2, have students rearrange the speed of sound in various materials from fastest to slowest. Ask students what materials could be used for soundproofing the classroom. You may also ask students to explain, in writing, orally, or dictated, why sound waves travel faster in one material than another. Logical/Visual 544 CHAPTER 16 Sound and Light

8 Loudness is determined by intensity. How do the sound waves change when you increase the volume on your stereo or television? The loudness of a sound depends partly on the energy contained in the sound waves. The intensity of a sound wave describes the rate at which a sound wave transmits energy through a given area of a medium. Intensity depends on the amplitude of the sound wave and your distance from the source of the sound. The greater the intensity of a sound is, the louder the sound will seem. However, a sound that has twice the intensity of another sound does not seem twice as loud. Humans perceive loudness on a logarithmic scale. So, a sound seems twice as loud when its intensity is 10 times the intensity of another sound. The quietest sound a human can hear is the threshold of hearing. The relative intensity of a sound is found by comparing the intensity of a sound with the intensity at the threshold of hearing. Intensity is measured in units called decibels, db. An increase in intensity of 10 db means a sound seems about twice as loud. A few common sounds and their decibel levels are shown in Figure 3. The quietest sound a human can hear is 0 db. A sound of 120 db is at the threshold of pain, so sounds louder than this can hurt your ears and give you headaches. Extensive exposure to sounds above 120 db can cause permanent deafness. What determines the intensity of a sound wave? (See Appendix E for answers to Reading Checks.) Cat purring, 30 db Threshold of hearing Vacuum cleaner, 70 db Figure 3 Sound intensity is measured on a logarithmic scale of decibels. How many times louder than a cat purring will a normal conversation seem? Nearby jet airplane, 150 db Threshold of pain Visual Literacy Figure 3 shows the decibel levels of several common sounds. Have students examine the figure. Discuss with students: What is the decibel level of normal conversation? (50 db) What is the level of a vacuum cleaner? (70 db) How many times louder would a vacuum cleaner sound than a normal conversation? (four times as loud; an increase of 10 db makes a sound seem twice as loud.) Visual Answer to caption question A normal conversation should seem about four times as loud as a cat purring. Loudness Some students believe that hitting an object harder will change the pitch of the sound. This actually affects the amplitude, and thus the intensity or loudness of the sound. The pitch depends on the frequency of the vibration, which is not affected by how hard an object is struck. Use a drum to demonstrate the relationship between loudness and how hard the drum is struck. Musical 0 db 30 db 50 db 70 db 90 db 120 db 150 db Normal conversation, 50 db Lawn mower, 90 db Basic Learners Sound Levels If you have or can borrow a sound level meter, let students measure the intensity of various sounds around school. Have them take note of the decibel (db) scale on the meter. These meters are inexpensive and available at local electronic stores. Kinesthetic SECTION 1 Sound 545

9 Visual Literacy Have students examine Figure 4. Ask students which of these mammals can hear the highest sounds? (dolphins) Which can hear the lowest? (elephants) Do the hearing ranges of mammals overlap? (yes) What frequency of sounds can be heard by all of the mammals shown? (70 Hz to 12,000 Hz) Visual Elephant Sounds Several years ago, scientist Katherine Payne of Cornell University discovered that elephants can make and hear sounds having pitches well below the range of human hearing, which ends at about 20 Hz. She found that elephants in the wild communicate over long distances with these sounds, which were in the range of 6 to 18 Hz. Few objects or materials can absorb the energy of sound waves of these frequencies, so the sounds can be heard by other elephants long distances away. Encourage interested students to learn more about elephant communication. Have students summarize their research in a poster to display in the classroom. Visual pitch (PICH) a measure of how high or low a sound is perceived to be, depending on the frequency of the sound wave infrasound (IN fruh SOWND) slow vibrations of frequencies lower than 20 Hz ultrasound (UHL truh SOWND) any sound wave with frequencies higher than 20,000 Hz Elephant Pitch is determined by frequency. Pitch is a measure of how high or low a sound is and depends on the sound wave s frequency. A high-pitched sound is made by something vibrating rapidly, such as a violin string or air in a flute. A low-pitched sound is made by something vibrating slowly, such as a cello string or the air in a tuba. In other words, high-pitched sounds have high frequencies, and low-pitched sounds have low frequencies. Trained musicians are capable of detecting subtle differences in frequency, even a change as slight as 2 Hz. Humans hear sound waves in a limited frequency range. The human ear can hear sounds from sources that vibrate as slowly as 20 vibrations per second (20 Hz) and as rapidly as 20,000 Hz. Any sound that has a frequency below the range of human hearing is infrasound. Any sound that has a frequency above the range of human hearing is ultrasound. Many animals can hear frequencies of sound outside the range of human hearing, as Figure 4 shows. What are the upper limit and the lower limit of frequencies that humans can hear? Human Figure 4 Humans can hear sounds that have frequencies of 20 Hz to about 20,000 Hz, but many other animals can hear sounds into the infrasound and ultrasound ranges. Dog Dolphin English Learners Pitch Versus Volume Students unfamiliar with musical terms may confuse the idea of high and low pitch with loud and soft sounds. All students should understand these words before proceeding. Also, students should understand that the words fast and slow when applied to vibration refer to the frequency, or oftenness, of the back and forth movement. 546 CHAPTER 16 Sound and Light

10 Musical Instruments Musical instruments, from deep-sounding tubas to twangy banjos, come in a wide variety of shapes and sizes and produce a wide variety of sounds. But musical instruments can be grouped into a small number of categories based on how they make sound. V Most instruments produce sound through the vibration of strings, air columns, or membranes. Musical instruments rely on standing waves. When you pluck the string of a guitar, as Figure 5 shows, particles in the string vibrate. Sound waves travel out to the ends of the string and then reflect back toward the middle. These vibrations cause a standing wave on the string. The two ends of the string are called nodes, and the middle of the string is called an antinode. You can change the pitch by placing your finger on the string anywhere on the guitar s neck. A shorter length of string vibrates more rapidly, and the standing wave has a higher fre quency. The resulting sound has a high pitch. Standing waves can exist only at certain wavelengths on a string. The primary standing wave on a vibrating string has a wavelength that is twice the length of the string. The frequency of this wave, which is also the frequency of the string s vibrations, is called the fundamental frequency. All musical instruments use standing waves to produce sound. The standing waves that form on the head of a drum are shown in Figure 5. In a flute, standing waves form in the column of air inside the flute. Opening or closing holes in the flute body changes the length of the air column, which changes the wavelength and frequency of the standing waves. Frequency and Pitch 10 min 1 Hold one end of a flexible metal or plastic ruler on a desk with about half of the ruler hanging off the edge. Bend the free end of the ruler, and then release it. Can you hear a sound? 2 Try changing the position of the ruler so that less hangs over the edge. How does the change in position change the sound produced? Figure 5 Musical instruments produce standing waves, which in turn produce sound waves in the air. Teacher s Notes Be sure that students do not bend the rulers too far. They should not cause the metal rulers to be permanently bent or cause the plastic rulers to break. Have students relate their observations to the idea of pitch. Materials per Group desk ruler, flexible metal or plastic Answers 1. Sample answer: Yes, I can hear the sound. 2. When less of the ruler vibrates, the ruler is vibrating at a higher frequency, so the sound has a higher pitch. Teaching Key Ideas Fundamental Frequency To explain how standing waves relate to fundamental frequency, point out that any object, when disturbed, will vibrate at certain natural frequencies that are characteristic of that object. Certain objects vibrate better than others and produce sounds of recognizable pitch. Ask students to name objects that fit into this category. (Responses may include bells, stretched strings, xylophone bars (metal), marimba bars (wood), crystal glassware.) Vibrations on a guitar string produce standing waves on the string. Vibrations on a drum head produce standing waves on the head. Differentiated Instruction Advanced Learners Music and Math A musical interval (two notes played together) can sound either consonant (harmonious or pleasing) or dissonant (inharmonious or harsh). In many consonant intervals, there is a whole-number ratio between the two frequencies, for example the octave (2:1), the perfect fifth (3:2), and the major third (5:4). Many dissonant intervals have irrational ratios. Encourage students to learn more about the relationship between mathematics and music and to share their results with the class. Logical Alternative Assessment Instruments of Other Cultures Ask students to research instruments from other cultures. Students may wish to focus on the instruments of one culture, or on one general type of instrument. Ask them to design a sales brochure or catalog for the instruments they learn about, including illustrations, information about what the instruments sound like and how the sounds are produced, and comparisons to familiar instruments. Visual SECTION 1 Sound 547

11 How Can You Amplify the Sound of a Tuning Fork? 20 min Teacher s Notes You should provide each group with at least one pair of forks of the same frequency. If you have a hollow resonance box, you may include it along with blocks of wood, metal bars, and so on. Caution students not to strike the tuning forks against anything hard. Dents and dings will eventually make a tuning fork useless. Tuning forks may be activated with a rubber stopper if rubber blocks are not available. Students can also get the second fork to vibrate by touching the bases of both forks simultaneously to a resonant surface. Materials per Group block, rubber objects, wood or metal tuning forks, 2 of the same frequency and at least 1 of a different frequency Answers to Analysis 1. Sample answer: The objects, such as desktops, boxes, tabletops, and sheets of poster board, were made of hard, thin materials and had a relatively large surface area. 2. Yes, this works when using two tuning forks with the same pitch. 3. Sample answer: Yes, if I touch the base of a ringing tuning fork to a resonant object and then touched a tuning fork of the same frequency to the object, the second tuning fork starts vibrating. 4. They both ring with the same frequency, or pitch. Procedure 1 Activate a tuning fork by striking the tongs of the fork against a rubber block. 2 Touch the base of the tuning fork to various wood or metal objects. Listen for any changes in the sound of the tuning fork. resonance (REZ uh nuhns) a phenomenon that occurs when two objects naturally vibrate at the same frequency 3 Activate the fork again. Then, try touching the base of the tuning fork to the bases of other tuning forks. Make sure that the tines of the forks are free to vibrate and are not touching anything. 4 If you find two tuning forks that resonate with each other, try activating one and holding it near the tines of the other one. Analysis 1. What are some characteristics of the objects that helped to amplify the sound of the tuning fork in step 2? 2. In step 3, could you make another tuning fork start vibrating by touching it with the base of your tuning fork? 3. In step 4, could you make another tuning fork vibrate without touching it with your tuning fork? 4. What is the relationship between the frequencies of the tuning forks that resonate with each other in steps 3 and 4? Instruments use resonance to amplify sound. When you pluck a guitar string, you can feel that the bridge and the body of the guitar vibrate. These vibrations, which are a response to the vibrating string, are called forced vibrations. The body of the guitar has natural frequencies, which are the specific frequencies at which it is most likely to vibrate. The sound produced by the guitar will be loudest when the forced vibrations cause the body of the guitar to vibrate at a natural frequency. This effect is called resonance. When resonance occurs, both the string and the guitar body are vibrating at the same frequency, which amplifies the sound. The guitar body has a larger area than the string and is in contact with more molecules in the air. So, the guitar body is better at transferring the vibrations to the air than the string is. The natural frequency of an object depends on the object s shape, size, mass, and the material from which the object is made. Complex objects such as guitars have many natural frequencies, so they resonate well at many pitches. However, some musical instruments, such as an electric guitar, do not resonate well and must be amplified electronically. Why does resonance amplify a sound? Showing Waves with a Stroboscope If your school has an adjustable strobe lamp, show students the standing wave on a vibrating string of a guitar. Adjust the strobe until students can see that the string vibrates not only in one segment but in two or more at the same time. A bright flashlight shining through the blades of a fan is a makeshift substitute, but the fan s speed must be continuously adjustable. Alternative Assessment Comparing Frequency and Intensity Have students use common objects or musical instruments to demonstrate the following: two sounds of different intensities but the same frequency; two sounds of different frequencies but about the same intensity two sounds of different pitches but about the same amplitude; two sounds of different amplitudes but the same pitch two sounds of different wavelengths but the same amplitude Kinesthetic/Musical 548 CHAPTER 16 Sound and Light

