Sound and Light. Cracking the Sound Barrier

Transcription

1 Sound and Light sections 1 Sound 2 Reflection and Refraction of Light 3 Mirrors, Lenses, and the Eye Lab Reflections of Reflections 4 Light and Color Lab Blocking Noise Pollution Virtual Lab How are colors created? Cracking the Sound Barrier Have you ever heard a sonic boom? A plane moving faster than the speed of sound makes a sonic boom. The sound waves from the plane add together to form a cone-shaped shock wave. Behind the cone are low-pressure regions that can cause water vapor to condense, forming the cloud you see here. Science Journal Write three things that you would like to learn about sound. AFP/CORBIS

2 Start-Up Activities What sound does a ruler make? Think of the musical instruments you ve seen and heard. Some have strings, some have hollow tubes, and others have keys or pedals. Musical instruments come in many shapes and sizes and are played with various techniques. These differences give each instrument a unique sound. What would an instrument made from a ruler sound like? STEP 1 STEP 2 Sound and Light Make the following Foldable to help you identify ways in which sound and light are different and alike. Fold one sheet of paper lengthwise. Fold into thirds. 1. Firmly hold one end of a thin ruler lying flat on a desk, allowing the free end to extend beyond the edge of the desk. 2. Gently pull up on and release the end of the ruler. What do you see and hear? 3. Vary the length of the overhanging portion and repeat the experiment several times. 4. Think Critically In your Science Journal, write instructions for playing a song with the ruler. Explain how the length of the overhanging part of the ruler affects the sound. STEP 3 STEP 4 Unfold and draw overlapping ovals. Cut the top sheet along the folds. Label the ovals as shown. Sound Both Light Constructing a Venn Diagram As you read the chapter, list the characteristics unique to sound under the left tab, the characteristics unique to light under the right tab, and those common to both under the middle tab. Preview this chapter s content and activities at gpescience.com 319 AFP/CORBIS

3 Sound Reading Guide Explain how sound travels through different mediums. Explain different properties of sound. Discuss the Doppler effect. Identify ways sound can be used. Sounds are one way that you learn about the world around you. Review Vocabulary vibration: rhythmic back-and-forth motion New Vocabulary intensity loudness decibel pitch Doppler effect Sound Waves An amusement park can be a noisy place. With all the racket of carousel music and booming loudspeakers, it can be hard to hear what your friends say. The many sounds you hear every day are different, but they do have something in common each sound is produced by an object that vibrates. Compression Rarefaction Figure 1 A vibrating tuning fork produces compressions and rarefactions that travel outward from the tuning fork. These compressions and rarefactions form a sound wave. Making Sound Waves When an object like the tuning fork in Figure 1 vibrates, it creates sound waves. Sound waves are compressional waves. Recall that a compressional wave is made of compressions and rarefactions. When an end of the tuning fork moves outward, it forms a compression on that side by pushing the molecules in air together. The compression moves away from the tuning fork as these molecules collide with other molecules in air. When the end of the tuning fork moves back, a rarefaction is formed where the molecules are farther apart. As the tuning fork vibrates, it produces a series of compressions and rarefactions that travels outward. This series of compressions and rarefactions is the sound wave that you hear. The Speed of Sound A sound wave moves in air as collisions between the molecules in air transfer energy from place to place. Sound waves also can move in other materials. The material in which a sound wave moves is called a medium. Just as in air, sound waves travel in solids, liquids, and other gases as a vibrating object transfers energy to the particles in the material. However, sound waves cannot travel in empty space where there are no particles. 320 CHAPTER 11 Sound and Light