12 Hearing and the Ear How do you hear waves and interpret them as different sounds? V The human ear is a sensitive organ that senses vibrations in the air, amplifies them, and then transmits signals to the brain. In some ways, the process of hearing is the reverse of the process by which a drum head makes a sound. In the ear, sound waves cause membranes to vibrate. Vibrations pass through three regions in the ear. Your ear is divided into three regions outer, middle, and inner as Figure 6 shows. Sound waves travel through the fleshy part of your outer ear and down the ear canal. The ear canal ends at the eardrum, a thin, flat piece of tissue. The vibrations pass from the eardrum through the three small bones of the middle ear the hammer, the anvil, and the stirrup. The vibrations cause the stirrup to strike a membrane at the opening of the inner ear, which sends waves through the spiral-shaped cochlea. Resonance occurs in the inner ear. The cochlea contains a long, flexible membrane called the basilar membrane. Different parts of this membrane vibrate at different natural frequencies. So, a wave of a particular frequency causes a specific part of the basilar membrane to vibrate. Hair cells near that part of the membrane then stimulate nerve fibers that send an impulse to the brain. The brain interprets this impulse as a sound that has a specific frequency and intensity. Mnemonics Create a sentence to help you remember the parts of the ear (eardrum, anvil, hammer, stirrup, and cochlea). The words in the sentence should begin with the letters E, A, H, S, and C. Topic: The Ear Code: HK80440 Teaching Key Ideas How the Human Ear Works Figure 6 shows the different parts of the ear. As students read the description of vibrations passing through the ear, have them follow the path of the vibrations in the figure. Then, ask students the following questions: What is the function of the outer ear? (to collect sound waves and direct them into the ear canal) What do the bones in the middle ear do? (The three bones act as levers to increase the size of vibrations.) What happens in the inner ear? (vibrations are converted into electrical signals for the brain to interpret) Visual Mnemonics Students can also include other parts of the ear, such as pinna and ear canal, in their mnemonic sentence. Encourage students to make a sentence that keeps the order of the parts correct. Sample answer: Each hamster ate several carrots. Verbal Figure 6 Sound waves are transmitted as vibrations through the ear. Keyword: HK8SALF6 3 In the inner ear, the basilar membrane vibrates. The movement of this membrane causes a signal to be sent to the brain. Students can interact with the figure by going to go.hrw.com and typing in the keyword HK8SALF6. 1 In the outer ear, sound waves cause the eardrum to vibrate. 2 In the middle ear, vibrations cause the stirrup bone to strike the outer membrane of the inner ear. Cochlear Implants Since the late 1980s, scientists around the world have used cochlear implants in over 70,000 people. While hearing aids merely amplify sound, cochlear implants actually bypass damaged parts of the inner ear, thereby restoring some level of hearing to people with profound deafness. Although the implants cannot fully restore hearing, they provide enough benefit to drastically improve the recipient s ability to communicate fluently in verbal conversation. The recipient of a cochlear implant wears a microphone in a hearing case behind the ear, and a sound processor about the size of a personal radio on a belt or in a pocket. The microphone picks up sounds and send them to the processor, which selects and arranges the sounds. A transmitter converts these sounds to electrical signals, which are sent to electrodes implanted in the cochlea and then to the brain. Researchers are currently working to improve hearing for cochlear-implant recipients and to find additional candidates for the use of this new technology. SECTION 1 Sound 549

13 Teaching Key Ideas Echolocation Animals such as bats, dolphins, and beluga whales use echolocation to find food and to locate objects in their path. An animal that uses echolocation sends out sound waves. The sound waves reflect off objects and are heard by the animal as an echo. The time needed for the echo to reach the animal lets the animal know how far away an object is. What Are Sonograms? In addition to sonograms and sonar, there are many other medical and nonmedical uses for ultrasound. Have interested students choose an application to research, and ask them to write a report with their findings. Discuss the following with students to give them some starting points for topic ideas. Kidney stones can be disintegrated with frequencies between 20,000 Hz and 30,000 Hz. Tumors have been treated by ultrasound beams from outside the body. The beams come to a focus at the site of the tumor and kill the tissue by heating it. Students may also look into some of the dangers of the misuse of ultrasound. In addition to heating, some frequencies can cause cells to disintegrate. In nonmedical applications, ultrasound can be used to locate hairline fractures in metal support beams and machinery. High-intensity ultrasonic waves passing through a liquid-filled tank are used to clean jewelry, dentures, or small machinery placed in the tank. Verbal Why It Matters What are sonograms? Ultrasound and Sonar If you shout over the edge of a rock canyon, the sound may be reflected by the rock walls. If the reflected sound reaches your ears, you will hear an echo. V Reflected sound waves are used to determine distances and to create images. Some ultrasound waves are reflected at boundaries. Ultrasound waves have frequencies greater than 20,000 Hz. At high frequencies, ultrasound waves can travel through most materials. But some sound waves are reflected when they pass from one type of material into another. The number of waves reflected depends on the density of the materials at each boundary. The reflected waves from different boundary surfaces can be made into a computer image called a sonogram. Ultrasound is used to see inside the human body because it does not harm living cells. For one to see the details in a sonogram, the wavelengths of the ultrasound must be slightly smaller than the smallest parts of the object being viewed. The higher the frequency of a wave is, the shorter the wave s wavelength is. Sound waves that have a frequency of 15,000,000 Hz have a wavelength of less than 1 mm when they pass through soft tissue. So, in a sonogram, you would be able to see details that are about 1 mm in size. Ultrasound imaging is used in medicine. The echoes of high frequency ultrasound waves between 1,000,000 Hz and 15,000,000 Hz are used to produce computerized images called sonograms. Using sonograms, doctors can safely view organs inside the body without having to perform surgery. Sonograms can be used to diagnose problems and to guide surgical procedures. Sonograms are often used to view unborn fetuses. Because ultrasound does not harm the mother or fetus, it is a common way to check the progress of a pregnancy. WRITING IN SCIENCE 1. Research one way, other than fetal ultrasound, that ultrasound is used in medicine. Write a onepage report that explains how ultrasound is used for this application. Answer to Your Turn 1. Answers may vary. Examples of applications may include the use of ultrasound to do diagnostic imaging such as an echocardiogram, or to find gallstones or certain types of tumors. Visual Literacy The image of the fetus is an example of a sonogram. Ask students to compare the sonogram to a photograph. (Sample answer: The sonogram is more grainy than a photograph. The sonogram is monochromatic and photographs are usually in color.) Explain to students that sonograms are harder to read than regular photographs. Tell students that ultrasound technicians and doctors must receive special training to learn how to read sonograms properly. Visual 550 CHAPTER 16 Sound and Light

14 Sonar is used to locate objects underwater. How can a person on a ship measure the distance to the ocean floor, which may be thousands of meters from the water s surface? Sonar is a system that uses reflected sound waves for measurement and can measure large distances. Using sonar, distance can be determined by measuring the time it takes for sound waves to be reflected from a surface. A sonar device on a ship sends a pulse of sound downward and measures the time, t, that it takes for the sound to be reflected back to the device from the ocean floor. The distance, d, can be calculated by using a form of the speed equation that solves for distance, using the average speed of sound in water, v. d = vt Because the sound waves travel to a surface and back to the device, the measured time must be divided by two to obtain the distance from the device to the surface. Fisherman and researchers can use sonar to detect fish. If a school of fish passes under the ship, the sound pulse will be reflected back much sooner than the sound from the ocean floor. Submarines can also be detected using sonar. Ultrasound works very well in sonar systems because the waves can be focused into narrow beams and can be directed more easily than other sound waves. Bats use reflected ultrasound to navigate in flight and to locate insects for food. Section 1 Review KEY IDEAS 1. Identify two factors that affect the speed of sound. 2. Explain why sound travels slower in air than in water. 3. Distinguish between infrasound and ultrasound. 4. Determine which two properties of a sound wave change when pitch gets higher. 5. Determine which two properties of a sound wave change when a sound gets louder. CRITICAL THINKING 6. Analyzing Information Your friend tells you that a clap of thunder has a sound intensity of about 130 db, so it is almost twice as loud as a vacuum cleaner. Explain whether or not your friend is correct. Echoes and Distance 10 min 1 Stand outside in front of a large wall, and clap your hands. Do you hear an echo? 2 Use a stopwatch to measure the time that passes between the time you clap your hands and the time you hear the echo. 3 The speed of sound in air is about 340 m/s. Use the approximate time from step 2 and the speed equation to estimate the distance to the wall. sonar (SOH NAHR) sound navigation and ranging, a system that uses acoustic signals and echoes to determine the location of objects or to communicate 7. Applying Ideas Why does an acoustic guitar generally sound louder than an electric guitar without an electronic amplifier sounds? 8. Inferring Relationships On a piano keyboard, the C5 key has a frequency that is twice the frequency of the middle C key. Is the string that vibrates to make the sound waves for the C5 key shorter or longer than the middle C string? Explain your reasoning. 9. Describing Events You hear a phone ring. Describe the process that occurs in your ear that results in sound waves from the phone being translated into nerve impulses that are sent to the brain. 10. Analyzing Methods To create sonograms, why are ultrasound waves used instead of audible sound waves? Teacher s Notes Have students work in pairs so that one student can handle the stopwatch as the other student claps. Tell students to count to three and clap and start the stopwatch simultaneously. Materials per Group stopwatch Answers 1. Sample answer: Yes, I heard an echo. 2. Sample answer: It took 1.2 s for the sound to return. 3. Sample answer: d = vt = (340 m/s 1.2 s)/2 = 204 m Reteaching Key Ideas Retracing the Process of Sound and Hearing Ask students what the first step is in sound production. (a vibration) Have a student draw a very simple sketch of this on the board. Then, ask students what happens to the sound. (Sound waves are carried through a medium.) Have another student draw a very simple sketch of this process on the board. Then, ask students what happens to the sound once it reaches your ear. (It is funneled by the outer ear, magnified by the middle ear, and changed into electrical signals by the inner ear.) Have another student draw a sketch of this process on the board. Visual/Verbal Formative Assessment In general, the speed of sound is fastest in A. solids. (Correct. The particles of matter are closest together in solids, so vibrations of sound waves travel through solids the fastest. However some solids, such as rubber, dampen vibrations.) B. liquids. (Incorrect. Although the particles in liquids are close together, sound generally travels faster in solids than in liquids.) C. gases. (Incorrect. The particles of gases are far apart, so vibrations do not travel through gases as quickly as they do through liquids and solids.) D. vacuums. (Incorrect. Sound cannot travel in vacuums.) Answers to Section Review 1. Sample answers: State of matter, nature of the medium, and temperature affect the speed of sound. 2. In water, the molecules are packed closer together; in air the molecules are farther apart and collide less often than in water. 3. Infrasound refers to sounds that are below the range of human hearing (frequencies lower than 20 Hz). Ultrasound refers to sounds above the range of human hearing (frequencies higher than 20,000 Hz) 4. When pitch gets higher, the frequency increases and the wavelength decreases. Answers continued on p. 581A SECTION 1 Sound 551

15 SECTION 2 In this section, students learn about the wave and particle characteristics of light. They also study the electromagnetic spectrum in detail, including the relationship of energy to frequency and applications of electromagnetic waves in communication, medicine, and other areas. Bellringer Use the Bellringer transparency to prepare students for this section. 2 The Nature of Light Key Ideas Key Terms Why It Matters V How do scientific models describe light? V What does the electromagnetic spectrum consist of? photon intensity radar Cell phones use radio waves, not sound waves, to send signals. Most of us see and feel light almost every moment of our lives. We even feel the warmth of the sun on our skin, which is an effect of infrared light. We are very familiar with light, but how much do we understand about what light really is? Two-Slit Experiment Obtain a helium-neon laser and a two-slit plate (sometimes called a Cornell plate) to reproduce the experiment shown in Figure 1. Show students that the light from the laser forms a single dot when it shines directly onto a wall, but forms a wide pattern of light and dark spots when it passes through the double slits. Explain to students that diffraction and interference of light waves produces the pattern that they see. Visual Figure 1 Light can produce an interference pattern. 2 The light waves diffract as they pass through the slits. Waves and Particles It is difficult to describe all of the properties of light with a single scientific model. V The two most common models describe light either as a wave or as a stream of particles. Light produces interference patterns as water waves do. In 1801, the English scientist Thomas Young devised an experiment to test the nature of light. He passed a beam of light through two narrow openings and then onto a screen on the other side of the openings. He found that the light produced a striped pattern on the screen, like the pattern in Figure 1. This striped pattern is an interference pattern that is formed when waves interfere with each other. 3 The diffracted light waves interfere both constructively and destructively. 1 Red light of a single wavelength passes through two tiny slits. 4 An interference pattern is created. The constructive interference results in the bright band of light, and the destructive interference results in the dark bands. Key Resources Teaching Transparencies TM44 Wave Frequency and Photon Energy E18 Electromagnetic Spectrum Visual Concepts The Dual Nature of Light Energy of a Photon Electromagnetic Spectrum Cross-Disciplinary Worksheet Real World Applications How Does Sunscreen Work? 552 CHAPTER 16 Sound and Light

16 Light can be modeled as a wave. Because the light in Young s experiment produced interference patterns, Young concluded that light consists of waves. The model of light as a wave is still used today to explain many of the basic properties of light and light s behavior. This model describes light as transverse waves that do not require a medium in which to travel. Light waves are also called electromagnetic waves because they consist of changing electric and magnetic fields. The transverse waves produced by these fields can be described by their amplitude, wavelength, and frequency. The wave model of light explains how light waves interfere with one another. It also explains why light waves may reflect when they meet a mirror, refract when they pass through a lens, and diffract when they pass through a narrow opening. The wave model cannot explain some observations. In the early part of the 20th century, physicists began to realize that some observations could not be explained by the wave model of light. For example, when light strikes a piece of metal, electrons may fly off the metal s surface. Experiments show that in some cases, dim, blue light may knock some electrons off a metal plate, while very bright, red light cannot knock off any electrons, as Figure 2 shows. According to the wave model, very bright, red light has more energy than dim, blue light has because the waves in bright light have greater amplitude than the waves in dim light. But this energy difference does not explain how blue light can knock electrons off the plate while red light cannot. Light can be modeled as a stream of particles. One way to explain the effects of light striking a metal plate is to assume that the energy of the light is contained in small packets. A packet of blue light carries enough energy to knock an electron off the plate, but a packet of red light does not. Bright, red light contains many packets, but no single packet has enough energy to knock an electron off the plate. In the particle model of light, these packets, or units of light, are called photons. A beam of light is a stream of photons. Photons are considered particles, but they are not like ordinary particles of matter. Photons do not have mass. They are like little bundles of energy. Unlike the energy in a wave, the energy in a photon is located in a specific area. What can the particle model of light explain that the wave model cannot explain? Struggling Readers Summarizing The speed of light and the waveparticle duality of light are fundamental ideas of science. To help students grasp these concepts, proceed in a stepwise fashion by having students read the subheadings under Waves and Particles one at a time. At the end of each subsection, stop and ask for volunteers to summarize that part. After the student finishes, ask students whether they agree on the summary given. If not, ask for other volunteers to offer their insights. Verbal Figure 2 The particle model of light can explain some effects that the wave model cannot explain. Bright, red light cannot knock electrons off this metal plate. Dim, blue light can knock electrons off the plate. The wave model of light cannot explain this effect, but the particle model can. photon (FOH TAHN) a unit or quantum of light Teaching Key Ideas Particle versus Wave Isaac Newton believed that light is made up of tiny particles, which he called corpuscles. He published this theory in his book Optics in One of Newton s contemporaries, Christiaan Huygens, argued that light is a wave. Both Newton and Huygens offered convincing evidence, and the subject was a matter of great debate. Later experiments (such as Young s double-split experiment) seemed to verify Huygens theory, which became widely accepted by scientists until Einstein proposed his photon theory in Einstein did not dispute the wave nature of light, but argued that in some cases, a particle model is more useful. Visual Literacy To help students understand the photon concept pictured in Figure 2, have them visualize a brick wall being struck by streams of thousands of ping-pong balls. No single ball has enough kinetic energy to chip the bricks. Then imagine the same wall being struck at the same speed by only a few steel bolts. Even though just a few strike the wall, each has enough energy to break chips out of the bricks. The many ping-pong balls represent bright red light and the few bolts represent dim blue light. The brightness of light corresponds to the number of photons striking a surface per unit of time, and the color of light corresponds to the energy per photon. SECTION 2 The Nature of Light 553

17 Booklet One way that students can make their booklets more useful is to number the pages and then cross-reference them. For example, on the page that describes which phenomena are explained by the wave model of light, students may write the number of the page that describes which phenomena are explained by the particle model. Teaching Key Ideas Accepting Two Models Some students may have difficulty understanding or accepting the idea of using two models of light. Tell students that scientists often use two or more models to represent the same phenomenon. Discuss some examples of more familiar dual models, such as maps and globes to represent the surface of Earth and the Bohr model and the electron-cloud model of the atom. Help students see that some models are useful in certain circumstances and other models are useful at different times. Answer to caption question The frequency increases as wave energy increases. Topic: Properties of Light Code: HK81227 Booklet Create a booklet, and label the cover Models of light. Label half of pages inside the booklet Wave model and the other half of the pages Particle model. Write details about each model on the appropriate pages of your booklet. Figure 3 The energy of photons of light is related to the frequency of electromagnetic waves. What happens to the frequency of the waves as the energy increases? The model of light used depends on the situation. Light can be modeled as either waves or particles. Which model is correct? The success of any scientific theory depends on how well the theory can explain various observations. Some effects, such as the interference of light, are more easily explained with the wave model. Other effects, such as light knocking electrons off a metal plate, are better explained by the particle model. The particle model also easily explains how light can travel across empty space without a medium. Most scientists currently accept both the wave model and the particle model of light. The model they use depends on the situation that they are studying. Some scientists think that light has a dual nature, which means that light can behave both as waves and as particles. In many cases, using either the wave model or the particle model of light gives good results. The energy of light is proportional to frequency. Whether modeled as a particle or as a wave, light is also a form of energy. Each photon of light can be thought of as carrying a small amount of energy. The amount of this energy is proportional to the frequency of the corresponding electromagnetic wave, as Figure 3 shows. A photon of red light, for example, carries an amount of energy that corresponds to the frequency of waves in red light, Hz. Ultraviolet photons have about twice as much energy as red light photons. So, the frequency of ultraviolet waves is about twice the frequency of red light waves. Likewise, a photon that has half the energy of red light, a photon of infrared light, corresponds to a wave that has half the frequency of a wave of red light. Type of wave Wavelength Wave frequency Photon energy Infrared Visible light m Hz J m Hz J Ultraviolet m Hz J Visual Literacy Use Figure 3 to remind students that light color relates to the energy per photon. In the wave model, color relates to frequency, as pointed out in the next subsection. Therefore, frequency and energy per photon correspond to each other. Alternative Assessment The Newton-Huygens Debate Have students research the arguments used by Newton and Huygens for their theories of light. Ask them to assess the validity of the arguments, and evaluate which arguments still hold today and which have been overturned by our modern understanding of light. Logical 554 CHAPTER 16 Sound and Light

18 The speed of light depends on the medium. In a vacuum, all light travels at the same speed and is represented by the variable c. The speed of light is very fast, about m/s (about 186,000 mi/s). There is no known entity in the universe that is faster than light. Light travels through transparent media, such as air, water, and glass. When light passes through a medium, however, the light travels slower than it does in a vacuum. Figure 4 shows the speed of light in several mediums. The brightness of light depends on intensity. Reading near a lamp that has a 100 W bulb is easier than reading near a lamp that has a 60 W bulb. A 100 W bulb is brighter than a 60 W bulb and helps you see. The quantity that measures the amount of light illuminating a surface is intensity. Intensity depends on the number of photons per second, or power, that pass through a certain area of space. The intensity of light decreases as distance from the light source increases because the light spreads out in spherical wave fronts. Imagine a series of spheres centered on a source of light, as Figure 5 shows. As light spreads out from the source, the number of photons or the power passing through a given area on a sphere decreases. So, the light is dimmer for an observer farther from the light source than for an observer closer to the light source. What does the intensity of light depend on? Figure 4 Speed of Light in Various Mediums Medium Speed of light ( 10 8 m/s) Vacuum Air Ice 2.29 Water 2.25 Quartz 2.05 Glass 1.97 Diamond 1.24 intensity (in TEN suh tee) the rate at which energy flows through a given area of space Teaching Key Ideas Speed of Light Unlike sound, which travels better in liquids and solids than in air, light travels more slowly as the density of the medium increases. When light passes through a medium, it encounters the many atoms that make up the medium. If light hits an atom, the light is scattered, which takes a very small amount of time. As light passes through the empty space between atoms in the medium, the light moves at its full speed of m/s. The speed of light in the medium is really an average speed that takes into account the scattering of the light as it encounters atoms. Visual Literacy The speed of light in air is slower than the speed of light in a vacuum, but the difference is very small, as students can see in Figure 4. The speed of light in a diamond is less than half the speed of light in a vacuum, but still on the same order of magnitude ( 10 8 m/s). Figure 5 Less light falls on each unit square as the distance from the source increases. English Learners Learning Prefixes Tell students that the prefix photo- means relating to light. Ask students to brainstorm a list of words that begin with photo-. Then, have them write definitions of the words and describe how the words relate to light. Verbal/Visual SECTION 2 The Nature of Light 555