4 The Speed of Sound in Different Materials The speed of a sound wave in a medium depends on the type of substance and whether it is a solid, liquid, or gas. For example, Table 1 shows the speed of sound in different mediums at room temperature. In general, sound travels slowest in gases and fastest in solids. Temperature and the Speed of Sound The speed of sound waves also depends on the temperature of the medium. As the temperature of a substance increases, its atoms and molecules move faster. At a higher temperature, atoms and molecules collide with each other more frequently. As a result, sound waves move faster. For example, in air at a temperature of 0 C, sound travels at a speed of 331 m/s, but in air at a temperature of 20 C, sound travels at 343 m/s. Amplitude and Energy of Sound Waves Think about the differences among sounds you hear. Some are loud; others are soft. The flute plays high notes, whereas the tuba is much lower. What properties of sound waves cause these differences in the sounds you hear? Recall that the amount of energy a wave carries corresponds to its amplitude. For a compressional wave, amplitude is related to the density of the particles in the compressions and rarefactions. A vibrating object makes a wave by transferring energy to the medium. More energy is transferred to the medium when the particles of the medium are forced closer together in the compressions and spread farther apart in the rarefactions. Look at Figure 2. A sound wave has a higher amplitude and carries more energy when particles in the medium are closer together in the compressions and more spread out in the rarefactions. Table 1 Speed of Sound in Different Mediums Medium Speed of Sound (m/s) Air (20 C) 343 Cork 500 Water 1,498 Brick 3,650 Aluminum 5,000 Figure 2 The amplitude of a sound wave depends on the density of the particles in the compressions and rarefactions. In a high-amplitude sound wave, particles are tightly packed in the compressions and far apart in the rarefactions. Compression Rarefaction High-amplitude sound wave In a low-amplitude sound wave, particles are less tightly packed in the compressions and not as far apart in the rarefactions. Compression Rarefaction Low-amplitude sound wave SECTION 1 Sound 321

5 Figure 3 The intensity of a sound wave decreases as the wave spreads out from the source of the sound. The energy of the wave is spread over a larger area as the wave spreads out. Intensity and Loudness Imagine you are listening to music from your CD player. Sound waves produced by the CD player travel through the air and transfer energy from the CD player to your ears. The energy that reaches your ears depends not only on the amplitude of the sound waves, but also on how close you are to the CD player. Figure 3 shows that as you get farther from a source of sound, the energy of the sound waves is spread over a greater area. The amount of energy transferred by a sound wave through a certain area each second is the intensity of the sound wave. As sound waves travel away from the source of the sound, the intensity of the waves decreases as the waves spread out. This means that as you get farther from the source, less energy reaches your ears each second. Loudness When you hear different sounds, you do not need special equipment to know which sounds have greater intensity. Your ears and brain can tell the difference. Loudness is the human perception of sound intensity. As the intensity of a sound wave increases, the loudness of the sound also increases. Figure 4 The decibel scale measures the intensity of sound. Identify where a normal speaking voice would fall on the scale. Whisper 15 Purring cat How are intensity and loudness related? A Scale for Sound Intensity The intensity of sound can be described using a measurement scale. Each unit on the scale for sound intensity is called a decibel, abbreviated db. On this scale, the faintest sound that most people can hear is 0 db. Sounds with intensity levels above 120 db may cause pain and permanent hearing loss. During some rock concerts, sounds reach this damaging intensity level. Wearing ear protection, such as earplugs, around loud sounds can help protect against hearing loss. Figure 4 shows some sounds and their intensity levels in decibels. Average home Loudness in Decibels Vacuum cleaner Power mower Pain threshold db Jet plane taking off Rustling leaves Noisy restaurant 322 CHAPTER 11 Sound and Light (l)mark A. Schneider/Visuals Unlimited, (cl)rafael Macia/Photo Researchers, (cr)david Young-Wolff/PhotoEdit, (r)superstock