19 Teaching Key Ideas Electromagnetic Spectrum Remind students that radiation in other parts of the electromagnetic spectrum is qualitatively no different from visible light. However, our eyes are sensitive to the frequencies in the visible spectrum. The unique properties mentioned in the text depend less on the nature of the radiation and more on the way the radiation interacts with matter. Emphasize the continuity of the visible spectrum by pointing out that it consists not only of red, orange, yellow, green, blue, and violet, but all intermediate colors as well. Visual Literacy Figure 6 shows the different parts of the electromagnetic spectrum. Ask students: Which end of the spectrum has higher frequency? (Frequency increases toward the right side of the spectrum.) Which end of the spectrum has longer wavelengths? (Wavelength increases toward the left side of the spectrum.) Which end of the spectrum has higher energy? (The right side has higher energy; the energy of electromagnetic waves is proportional to frequency.) Visual Topic: Electromagnetic Spectrum Code: HK80482 radar (RAY DAHR) radio detection and ranging, a system that uses reflected radio waves to determine the velocity and location of objects Figure 6 The Electromagnetic Spectrum The Electromagnetic Spectrum Light fills the air and space around us. Our eyes can detect light waves that have wavelengths of 400 nm (violet light) to 700 nm (red light). But the visible spectrum is only a small part of the electromagnetic spectrum, as Figure 6 shows. We live in a sea of electromagnetic waves that range from radio waves given off by TV stations to the sun s ultraviolet waves. V The electromagnetic spectrum consists of waves at all possible energies, frequencies, and wavelengths. Although all electromagnetic waves are similar in certain ways, each part of the electromagnetic spectrum has unique properties. Many modern technologies, including radar guns and cancer treatments, use electromagnetic waves. Radio waves are used in communications and radar. Radio waves are the longest waves in the electromagnetic spectrum and have wavelengths from tenths of a meter to thousands of meters. This part of the electromagnetic spectrum includes TV signals, and AM and FM radio signals. Air traffic control towers at airports use radar to find the locations of aircraft. Radar is a system that uses radio waves to find the locations of objects. Antennas at the control tower emit radio waves. The radio waves bounce off the aircraft and return to a receiver at the tower. Many airplanes are equipped with special radios called transponders. These radios receive a signal from the tower and send a new signal back to the tower. This signal gives the plane s location and elevation. Radar is also used by police to monitor the speed of vehicles. Radio waves are used in radar systems to monitor air traffic. Microwaves are used to send phone signals long distances. Infrared waves are used by special cameras to image the temperature differences in an object. The Radio Spectrum The frequencies at which radio and television stations broadcast in the United States are assigned by the Federal Communications Commission (FCC). In fact, the FCC has assigned frequencies for all devices that use radio waves, including garage door openers, radio controlled toys, and baby monitors. Have students research how the FCC has divided the radio-wave spectrum and make a poster showing what they learned. Alternative Assessment Electromagnetic Wave Applications Have students research electromagnetic wave applications. Ask each student to focus on one application that is not discussed in the student text. Have them prepare a poster that illustrates the application, including how it works, what type of electromagnetic waves are used, and why that type works best. Display student posters around the classroom. Verbal/Visual 556 CHAPTER 16 Sound and Light

20 Microwaves are used in cooking and communication. Electromagnetic waves that have wavelengths in the range of centimeters are known as microwaves. Microwaves are used to carry telecommunication signals over long distances. Space probes use microwaves to transmit signals back to Earth. Microwaves are reflected by metals and are easily transmitted through air, glass, paper, and plastic. However, the water, fat, and sugar molecules in food all absorb microwaves. The absorbed microwaves can cook food. Microwaves can travel about 3 to 5 cm into most foods. They are absorbed as they go deeper into food. The energy from the absorbed waves causes water and other molecules to rotate. The energy of these rotations spreads throughout the food and warms it. Infrared light can be felt as warmth. Electromagnetic waves that have wavelengths slightly longer than wavelengths of red visible light are in the infrared (IR) part of the spectrum. Infrared light from the sun warms you. Infrared light from heat lamps is used to keep food warm. Devices and photographic film that are sensitive to infrared light can reveal images of objects. An infrared sensor can measure the heat energy that objects radiate. These data can then be used to create images that show temperature variations. Remote infrared sensors on weather satellites can record temperature changes in the atmosphere and track the movement of clouds. Many computers can detect infrared signals from external devices, such as a computer mouse. Name three devices that use infrared light. Academic Vocabulary detect (dee TEKT) to discover the presence of something Visible light, the colors that we can see, is a small part of the electromagnetic spectrum. Ultraviolet light is emitted by the sun and has more energy than visible light does. X rays are absorbed by bones and make bright areas in X-ray images. Gamma rays, such as those produced by nuclear reactions, have the highest energy of any waves in the electromagnetic spectrum. Real-World Connection Microwave Ovens Although microwaves are reflected by metals, powdered metals absorb a significant amount of microwaves and become very hot. This technology is used in packaging microwavable foods. A paper or plastic sheet containing metal powder becomes hot and browns or fries the food. Teaching Key Ideas Dividing the Electromagnetic Spectrum Be sure students understand that the division point between microwaves and radio waves is arbitrarily set, as are other division points on the electromagnetic spectrum. There is no significant difference between the waves on either side of a division point. The exception is visible light, which is limited by the eye s sensitivity. Teaching Key Ideas Energy and Penetrating Power Reiterate the connection between color and energy by pointing out that ultraviolet light can damage cells because of the high energy of its photons, which also gives it penetrating ability. Call students attention to the fact that X rays lie just beyond the ultraviolet part of the spectrum and have enough penetrating power to pass completely through the body. Also point out that X rays are even more damaging to cells than UV rays of the same intensity. Special Education Students Sunscreen and SPF Have students examine labels from different kinds of sunscreen, and develop a chart listing the different kinds and the SPF for each. By using information from the chapter, have students add a column to their chart that describes the effects on the skin that can be avoided when each kind of sunscreen is used. Students should also be able to explain why a person can still get a sunburn on a cloudy day. Logical/Visual SECTION 2 The Nature of Light 557

21 Reteaching Key Ideas Comparing Models Have students make a two-column chart that compares the wave model of light and the particle model of light. Tell students to include evidence supporting both models in their chart. Also have students describe how the electromagnetic spectrum is arranged in both models. (In the wave model, the electromagnetic spectrum is arranged by wavelength and frequency. In the particle model, the electromagnetic spectrum is arranged by photon energy.) Logical/Visual Formative Assessment Photon energy increases as A. wavelength increases. (Incorrect. As the wavelength of light increases, the photon energy decreases.) B. intensity decreases. (Incorrect. The energy of light does not depend on intensity. As the intensity of light decreases, the brightness of the light decreases.) C. frequency increases. (Correct. Photon energy is directly proportional to frequency. As the frequency of light increases, photon energy increases.) D. frequency decreases. (Incorrect. Photon energy is directly proportional to frequency. As the frequency of light decreases, photo energy decreases.) Figure 7 We cannot see ultraviolet light, but it can still damage the skin. Sunscreens protect skin by absorbing or blocking ultraviolet light before it reaches the skin. Section 2 Review KEY IDEAS 1. State one piece of evidence that supports the wave model of light and one piece of evidence that supports the particle model of light. 2. Name the regions of the electromagnetic spectrum from the shortest wavelengths to the longest wavelengths. 3. Determine which photons have more energy, those associated with microwaves or those associated with visible light. 4. Determine which band of the electromagnetic spectrum has the following: a. the lowest frequency c. the most energy b. the shortest wavelength d. the least energy Sunlight contains ultraviolet light. The invisible light that lies just beyond violet light falls into the ultraviolet (UV) part of the spectrum. Ultraviolet light has higher energy and shorter wavelengths than visible light does. Although humans cannot see UV light, many insects can see it. Nine percent of the energy emitted by the sun is ultraviolet light. Earth s atmosphere, in particular the ozone layer, absorbs much of this UV light. The UV light that makes it through the ozone layer has enough energy that some of the energy can pass through thin layers of clouds. As a result, you can get a sunburn, as Figure 7 shows, on overcast days. X rays and gamma rays are used in medicine. Beyond the ultraviolet part of the spectrum are waves called X rays, which have higher energy and shorter wavelengths than ultraviolet waves do. X rays have wavelengths less than 10 8 m. Gamma rays are the electromagnetic waves with the highest energy and have wavelengths shorter than m. An X-ray image of bones is made by passing X rays through the body. Most of them pass right through, but a few are absorbed by bones and other tissues. The X rays that pass through the body to a photographic plate produce an image. X rays are useful tools for doctors, but they can also be dangerous. Both X rays and gamma rays have very high energies, so they may kill living cells or turn them into cancer cells. However, gamma rays can also be used to treat cancer by killing the diseased cells. CRITICAL THINKING 5. Applying Concepts A certain molecule can absorb the energy of red light but cannot absorb the energy of green light. Does the wave model or the particle model of light better explain this statement? Explain. 6. Applying Ideas You and a friend are looking at the stars, and you notice two stars close together, one bright and one fairly dim. Your friend comments that the bright star emits much more light than the dimmer star. Is your friend correct? Explain your answer. 7. Evaluating Conclusions You and a friend decide to go hiking on a cloudy day. Your friend claims that she does not need any sunscreen because the sun is not shining. What is wrong with her reasoning? Answers to Section Review 1. Answers may vary. Interference, reflection, and refraction support the wave model. The fact that blue light can knock electrons off a metal plate while red light cannot (the photoelectric effect) supports the particle model. 2. The regions of the electromagnetic spectrum from shortest to longest wavelength are gamma rays, X rays, ultraviolet light, visible light (from violet to red), infrared light, microwaves, and radio waves. 3. Photons of visible light have higher energy than photons associated with microwaves because their frequency is greater. 4. a. Radio waves have the lowest frequency. b. Gamma rays have the shortest wavelength. c. Gamma rays have the most energy. d. Radio waves have the least energy. 5. The particle model of light explains this better than the wave model because only certain packets of energies are absorbed. 6. He may be wrong because there is no way to judge. The star that appears brighter may actually emit less light but be much closer than the dimmer star. 7. Ultraviolet light from the sun has enough energy to make it through clouds, so it could still cause a sunburn. 558 CHAPTER 16 Sound and Light

22 Why It Matters How do cell phones work? If you looked inside of a cellular phone, you would find a small radio wave transmitter/receiver, or transceiver. Cellular phones communicate with one of an array of antennas mounted on towers or tall buildings. The area covered by each antenna is called a cell. As the user moves from one cell to another, the phone switches to communicate with the antenna in that cell. As long as the phone is not too far from a cellular antenna, the user can make and receive calls. The antenna is connected to a base station, which is also a transceiver. The base station sends the call to the mobile telephone switching office, which routes the call. 5 The base station sends a signal to your friend s phone. How Do Cell Phones Work? Students may be surprised to learn that the cell covered by each antenna is approximately only 26 km 2 a circular area that has a radius of approximately 3 km. So if a person in a moving car is using a cell phone, the antenna that handles the call changes frequently. Ask students to learn why dividing an area into relatively small cells is advantageous. Have students write what they learn in a one-page paper. Verbal 2 The call is sent to the mobile telephone switching office (MTSO). 3 Depending on its destination, the call is routed through a wire cable, fiber-optic cable, microwave towers, or communication satellites. 4 The telephone signal arrives at another switching office and is sent to a base station near your friend. Topic: Telephone Technology Code: HK81499 Visual Literacy The diagram on this page shows how a signal travels from one cell phone to another. In the diagram, the signal is shown as waves traveling from one component to another in a single path. Be sure students understand that the signals sent out by certain components, such as the cell phone and the base stations, are actually sent out in all directions. Answers to Your Turn 1. The signal from my phone goes to the nearest cell phone tower first. 2. Answers may vary. 1 Your telephone call is picked up by the nearest cell phone tower. UNDERSTANDING CONCEPTS 1. When you place a call on a cellular phone, where is the first place that the radio signal goes? ONLINE RESEARCH 2. Research the development of the cellular phone and the timeline of the history of this development. Advanced Learners Benefits and Drawbacks of Cell Phones Organize students into two teams and ask the teams to debate the benefits and drawbacks of cell phones. Randomly assign which team will argue the benefits and which team will argue the drawbacks. Give the teams a few days to do research and to organize their arguments. Then, have the teams debate orally in class. Verbal/Interpersonal SECTION 2 The Nature of Light 559

23 SECTION 3 This section begins with the law of reflection. Next, students learn how flat and concave mirrors form virtual and real images. The section concludes with a discussion of color, including why objects appear as different colors and how colors can be added or subtracted. 3 Reflection and Color Key Ideas Key Terms Why It Matters V How do objects interact with incoming light? V How can you see an image in a mirror? V Why do we see colors? light ray virtual image real image An object s color comes from the light that is reflected by the object. Bellringer Use the Bellringer transparency to prepare students for this section. Adding Colors Cover one lens of a high-intensity flashlight with a green filter and a second flashlight lens with a red filter. In a darkened room, turn the green light on, and shine it on a white sheet, white wall, or overhead screen. Turn the red light on, and shine it on a different area of the screen. Ask the students what colors they see. Ask students to predict what color they will see if you overlap the green and red light. The students will probably answer brown. Overlap the two colors. (Yellow will appear.) Visual Answer to caption question The light is focused into the large box in front of the mirror. (Teacher s Note: The large box is a large furnace.) light ray (LIET RAY) a line in space that matches the direction of the flow of radiant energy Figure 1 This solar collector in the French Pyrenees uses mirrors to reflect and focus light. Where is the light focused? You may be used to thinking about light bulbs, candles, and the sun as objects that send light to your eyes. But all of the other objects that you see, including this book, also send light to your eyes. Otherwise, you would not be able to see them. Light from the sun differs from light from a book. The sun emits its own light. The light from a book is light that is given off by the sun or a lamp and that then bounces off the book. Reflection of Light Mirrors, such as those on the solar collector shown in Figure 1, reflect almost all incoming light. V Every object reflects some light and absorbs some light. The way light is reflected depends on the surface of the object. Because of the way mirrors reflect light, you can see an image of yourself in a mirror. Light can be modeled as a ray. It is useful to use another model for light, the light ray, to describe reflection, refraction, and many other ways light behaves. A light ray is an imaginary line running in the direction that the light travels. The direction of the light ray is the same as the direction of wave travel in the wave model of light or as the path of photons in the particle model of light. Light rays do not represent a full picture of the complex nature of light but are good for showing how light will behave in many cases. The study of light in cases in which light behaves like a ray is called geometrical optics. Using light rays, one can trace the path of light in geometrical drawings called ray diagrams. What are two behaviors of light that light rays are used to model? Key Resources Teaching Transparencies TM45 Law of Refl ection TM46 Flat Mirror Visual Concepts Refl ection Law of Refl ection Comparing Real and Virtual Images Additive Color Mixing Datasheets Curved Mirror Filtering Light Cross-Disciplinary Worksheet Integrating Space Science The Refracting Telescope at Yerkes 560 CHAPTER 16 Sound and Light

24 Rough surfaces reflect light rays in many directions. Many of the surfaces that we see every day, such as paper, wood, cloth, and skin, reflect light but do not appear shiny. When a beam of light is reflected, the path of each light ray in the beam changes from its initial direction to another direction. If a surface is rough, light striking the surface will be reflected at all angles, as Figure 2 shows. This reflection of light into random directions is called diffuse reflection. Smooth surfaces reflect light rays in one direction. When light hits a smooth surface, such as a polished mirror, the light does not reflect diffusely. Instead, all of the light hitting a mirror from one direction is reflected together into a single, new direction, as the bottom of Figure 2 shows. The new direction of the light rays is related to the old direction in a definite way. The angle of the light rays reflecting off the surface, called the angle of reflection, is the same as the angle of the light rays striking the surface, called the angle of incidence. This equality of angles is the law of reflection. Law of reflection The angle of incidence equals the angle of reflection. Both of these angles are measured from a line that is perpendicular to the surface at the point where the light hits the surface. This line is called the normal. Figure 3 shows a ray diagram that illustrates how the law of reflection works. Incoming light Normal Reflected light Figure 2 Surfaces affect how light is reflected. Light rays that are reflected from a rough surface are reflected in many directions. Light rays that are reflected from a smooth surface are reflected in the same direction. Topic: Reflection Code: HK81282 Figure 3 When light hits a surface, the angle of incidence (θ) equals the angle of reflection (θ ). Visual Literacy Explain that the small arrows on the rays in Figure 2 indicate direction of travel. Point out that in both cases, the incoming light rays are parallel. After reflecting from the rough surface, the rays are no longer parallel, but the rays reflected from the smooth surface remain parallel. Angles of Reflection You will need an aquarium (5 or 10 gal), milk, small mirror, aluminum foil, laser pointer, transparent plastic protractor, concave mirror, and a convex mirror. Step 1 Fill the aquarium about two-thirds full with water. Add three drops of milk to the water, and stir. Shine the laser into the water to see if the beam is visible. Add more milk as needed. Step 2 Simulate the first image in Figure 2 with the shiny side of crumpled aluminum foil and the second image in Figure 2 with a flat mirror lying at the bottom of the tank. Step 3 Demonstrate the angles of incidence and reflection with the flat mirror on the bottom of the tank. Move the light source to achieve various angles to show the validity of the law of reflection. Tape a transparent protractor to the outside of the tank to measure angles. Continued on the next page Alternative Assessment Making a Periscope Organize students into groups. Give each group a shoebox, two small hand mirrors, some modeling clay, and a pair of scissors. Have students cut a 3 cm hole on the left side of each end of the box (so the holes will not be directly opposite each other). Then, tell students to arrange the mirrors inside of the box with the modeling clay in such a way that someone can look straight into one hole and see out of the other hole. Ask students to explain how this device works. Logical/Interpersonal Teaching Key Ideas Reflection and Absorption Take your students outside on a sunny day. Find a shiny object that is in the sun, such as a school bus with windows or metal mailbox. Ask students what happens to the sunlight that hits the object. Students should recognize that light reflects off the object. Tell students that they can see a reflection in the object because light reflects off it and that any glare they see is reflected light. Help students understand that light is also absorbed by the object. Have students touch the object and tell them that the object feels warm because of the light energy it absorbed. Kinesthetic/Verbal SECTION 3 Reflection and Color 561

25 Continued from previous page Step 4 Place the convex ( wideangle ) mirror at the bottom of the tank. Shine the laser pointer vertically downward onto the mirror, and move it from side to side over the mirror. Rays will reflect increasingly outward as you move the beam in any direction away from the center of the mirror. Reverse the rays by shining the beam onto the mirror from many different angles. Ask students why this type of mirror is called a wide-angle mirror. Step 5 Place a concave mirror at the bottom of the tank. Again, shine the beam straight down at the mirror, and move it back and forth. Students will observe that all of the rays reflect inward through a common point. Use two beams if you can to show that they cross at a common point no matter where the beams strike the mirror. Step 6 Place both mirrors in the tank and adjust a flashlight to produce a straight beam. Shine the beam into the mirrors. Students will see the light spread from the convex mirror and focus to a point from the concave mirror. Prefixes The word reflection contains the prefix re-. What is the meaning of this prefix? How can it help you understand the word reflection? virtual image (VUHR choo uhl IM ij) an image from which light rays appear to diverge, even though they are not actually focused there; a virtual image cannot be projected on a screen real image (REE uhl IM ij) an image that is formed by the intersection of light rays; a real image can be projected on a screen Figure 4 Flat mirrors create virtual images. Mirrors When you look into a flat mirror, you see an image of yourself that appears to be behind the mirror. You see a twin or copy of yourself standing on the other side of the glass, but your image is flipped from left to right. You also see a whole room, a whole world of space beyond the mirror. V Mirrors reflect light as described by the law of reflection, and this light reaches your eyes. The type of image you perceive depends on the type of mirror. Flat mirrors form virtual images by reflection. The ray diagram in Figure 4 shows the path of light rays striking a flat mirror. When a light ray is reflected by a flat mirror, the angle at which it is reflected is equal to the angle of incidence, as described by the law of reflection. When the reflected rays reach your eyes, your eyes sense light coming from certain directions. Your brain interprets the light as if it traveled in straight lines from an object to your eyes. So, you perceive an image of yourself behind the mirror. Of course, there is not actually a copy of you behind the mirror. The image that you see, called a virtual image, results from the apparent path of the light rays, not an actual path. The virtual image appears to be as far behind the mirror as you are in front of the mirror. Why is the image that a flat mirror creates called a virtual image? A virtual image appears behind a flat mirror. Prefixes Have students write other words that start with re- to help them determine the meaning of the prefix. (Sample answers: return, rebound, react, recoil) Then, ask students what re- means. (Sample answer: Re- means back.) Verbal A ray diagram shows where the light actually travels as well as where you perceive that it has come from. Advanced Learners Mirror Height Have students use what they know about reflection to explain why a plane mirror must be at least half a person s height for the person to see his or her full image in the mirror. Have students use diagrams or mirrors in their explanations. (Note: The mirror should be hung so that the top of the mirror is midway between the top of the person s head and his or her eyes.) (The angle of incidence equals the angle of reflection, so a person can see the top of his or her head by looking at a point on the mirror halfway between his or her eyes and the top of his or her head. A person can see his or her feet by looking at a point on the mirror halfway between his or her eyes and feet. Together, these two images add up to half the person s height) Logical/Visual 562 CHAPTER 16 Sound and Light

26 Curved Mirror Procedure 1 Observe the reflection of a short pencil in the inner part of a large, stainless steel spoon. 2 Slowly move the spoon closer to the pencil. Note any changes in the appearance of the pencil s reflection. 3 Repeat steps 1 and 2 by using the other side of the spoon as the mirror. Curved mirrors can distort images. If you have ever looked into the passenger s side mirror on a car or a dressing table mirror, you have used a curved mirror. The images created by these mirrors are distorted so that they are smaller or larger than the real object. Curved mirrors create images by reflecting light according to the law of reflection. But because the surface is not flat, the line perpendicular to the mirror (the normal) points in different directions for different parts of the mirror. Mirrors that bulge out are called convex mirrors. Convex mirrors, such as that on the passenger s side of the car shown in Figure 5, make images appear smaller than they actually are. Indented mirrors are called concave mirrors. Some concave mirrors magnify objects so that the image created is larger than the object. Concave mirrors can create real images. Concave mirrors are used to focus reflected light. A concave mirror can form one of two kinds of images. It may form a virtual image behind the mirror or a real image in front of the mirror. When light rays from an object are focused onto a small area, a real image forms. If a piece of paper is placed at the point where the light rays come together, the real image appears on the paper. If you placed a piece of paper behind a mirror where the virtual image seemed to appear, you would not see the image on the paper. This example shows the primary difference between a real and a virtual image. Light rays exist at the point where the real image appears. A virtual image appears to exist in a certain place, but no light rays exist there. Analysis Academic Vocabulary 10 min 1. Which side of the spoon is a concave mirror? Which side is a convex mirror? 2. What differences in the reflected images did you observe? 3. How does distance affect an object s image in concave and convex mirrors? distort (di STAWRT) to change the natural appearance of something Figure 5 Images formed by a convex mirror are smaller than the original object. Teacher s Notes This experiment works best with a large spoon that is very shiny and that has a large radius of curvature. Materials per Group pencil, short spoon, large, stainless steel Answers to Analysis 1. The front side of the spoon curves inward and is the concave mirror. The back side of the spoon curves outward and is a convex mirror. 2. The image in the concave part of the spoon was small and upside down, except when the pencil was very close to the spoon. The image was smaller than the pencil in the convex part of the spoon. 3. Answers may vary. In the concave part of the spoon, the image of the pencil was larger than the pencil when the pencil was very close to the mirror. As the distance between the pencil and the concave mirror increased, the image inverted and became smaller than the actual pencil. For the convex mirror, the image was always right side up and smaller, and the image became smaller the farther the pencil was from the spoon. Teaching Key Ideas Images in Mirrors The ray diagram in Figure 4 does not show every light ray involved in producing the image. One ray from the knee and one ray from the top of the head are shown as they reflect off the surface and to the eye of the observer. The dotted lines represent the straightline paths that your brain calculates the rays have taken. Differentiated Instruction Struggling Readers Distinguishing Words Help students understand the difference between the terms concave and convex. Have students draw items with concave and convex parts and write the terms concave and convex as parts of the drawings in place of concave and convex lines. For example, a student might draw a crescent moon where one section of the inner edge is replaced with the term concave and one section of the outer edge is replaced with the term convex. Verbal/Visual Basic Learners Law of Reflection Give students a diagram of a plane mirror with three rays of light approaching it at different angles. Have them draw the resulting reflected rays. Evaluate to see if students understand the law of reflection. Repeat with a concave mirror, but have the rays parallel to each other and the middle ray striking the exact center of the mirror. (The reflected rays should meet at a common point. The ray in the center is reflected straight back along the same path.) Visual SECTION 3 Reflection and Color 563