6 Pitch and Frequency You might be familiar with the music scale do, re, mi, fa, sol, la, ti, do shown in Figure 5. If you were to sing this scale, you would hear the pitch of your voice get higher. Pitch is the human perception of the frequency of sound waves. Pitch gets higher as the frequency of the sound waves increases. C do 262 Hz D re 294 Hz E mi 330 Hz F fa 349 Hz G sol 392 Hz A la 440 Hz B ti 494 Hz C do 523 Hz Frequency Frequency is a measure of how many wavelengths pass a particular point each second. For a compressional wave, such as sound, the frequency is the number of compressions or the number of rarefactions that pass by each second. Frequency is measured in hertz (Hz). A frequency of 1 Hz means that one complete wavelength passes by in 1 s. A healthy human ear can hear sound waves with frequencies from about 20 Hz to 20,000 Hz. The human ear is most sensitive to sounds in the range of 440 Hz to about 7,000 Hz. In this range, most people can hear much fainter sounds than they can at higher or lower frequencies. Ultrasonic and Infrasonic Waves Most people can t hear sound frequencies above 20,000 Hz, which are called ultrasonic waves. Dogs can hear sounds with frequencies up to about 35,000 Hz, and bats can detect frequencies higher than 100,000 Hz. Even though humans can t hear ultrasonic waves, they have a number of uses. For example, ultrasonic waves directed into the human body are used in medical diagnosis and treatment. Ultrasonic waves are reflected by objects underwater, and can be used to determine the size, shape, and depth of underwater objects. Infrasonic, or subsonic, waves have frequencies below 20 Hz too low for most people to hear. These waves are produced by sources that vibrate slowly, such as wind, heavy machinery, and earthquakes. Although you can t hear infrasonic waves, you might feel them as a rumble inside your body. The Doppler Effect Imagine that you are standing at the side of a racetrack with race cars zooming past. As they move toward you, the pitches of their engines become higher. As they move away, the pitches become lower. The change in pitch or frequency due to the relative motion of a wave source is called the Doppler effect. Figure 5 Every note has a different frequency, which gives it a distinct pitch. Describe how pitch changes when frequency increases. Comparing Intensity Procedure 1. Tie the middle of a 50-cm length of string to a metal object such as a spoon. 2. Wrap each end of the string around a finger on each hand. Place one of these fingers in each ear. 3. Swing the object so it bumps against a table or a chair. Note the sound. 4. Take your fingers out of your ears and repeat step 3. Analysis 1. Compare the sounds you heard in both trials. 2. Compare the intensity of the sound waves that reached your ears in both trials. SECTION 1 Sound 323

7 Figure 6 The Doppler effect occurs when the source of a sound wave is moving relative to a listener. The race car creates compression A, which spreads through the air in all directions from the point where it was created. Compression A Compression A The car is closer to the flagger when it creates compression B. Compressions A and B are closer together in front of the car, so the flagger hears a higher-pitched sound. Compression B Red Shift The Doppler effect can also be observed in light waves emanating from moving sources although the sources must be moving at tremendous speeds. Astronomers have learned that the universe is expanding by observing the Doppler effect in light waves. Research the phenomenon known as red shift and explain in your Science Journal how it relates to the Doppler effect. A Moving Source of Sound As a race car moves, it produces sound waves in the form of compressions and rarefactions. In Figure 6A, the race car creates a compression, labeled A. Compression A moves through the air toward the flagger. By the time compression B leaves the race car in Figure 6B, the car has moved forward. Because the car has moved since the time it created compression A, compressions A and B are closer together in front of the car than they would be if the car had stayed still. Because the compressions are closer together, more compressions pass by the flagger each second than if the car had been at rest. As a result, the flagger hears a higher pitch. Figure 6B also shows that the compressions behind the moving car are farther apart, resulting in a lower frequency. The flagger hears a lower pitch after the car passes. A Moving Listener You also can hear the Doppler effect when you are moving past a sound source that is standing still. Suppose you were riding in a school bus and passed a building with a ringing bell. The pitch would become higher as you approached the building and lower as you rode away from it. The Doppler effect happens any time the source of a sound is changing position relative to the listener. It occurs no matter whether it is the sound source or the listener that is moving. The faster the change in position, the greater the change in frequency and pitch. How does pitch change if you are moving away from a sound source? 324 CHAPTER 11 Sound and Light

8 Using Sound When sound waves strike an object, they can be absorbed by the object, transmitted through the object, or reflected from the object. By detecting the sound waves reflected from an object, the size, shape, and location of an object can be determined. Echolocation and Sonar Some species of bats, as well as dolphins, whales, and other animals, use sound waves to detect their prey. Echolocation is the process of locating objects by emitting sounds and detecting the sound waves that reflect back. Sonar is a system that uses the reflection of underwater sound waves to detect objects. First, a sound pulse is emitted underwater. The sound waves travel in the water and are reflected when they strike an object, such as a fish or a ship. An underwater microphone detects the reflected waves. Because the speed of sound in water is known, the distance to the object can be calculated by measuring how much time passes between when the sound pulse is emitted and when the reflected signal is received. Topic: Sonar Visit gpescience.com for Web links to information about ways sonar is used. Activity Research to find out the different types of sonar. Make a poster that describes active and passive sonar. USING SONAR A sonar pulse returns in 3.00 s from a sunken ship that is directly below. Find the depth of the ship if the speed of the pulse is 1,500 m/s. Hint: The sonar pulse travels a distance equal to twice the depth of the ship, so use the equation d = st/2 to find the depth. IDENTIFY known values and the unknown value Identify the known values: speed of sound in water means s 1,500 m/s time for round trip Identify the unknown value: depth of the ship means means Solve a Simple Equation t 3.00 s d? m SOLVE the problem Substitute the known values s 1,500 m/s and t 3.00 s into the equation for depth: d s t (1,500 m /s)(3.00 s) 1 (4,500 m) 2,250 m CHECK your answer Check your answer by dividing the distance by the time and multiplying by 2. The result should be the speed of sound given in the problem. Find the speed of a sonar pulse that returns in 2.0 s from a ship at a depth of 1,500 m. For more practice problems go to page 879, and visit Math Practice at. gpescience.com SECTION 1 Sound 325

9 Figure 7 Ultrasonic waves are directed into a pregnant woman s uterus to form images of her fetus. Ultrasound in Medicine High-frequency sound waves can also be used to remove dirt buildup on jewelry and glassware. One of the most important uses of ultrasonic waves, though, is in medicine. Using special instruments, medical professionals can send ultrasonic waves into a specific part of a patient s body. Reflected ultrasonic waves are used to examine different body parts and to detect and monitor certain types of heart disease and cancer. Ultrasound imaging also is used to monitor the development of a fetus, as shown in Figure 7. However, ultrasound does not produce good images of the bones and lungs, because hard tissues and air absorb the ultrasonic waves instead of reflecting them. High-frequency sound waves can be used to treat certain medical problems. For example, sometimes small, hard deposits of calcium compounds or other minerals form in the kidneys, making kidney stones. In the past, physicians had to perform surgery to remove kidney stones. Now ultrasonic treatments are commonly used to break them up instead. Bursts of ultrasound create vibrations that cause the stones to break into small pieces that can easily pass out of the body with the urine. A similar treatment is available for gallstones. Summary Sound Waves Sound waves are compressional waves that are produced by vibrating objects. Sound usually travels fastest in solids and slowest in gases. Sound travels faster as the temperature of the medium increases. Properties of Sound The amplitude of sound waves increases as the density of particles increases in the compressions and decreases in rarefactions Loudness is the human perception of the intensity of sound waves. Pitch is the human perception of the frequency of sound waves. The Doppler effect is the change in the frequency of a sound wave when the source of the sound is moving relative to a listener. Self Check 1. Explain how sound travels from your vocal cords to your friend s ears when you talk. 2. Explain how the loudness of a sound wave depends on the intensity of the sound wave. 3. Draw and label a diagram that explains the Doppler effect. 4. Describe at least three uses of ultrasonic technology in medicine. 5. Think Critically How could sonar technology be used to locate deposits of oil and minerals? 6. Calculate Time How long would it take a sound wave from a car alarm to travel 1.0 km in air if the air temperature were 0 C? 7. Calculate Distance If sound travels in the ocean with a speed of 1,500 m/s, how far will a sonar pulse travel in 45 s? 326 CHAPTER 11 Sound and Light Doug Martin More Section Review gpescience.com