27 Teacher s Notes You may be able to obtain used but still functioning filters from your school s theater department. Materials per Group filters, colored, 4 Answers 1 2. Answers may vary depending on the filters used and the objects being viewed. Sample answer: When I looked through the yellow filter, the color of the object changed. Objects looked more yellow, and some objects looked darker. 3. Answers may vary. When I combined the yellow and the blue filter, the objects looked greener and some objects appeared to be darker. The colors of the objects were different than when I looked at them through a single filter because each filter absorbs certain wavelengths of light, so those wavelengths are subtracted out from what I see. Fewer colors pass through two combined filters than through one filter. 4. When I look through all of the filters at once, the objects are very dark and have very little color. Filtering Light 20 min 1 Look through one of four colored filters red, blue, yellow, or green at an object across the room. Describe the object s color. 2 Repeat step 1 with each one of the filters. 3 Look at the same object through two filters placed together. Describe the object s color. Why does the object s color differ from the color when one filter is used? 4 Place the red, blue, and yellow filters together, and look at the same object. Describe what you see. Why do you see this? Figure 6 A Rose in White and Red Light Seeing Colors V The colors that you perceive depend on the wavelengths of visible light that reach your eyes. When you see light that has a wavelength of about 550 nm, your brain interprets the light as green. If the light comes from the direction of a leaf, then you will think that the leaf is green. A leaf does not emit light. And in the darkness of night, you may not be able to see the leaf at all. The leaf reflects green light from another light source. Objects have the color of the wavelengths they reflect. If you pass the light from the sun through a prism, the prism separates the light into a rainbow of colors. White light from the sun actually contains light that has all of the wavelengths in the visible region of the electromagnetic spectrum. When white light strikes a leaf, as Figure 6 shows, the leaf reflects light that has a wavelength of about 550 nm, which corresponds to the color green. The leaf absorbs light at other wavelengths, so those wavelengths are not reflected. When the light reflected from the leaf enters your eyes, your brain interprets the light as green. When you look through a transparent object, such as a color filter, you see the color of the light that passes through the filter. A green filter transmits green light and absorbs other colors of light. Likewise, the petals of a red rose reflect red light and absorb other colors. So, the petals appear to be red. If you view a rose and its leaves under red light, as Figure 6 shows, the petals will appear red but the leaves will appear black. Under white light, the petals of a rose reflect red light, while the leaves reflect green light. Under red light, the petals still look red, but the leaves look black because there is no green light for them to reflect. Teaching Key Ideas Color Perception When the brain receives signals from certain combinations of photoreceptor cells in the retina, it interprets them as the color green. These receptors are three kinds of cone cells, one each for red, green, and blue. Be sure students understand that there is nothing inherently green about electromagnetic waves in one part of the visible spectrum. Green is just the way our brains interpret certain signals. Colors of Light Most students believe that white light is colorless and pure, and that a color filter adds color to a white beam. These students have trouble understanding that white light is actually a mixture of colored light. You can show students a prism separating white light into a rainbow of colors to dispel this misconception. 564 CHAPTER 16 Sound and Light

28 Answer to caption question The center of the lights is white because all of the colors in light combine to form white light. The center of the filters is black because filters subtract out light, so no light is getting through, and black is the absence of light. Red, green, and blue lights can combine to produce yellow, magenta, cyan, or white lights. Mixtures of colors produce other colors. Most of the colors that we see are not pure colors. They are mixtures of primary colors. Televisions and computer monitors display many colors by combining light of the additive primary colors red, green, and blue. Mixing light of two of these colors can produce the secondary colors yellow, cyan, and magenta, as shown on the left of Figure 7. Mixing light of the three additive primary colors makes white light. Because pigments and filters absorb light, the opposite effect happens when they are mixed. The subtractive primary colors yellow, cyan, and magenta can be combined to create red, green, and blue, as shown in the right of Figure 7. If filters or pigments of all three colors are combined in equal proportions, all visible light is absorbed. No light gets to your eyes, so you see black. Black is not a color. It is the absence of color. Section 3 Review KEY IDEAS 1. List three examples of the diffuse reflection of light. 2. Describe the law of reflection in your own words. 3. Draw a ray diagram that represents a light reflecting off of a flat surface to illustrate the law of reflection. 4. Discuss how reflection from objects that appear blue differs from objects that appear yellow. 5. Explain why a plant may look green in sunlight but black under red light. Yellow, magenta, and cyan filters can be combined to produce red, green, blue, or black. CRITICAL THINKING Figure 7 Colors combine to produce other colors. Why is the center of the lights white and the center of the filters black? 6. Analyzing Information A friend says that only mirrors and other shiny surfaces reflect light. Explain what is wrong with this reasoning. 7. Applying ideas How does a flat mirror form a virtual image? 8. Forming Models A convex mirror can be used to see around the corner of a hallway. Draw a simple ray diagram that illustrates how this works. Reteaching Key Ideas What Happens to Light? Gather a variety of objects such as mirrors, a colored T-shirt, an orange, white paper, and black patent-leather shoes. Ask students to describe what happens to the light that strikes the object. Students should recognize that all the objects reflect and absorb light. They should also note that some objects reflect light diffusely and some objects reflect objects normally (non-diffusely). Students should be able to explain why objects appear to be different colors. Logical/Visual Formative Assessment Which kind of mirror can create real images? A. a flat mirror (Incorrect. Flat mirrors can form only virtual images.) B. a concave mirror (Correct. Concave mirrors can form real images and virtual images.) C. a convex mirror (Incorrect. Convex mirrors can form only virtual images.) D. a diffuse mirror (Incorrect. Diffuse mirror is not a type of mirror.) Answers to Section Review 1. Examples may include bicycle reflectors, clothing, and paper. Almost anything that is visible, except a direct source of light, reflects light. Only polished surfaces, such as mirrors, reflect light non-diffusely. 2. Sample answer: The law of reflection states that the angle of the reflected light ray is equal to the angle of the incoming light ray. 3. Ray diagrams should resemble the diagram for reflection from a smooth surface in Figure 2. The angle of incidence should approximately equal the angle of reflection. 4. Blue objects reflect blue light and absorb colors in the rest of the visible spectrum. Yellow objects reflect yellow light and absorb other colors. 5. A green leaf reflects green light. Light from the sun contains all colors, so the leaf looks green. Red light contains no green, so the leaf reflects no light and appears black. 6. You see all objects by light reflected from them. Therefore, anything you can see, except for direct sources of light, reflects light. 7. Light that reaches your eyes from the mirror comes from in front of the mirror. However, because the path of the rays goes through the mirror, you see an image that appears to be behind the mirror. 8. Diagrams should show how reflected light is directed outward by a convex mirror. SECTION 3 Reflection and Color 565

29 SECTION 4 This section discusses the refraction of light as it passes between mediums. Students also learn about lenses and how they function in microscopes and the human eye. The section also covers dispersion and prisms. 4 Refraction, Lenses, and Prisms Key Ideas Key Terms Why It Matters V What happens to light when it passes from one medium to another medium? V What happens when light passes through a lens? V How can a prism separate white light into colors? lens magnification prism dispersion Your eyes contain lenses that bend and focus light to allow you to see objects around you. Bellringer Use the Bellringer transparency to prepare students for this section. Light travels in straight lines through empty space. But we also see light that passes through various media, such as air, water, or glass. The direction of a light wave may change when the light passes from one medium into another. Refraction You can demonstrate refraction using an aquarium (5 or 10 gal) or other large transparent container, milk, a small mirror, a laser pointer or focusable flashlight, and two dusty chalkboard erasers. Fill the aquarium about two-thirds full with water. Add three drops of milk to the water, and stir. Shine the laser into the water to see if the beam is visible. Add more milk as needed. Shine the laser into the water at about a 30 angle from the normal. Hold the erasers over the aquarium and gently tap them together. Let students observe how the light ray bends as it enters the water. Shine the laser from various different angles to show students how the path of the light ray in the water depends on the angle at which the light strikes the water s surface. Figure 1 Light waves can change direction. Refraction of Light V Light waves bend, or refract, when they pass from one transparent medium to another. Light bends when it changes mediums because the speed of light differs in each medium. If light meets the boundary of two mediums at an angle to the normal, as in Figure 1, the light changes direction. When light moves from a material in which its speed is high to a material in which its speed is lower, such as from air to glass, the ray is bent toward the normal. If light moves from a material in which its speed is low to one in which its speed is higher, the ray is bent away from the normal. The path of a light ray bends toward the normal when the light ray moves from air into glass. The path of a light ray is bent away from the normal when the ray passes from glass into air. Key Resources Teaching Transparencies P16 Refraction P17 The Eye Visual Concepts Refraction Dispersion of Light Converging and Diverging Lenses Parts of the Human Eye Datasheet Water Prism Science Skills Worksheets Angles and Degrees The Area of a Circle 566 CHAPTER 16 Sound and Light

30 Refraction makes objects appear to be in different positions. When a cat looks at a fish underwater, the cat perceives the fish as closer than it actually is, as the ray diagram in the top of Figure 2 shows. On the other hand, when the fish looks at the cat above the surface, the fish perceives the cat as farther than it really is, as Figure 2 also shows. The images that the cat and the fish see are virtual images like the images that form behind a mirror. The light rays that pass from the fish to the cat bend away from the normal when they pass from water to air. But the cat s brain interprets the light as if it traveled in a straight line, and thus the cat sees a virtual image. Similarly, the light from the cat to the fish bends toward the normal as it passes from the air into water, which causes the fish to see a virtual image. Why does the fish seem closer than it is? Refraction in the atmosphere creates mirages. Have you ever seen what looks like water on the road on a hot, dry summer day? If so, then you may have seen a mirage like the one shown in Figure 3. A mirage is a virtual image that is caused by refraction of light in the atmosphere. The air temperature affects the speed at which light travels. When light from the sky passes into the layer of hot air just above the asphalt on a road, the light refracts and bends upward away from the road. This refraction creates a virtual image of the sky coming from the direction of the road. Your mind may assume that a reflection was caused by water. Figure 2 Refraction creates virtual images. To the cat on the pier, the fish appears to be closer than it actually is. To the fish, the cat seems to be farther from the surface than it actually is. Figure 3 A mirage is produced when light bends as it passes through air at different temperatures. Teaching Key Ideas Light Refraction Steering bulldozers and tanks is analogous to light refraction. Ask students if they have ever steered a track-driven vehicle. A tracked vehicle changes direction by the driver either applying a brake to slow down the track on one side or by speeding up the track on the other side (or both actions for sharp turns). Another analogy is making a turn while rowing a boat. Visual Literacy Figure 2 on this page, like Figure 4 in Section 3, shows both light rays and dotted lines. The dotted lines show the perceived, or apparent, path of the light rays. Ask students whether the images in the figure are virtual images or real images. (They are virtual images.) Visual Life Science Connection Archerfish and Refraction Archerfish (Toxotes jaculator) have the ability to correct for the refraction of light between air and water, and they have the ability to judge the distance to their prey. Archerfish knock insects and other small prey from overhanging vegetation by spitting jets of water at them. Archerfish have a groove in the roof of their mouth. When the tongue is pressed against the groove and the gills are squeezed shut, a jet of water is produced. These fish can hit prey more than 1.5 m away. Differentiated Instruction Advanced Learners Index of Refraction The index of refraction for a medium is calculated by dividing the speed of light in a vacuum by the speed of light in the medium. When light moves from a medium with a lower index to one with a higher index, light rays bend toward the normal. Conversely, when it moves from a higher to a lower index, light rays bend away from the normal. Have students use a reference source to find the speed of light in various mediums and ask them to calculate the index of refraction for each medium. Then, have students determine whether light will bend toward or away from the normal for different combinations of mediums. Remind them that the higher the index, the more the light bends. Logical SECTION 4 Refraction, Lenses, and Prisms 567

31 Teaching Key Ideas Using Converging Lenses Give each pair of students a magnifying lens. Tell students to use the lens to look at an object. First, tell students to use the lens to see an image that is larger than the object. Have students draw diagrams showing the relative positions of their eye, the magnifying lens, and the object. Then, challenge students to use the lens to see an image that is smaller than the object. Ask them to draw a diagram for this situation, too. Tell students that the larger image is a virtual image and the smaller image is a real image. Kinesthetic/Visual Eyeglass Lenses Students can determine the correction in eyeglasses with the following tests. Hold the lens horizontally above a page of type, and move the lens back and forth. If the lens is converging (used to correct farsightedness), the type will seem to move opposite the direction that the lens moves. With a diverging lens (for nearsightedness), the type seems to move in the same direction as the lens moves. If a lens has correction for astigmatism, rotating the lens over the type will cause the type to distort as the lens turns. Kinesthetic Converging Lens When light rays pass through a lens that is thicker at the middle, the rays are bent inward. Figure 4 The way the surface of a lens curves affects how light rays bend as the rays pass through the lens. lens (LENZ) a transparent object that refracts light waves such that they converge or diverge to create an image magnification (MAG nuh fi KAY shuhn) the increase of an object s apparent size by using lenses or mirrors Figure 5 A magnifying glass makes a large virtual image of a small object. Diverging Lens When light rays pass through a lens that is thicker at the ends, the rays are bent outward. Lenses You may not realize that you use the refraction of light every day. Human eyes, as well as cameras, contact lenses, eyeglasses, and microscopes, contain parts that bend light. Light traveling at an angle through a thin, flat medium is refracted twice once when it enters the medium and again when it reenters the air. So, the position of a light ray that exits the medium is shifted, but the light ray is still parallel to the original light ray. However, if the medium has a curved surface, the exiting rays will not be parallel to the original ray. V When light passes through a medium that has a curved surface, a lens, the light rays change direction. Each light ray strikes the surface of the curved surface at a slightly different angle, so angles at which the rays are bent differ. The effects of a converging lens and a diverging lens on light rays are shown in Figure 4. A converging lens bends light inward. This type of lens can create either a virtual image or a real image, depending on the distance from the lens to the object. A diverging lens bends light outward and can create only a virtual image. Which type of lens can create a real image? Lenses can magnify images. A magnifying glass is a familiar example of a converging lens. A magnifying glass reveals details that you would not usually be able to see, such as the small parts of the flower in Figure 5. The large image that you see through the lens is a virtual image. Magnification is any change in the size of an image compared with the size of the object. Magnification can produce an image that is larger than the object. If you hold a magnifying glass over a piece of paper in bright sunlight, you can see a real image of the sun on the paper. By adjusting the height of the lens above the paper, you can focus the light rays together into a small area, called the focal point. At the focal point, the image of the sun may contain enough energy to eventually set the paper on fire. English Learners Vocabulary The words converge and diverge derive from the Latin verb vergere, to slant, slope, or incline. The prefix con- means together, and the prefix di- means apart. Thus, a converging lens slants light rays together, and a diverging lens slants light rays apart. 568 CHAPTER 16 Sound and Light

32 Microscopes use multiple lenses. A compound light microscope, shown in Figure 6, uses multiple lenses to provide greater magnification than a single magnifying glass can. The objective lens is closest to the object and forms a large, real image of the object. The eyepiece then acts like a magnifying glass and creates a larger virtual image that you see when you look through the microscope. The eye depends on refraction and lenses. Without refraction of light, you would not be able to see at all. The way a human eye bends light, which is shown in Figure 7, is similar to the way a simple camera operates. Light enters a camera through a large lens, which focuses the light into an image on the film at the back of the camera. Light first enters the eye through a transparent tissue called the cornea. The cornea is responsible for 70% of the refraction of light in the eye. After the cornea, light passes through the pupil. Then, light travels through the lens, which is composed of fibers. The curvature of the lens determines how much the lens refracts light. Muscles can adjust the curvature of the lens until an image is focused on the retina. The retina is composed of tiny, light-sensitive structures called rods and cones. When light strikes the rods and cones, signals are sent to the brain where they are interpreted as images. Most cones are in the center of the retina, and most rods are on the outer edges. The cones are responsible for color vision, but they respond only to bright light. Therefore, you cannot see color in very dim light. The rods are more sensitive to dim light but cannot resolve details well. So, you can glimpse faint movements from the corners of your eyes. 3 Light is refracted again by the lens, which is made up of transparent fibers. 2 Light passes through a hole in the colorful iris known as the pupil. 4 The refracted light is focused onto the back surface of the eye, the retina. 5 The light is detected by rods and cones in the retina. Figure 6 A compound light microscope uses several lenses to produce a highly magnified image. Figure 7 Eyes focus light. Visual Literacy Figure 6 shows that light from a point on the specimen is focused to a point somewhere inside the body tube. If a screen were placed at this point, a small image of the specimen would appear on it. However, a real image can be viewed without a screen. A magnifying lens the eyepiece at the top of the tube lets you view a magnified image of the alreadymagnified real image inside the tube. A telescope works in the same way as a microscope, except that the objective lens is large to capture a lot of light, and it brings light to a focus much farther from the lens. The Eye Lens Some students were probably taught in the past that the lens of the eye is solely responsible for forming an image on the retina. Help them overcome this error by calling their attention to the fact that the curved cornea is a lens too. The lens is important, because muscles change its curvature to adjust the focus for nearby and distant objects. Some students may be interested in exploring the use of surgery to reshape the cornea to correct vision defects that would otherwise be corrected by contact lenses or eyeglasses. 1 The cornea is a transparent membrane that covers the eye and refracts light. 6 The optic nerve carries signals to the brain. Advanced Learners Vision Problems Encourage interested students to learn why certain vision problems occur. For example, students can find out what causes nearsightedness, farsightedness, and colorblindness. Have students summarize their research in a poster that they can present to the class. Verbal/Visual Nearsighted and Farsighted Projector To simulate normal vision, have students focus an image from an overhead projector onto a screen. Then, to simulate a nearsighted eye, have students increase the distance between the projector and the screen. The image becomes blurry because it is focused in front of the screen. Then, have students move the projector closer to the screen than it was originally. This simulates farsightedness: the image is now focused behind the screen, making the image blurry. Visual SECTION 4 Refraction, Lenses, and Prisms 569

33 Dispersion For this demonstration you will need an aquarium, prism, slide projector, and a focusable halogen flashlight. Step 1 Darken the room for this demonstration. Focus the flashlight to produce a beam that does not diverge. Shine the beam through a prism, as shown in Figure 8. Arrange the prism and beam so that a light spectrum appears on a lightcolored wall or on white paper. Step 2 Fill the aquarium with clear water. Shine the projector beam through the side of the tank so that it refracts, as shown in the diagram below. Arrange the aquarium so that a bright spectrum falls on a light-colored wall. You will need to adjust the lens of the projector to produce the narrowest possible beam. If you do not get a good spectrum, place a sheet of aluminum foil with a 1/8-inch wide vertical slit over the projector lens. This should produce a narrow beam. You may have to adjust the placement of the projector for best results. Spectrum Water Prism prism (PRIZ uhm) in optics, a system that consists of two or more plane surfaces of a transparent solid at an angle with each other dispersion (di SPUHR zhuhn) in optics, the process of separating a wave (such as white light) of different frequencies into its individual component waves (the different colors) Topic: Refraction Code: HK min 1 Fill a shallow dish with about 3 cm of water. 2 Place a flat mirror in the tray so that about half of the mirror is under the water. The mirror should be placed at an angle and should lean against the side of the dish. 3 Use a white piece of paper to capture the sunlight that is reflected by the mirror. Change the distance of the paper from the mirror and the angle of the paper until you see colors in the reflection. What colors do you see? 4 Why are the colors of the reflected sunlight separated by the water? Dispersion and Prisms A prism, such as the one in Figure 8, can separate white light into its component colors. Water droplets in the air can also separate the color in white light to produce a rainbow. V A prism can separate the colors of light because the speeds of light waves traveling through the medium depend on the wavelengths of light. Different colors of light are refracted by different amounts. Light waves of all wavelengths travel at the same speed ( m/s) in a vacuum. But when a light wave travels through a medium, the speed of the light wave depends on the light wave s wavelength. The colors in the visible spectrum, in order of longest to shortest wavelength, are red, orange, yellow, green, blue, and violet. In the visible spectrum, violet light has the shortest wavelength and travels the slowest. Red light has the longest wavelength and travels the fastest. Because violet light travels slower than red light, violet light bends more than red light when it passes from one medium to another. When white light passes from air to the glass in the prism, violet bends the most and red bends the least. When the light exits the prism, the light is separated into the colors in the visible spectrum. This effect in which light separates into different colors because of differences in wave speed is called dispersion. Figure 8 A prism separates white light into its component colors. White light enters the prism. In the light exiting the prism, the colors are separated. Notice that violet light is bent more than red light. Looking down from above Teacher s Notes If you cannot use sunlight for this lab, a bright light, such as a flashlight, may be used. If you use an artificial light source, you should have the students cut a slit in a piece of cardboard or black construction paper and shine the light through the slit. Materials per Group dish, shallow mirror, flat paper, white, 1 sheet sunlight water Answers 3. Answers may vary depending on quality of reflection. Sample answer: I see red, orange, yellow, green, blue, and violet. 4. When light travels through the water, the wavelengths of visible light travel at different speeds. Red light travels fastest and is bent the least, and violet travels the slowest and is bent the most. As a result, the colors are dispersed, or spread out. 570 CHAPTER 16 Sound and Light

34 Booklet Remind students to review Section 3 when working on the pages for mirrors. Be sure students note which light interaction (reflection or refraction) is caused by each object. Rainbows are caused by dispersion and reflection. Rainbows, such as the one in Figure 9, may form any time that water droplets are in the air. When sunlight strikes a droplet of water, the light is dispersed into different colors as it passes from the air into the water. If the angle at which the refracted light rays meet the back surface of the water droplet is small enough, the rays can be reflected. Some of the light will reflect back through the droplet. The light disperses further when it passes out of the water back into the air. When light finally leaves the droplet, violet light emerges at an angle of 40, red light emerges at 42, and the other colors are in between these angles. We see light from many droplets as arcs of color, which form a rainbow. Only the red light from droplets higher in the air and only the violet light from lower droplets reaches your eyes. So, the colors of a rainbow are separated in space. Figure 9 Sunlight is dispersed and reflected by water droplets to form a rainbow. Booklet Create a booklet you can use to compare mirrors, lenses, and prisms. Label the cover Mirror, Lenses, and Prisms. Label the first two pages Mirrors, the next two pages Lenses, and the last two pages Prisms. Reteaching Key Ideas Reviewing Refraction Give groups a converging lens, a diverging lens, a prism, and a bright flashlight. Instruct students to experiment with their materials and then work together to draw diagrams that show and explain how refraction affects the light that passes through the prism and the lenses. Interpersonal/Visual Section 4 Review KEY IDEAS 1. Draw a ray diagram that shows the path of light when the light travels from air into glass. 2. Describe how a mirage is formed. 3. Explain how a simple magnifying glass works. 4. Explain why light is dispersed by a prism. CRITICAL THINKING 5. Explaining Events How does your eye focus light on the retina? 6. Applying Ideas A spoon partially immersed in a glass of water may appear to be bent. Is the image of the spoon in the water a real image or a virtual image? 7. Forming Conclusions In the dispersed light that exits a glass prism, green light is closer to violet light than yellow light is. Does green light or yellow light travel faster through the glass prism? 8. Applying Ideas If light traveled at the same speed in raindrops as it does in air, could rainbows exist? Explain your reasoning. Formative Assessment Which of the following correctly describes the path of light as it passes from air into a glass prism and then back into air? A. The light will bend toward the normal as it passes into the prism and then bend away from the normal as it passes back into air. (Correct. During refraction, light bends toward the normal when passing into a medium.) B. The light will bend away from the normal as it passes into the prism and then bend toward the normal as it passes back into air. (Incorrect. The speed of light in air is higher than the speed of light in glass.) C. The light will bend away from the normal as it passes into the prism and then bend away from the normal as it passes back into air. (Incorrect. When light enters a new medium, it will refract either toward or away from the normal. When light exits the medium, it will refract in the opposite direction.) D. The light will bend toward the normal as it passes into the prism and then will not bend when it passes back into air. (Incorrect. The speed of light is different in air from that in glass, so refraction should happen when light passes from air to glass and when light passes from glass to air.) Answers to Section Review on p. 581A SECTION 4 Refraction, Lenses, and Prisms 571

35 Detecting Counterfeit Money Tell students that the newly designed bills have other distinctive features. For example, each of the new bill denominations is shaded with different colors. The $10 bill is shaded with orange, yellow, and red; the $20 bill is shade with green, peach, and blue; and the $50 bill is shaded with red and blue. Also, the security threads in the bills are placed in different locations and glow different colors when viewed under ultraviolet light. The security thread in the $10 bill is located to the right of the portrait and glows orange. The security thread in the $20 bill is located on the far left of the portrait and glows green. The security thread in the $50 bill is located very close to the right side of the portrait and glows yellow. Ask students why these features are useful. (Sample answer: The shading in the bills makes it easier for people to tell the difference between different denominations and makes it more difficult to counterfeit the bills. Varying the location and behavior of the security thread makes it easy to identify counterfeit bills by placing the bills under ultraviolet light.) Logical Why It Matters Detecting Counterfeit Money You might think that high-quality photocopiers, scanners, and printers would make it easy to print fake money. However, newer 10, 20, and 50 dollar bills have security features that cannot be duplicated by an ink-jet printer. The United States Treasury introduced a redesigned 10 dollar bill in This bill, along with 20 and 50 dollar bills, was updated to include security features that make counterfeit money harder to make and easier to detect. Several of the security features depend on how light interacts with the paper and inks used to make money. These features include a security thread, a watermark, and features that are printed using color-shifting ink. A plastic strip known as a security thread is embedded in the paper. If you hold the bill up to the light and look to the right of the portrait of Alexander Hamilton, you will be able to see the words USA TEN printed on the thread. This strip glows orange when the bill is held under ultraviolet light. It may be obvious that more colors are used in the new bills. But if you look closely at a bill, you will notice that the patterns are made by very fine lines. The colors appear darker where the lines are more closely spaced. The bills even include text, called microprinting, that you cannot read without a magnifying lens. The fine patterns and microprinting make it hard for counterfeiters to reproduce the quality of the printing on real money. Have you ever left a dollar bill in your pocket and found that it is still in one piece after it has been through the washing machine? If money were printed on regular paper, it would have fallen apart. All paper money is printed on special paper made from linen and cotton, which is known as rag paper. This paper is very thin and does not feel like other papers. Counterfeiters cannot buy this paper. It is made only for the U.S. government. Alternative Assessment Brochure Have students make brochures that teach people how to identify counterfeit money. Students should include information they learned in this feature and information that they gather through research. Encourage students to distribute their brochures to small business owners in your area. Visual 572 CHAPTER 16 Sound and Light

36 Color-shifting ink is used to print the number 10 in the lower right of the bill. This ink contains metal particles that reflect light. Only certain wavelengths make it out of the ink to produce the color you see. The other light waves destructively interfere with one another. The light waves that you see also depend on the angle of the light. When you tilt the bill, you see a different color. The color on the new bills shifts from copper to green. Visual Literacy Instruct students to study the image on these pages and compare it to an older version of the $10 bill. If older versions of the $10 bill are hard to find, have students compare the image to $1 or older $5 bills which should be easier to find. (A new design for the $5 bill will be issued in early 2008.) Ask students to make a list of differences between the appearance of the older bills and the appearance of the redesigned bills. (Sample answers: The portraits in the older bills are smaller and in an oval. Older bills do not have watermarks or color variations. Newer bills have a large number in the lower right corner on the back.) Visual If you hold the bill up to the light and look in the white oval, you will see another image of Alexander Hamilton. This image is a watermark. The image is not created with ink. It is part of the paper. The thickness of the paper is varied to make a watermark. Less light comes through the thicker parts of the paper, which creates an image that is darker than the surrounding area. The watermark can be seen through both sides of the paper. UNDERSTANDIING CONCEPTS 1. If you tilt a 10 dollar bill, what should you see change? CRITICAL THINKING 2. Suppose counterfeiters use paper that is similar to the paper that the government uses to print money. Do you think that they would be able to successfully make fake money? Explain your reasoning. Topic: Printing Processes Code: HK81700 Answers to Your Turn 1. The color of the 10 in the lower right corner will shift from copper to green. 2. Sample answer: I do not think that they would be able to successfully counterfeit money because the paper would need to have the watermark and the security thread embedded in it. Also, they would need to find a source of the color-shifting ink and a way to print the special ink. SECTION 4 Refraction, Lenses, and Prisms 573

37 50 min Time Required 1 lab period Lab Ratings Teacher Prep Student Set-Up Concept Level Clean Up Skills Acquired Collecting data Communicating Interpreting Measuring Organizing and analyzing data Scientific Methods In this lab, students will: Make observations Analyze the results Draw conclusions Communicate results Tips and Tricks A 15 W bulb works well for the experiments. Each setup can accommodate 2 3 students. In step 4, a partially darkened room produces a closer approximation of f. As a result, the students may have some difficulty measuring the actual size of the image in step 7. Emphasize the relationship between f measured in step 1 and 1 _ f calculated in the analysis of the results. What You ll Do V Observe images formed by a convex lens. V Measure the distance of objects and images from the lens. V Analyze your results to determine the focal length of the lens. What You ll Need cardboard screen, 10 cm 20 cm convex lens, 10 cm to 15 cm focal length lens holder light box with light bulb meterstick ruler, metric screen holder supports for meterstick Safety Lenses and Images As an optical engineer for a camera company, you have been given a lens for which your job is to figure out the focal length. Based on the specifications you obtain by doing an experiment, a new model of camera will be designed that uses that lens. Procedure Preparing for Your Experiment 1 The shape of a lens determines the size, position, and types of images that it may form. When parallel rays of light from a distant object pass through a converging lens, they come together to form an image at a point called the focal point. The distance from this point to the lens is called the focal length. In this experiment, you will find the focal length of a lens. Then, verify this value by forming images, measuring distances, and using the lens formula below. 1_ + 1_ = 1_ d o d i f where d o = object distance, d i = image distance, and f = focal length 2 On a clean sheet of paper, make a data table like the one shown. 3 Set up the equipment as illustrated in the figure below. Make sure the lens and screen are securely fastened to the meterstick. Determining Focal Length 4 Stand about 1 m from a window, and point the meterstick at a tree, parked car, or similar object. Slide the screen holder along the meterstick until a clear image of the distant object forms on the screen. Measure the distance between the lens and the screen in centimeters. This distance is very close to the focal length of the lens that you are using. Record this value at the top of your data table. Safety Cautions Students should be cautioned that the light bulb will become hot to the touch during the course of the lab. Only devices that are UL-listed should be used. The condition of the wiring and adequate grounding should be checked prior to use. 574 CHAPTER 16 Sound and Light Sample Data: Objects, Lenses, and Images Focal length of lens: 10.2 cm Object distance do (cm) Image distance di (cm) 1_ d o 1_ d i 1_ d o + 1 _ d i 1_ f Size of object (mm) Trial Trial Trial Size of image (mm)

38 Sample Data Table: Objects, Lenses, and Images Focal length of lens, f: cm Trial 1 Trial 2 Object distance, d o (cm) Image distance, d i (cm) 1_ d o 1_ d i 1_ d o + 1_ d i 1_ f Size of object (mm) Size of image (mm) Answers to Analysis 1. Results may vary. See sample data table. 2. The value of _ 1 + _ 1 is generally d o d i very close to the value of _ 1 f. Trial 3 Answer to Communicating Your Results 3. If the object distance is greater than the image distance, the image will be smaller than the object. Forming Images 5 Set up the equipment as illustrated in the figure. Place the lens more than twice the focal length from the light box. 6 Move the screen along the meterstick until a clear image forms. Record the distance from the light to the lens, d o, and the distance from the lens to the screen, d i, in centimeters as Trial 1 in your data table. Also, record the height of the object and of the image in millimeters. The object in this case may be either the filament of the light bulb or a cut-out shape in the light box. 7 For Trial 2, place the lens exactly twice the focal length from the object. Slide the screen along the stick until a clear image is formed, as in step 6. Record the distances from the screen and the sizes of the object and image as you did in step 6. 8 For Trial 3, place the lens at a distance from the object that is greater than the focal length but less than twice the focal length. Adjust the screen, and record the measurements as you did in step 7. Answer to Application Yes, the experiments showed that the lens forms images according to the standard lens equations. The minimum length of the camera would have to be the focal length of the lens, plus whatever additional space is needed for the camera housing and other components that would go behind the camera s image-capturing mechanism. Analysis 1. Analyzing Data Perform the necessary calculations to complete your data table. 2. Analyzing Data How does 1_ + 1_ compare with 1_ d o d i f three trials? Communicating Results in each of the 3. Drawing Conclusions If the object distance is greater than the image distance, how will the size of the image compare with the size of the object? Application Does the lens that you tested conform to the lens equations for image formation? If a camera that contained this lens was made, what would the minimum length of the camera have to be? Explain your answer. Key Resources Virtual Investigation Classroom Lab Video/DVD Holt Lab Generator CD-ROM Search for any lab type, standard, diffi cultly level, or time. Edit any lab to fi t your needs, or create your own labs. Use the Lab Material QuickList software to customize your lab materials list. Datasheet Lenses and Images Observation Lab Mirror Images CBL TM Probeware Lab Choosing a Pair of Sunglasses Chapter Lab 575