10 Reflection and Refraction of Light Reading Guide Describe how light waves interact with matter. Explain the difference between regular and diffuse reflection. Define the index of refraction of a material. Explain why a prism separates white light into different colors. The images you see every day are due to the behavior of light waves. Review Vocabulary visible light: an electromagnetic wave with wavelengths between about 400 and 700 billionths of a meter New Vocabulary opaque translucent transparent index of refraction The Interaction of Light and Matter Look around your darkened room at night. You know that some of the objects are brightly colored, but they look gray or black in the dim light. Turn on the light, and you can see all the objects in the room clearly, including their colors. What you see depends on the amount of light in the room and the color of the objects. For you to see an object, it must reflect or emit some light that reaches your eyes. Absorption, Transmission, and Reflection Objects can absorb light, reflect light, and transmit light allow light to pass through them. The type of matter in an object determines the amount of light it absorbs, reflects, and transmits. For example, the opaque material in the top candleholder in Figure 8 only absorbs and reflects light no light passes through it. As a result, you cannot see the candle inside. Materials that allow some light to pass through them, like the material of the middle candleholder in Figure 8, are described as translucent. You cannot see clearly through translucent materials. Transparent materials, such as the bottom candleholder in Figure 8, transmit almost all the light striking them, so you can see objects clearly through them. Only a small amount of light is absorbed and reflected by transparent materials. Figure 8 These candleholders interact with light differently. Opaque Translucent Transparent SECTION 2 Reflection and Refraction of Light 327 First Image

11 Mirror Figure 9 According to the law of reflection, light is reflected so that the angle of incidence always equals the angle of reflection. i r i r Normal Normal i = angle of incidence r = angle of reflection Reflection of Light Have you glanced in a mirror today? You see your reflection in the mirror when light is reflected off you, strikes the mirror, and is reflected off the mirror into your eye. Because light behaves as a wave, it obeys the law of reflection, as shown in Figure 9. Recall that, according to the law of reflection, the angle at which a light wave strikes a surface is the same as the angle at which it is reflected. Light reflected from any surface a mirror or a sheet of paper obeys this law. Regular and Diffuse Reflection Why can you see your reflection in a store window but not in a sheet of paper? The answer has to do with the smoothness of the surfaces. A smooth, even surface such as a pane of glass produces a sharp image by reflecting parallel light waves in only one direction. Reflection of light waves from a smooth surface is regular reflection. To cause a regular reflection, the roughness of a surface must be less than the wavelengths it reflects. A sheet of paper has an uneven surface that causes incoming parallel light waves to be reflected in many directions, as shown in Figure 10. Reflection of light from a rough surface is diffuse reflection. Scattering Diffuse reflection is a type of scattering that occurs when light waves traveling in one direction are made to travel in many different directions. Scattering also occurs when light waves traveling through the air reflect off small particles. An example is light scattering off the small water droplets that make up a cloud. Scattering causes the cloud to appear white, even though the droplets are transparent. Figure 10 A sheet of paper has an uneven surface that produces a diffuse reflection. Explain Use the law of reflection to explain why a rough surface causes parallel light waves to be reflected in many directions. What is scattering of light? 328 CHAPTER 11 Sound and Light

12 (t)stephen Frisch/Stock Boston/PictureQuest, (b)david Parker/Photo Researchers Refraction of Light What occurs when a light wave passes from one material to another from air to water, for example? Recall that refraction is caused by a change in the speed of a wave when it passes from one material to another. If the light wave is traveling at an angle to the boundary between the materials and the speed of light is different in the two materials, the wave will be bent, or refracted. Why does refraction occur? The Index of Refraction The amount of bending that takes place depends on the speeds of light in both materials. The greater the change in speed, the more the light will be bent as it passes at an angle from one material to the other. Figure 11 shows an example of refraction. Every material has an index of refraction which is the ratio of the speed of light in a vacuum to the speed of light in the material. The index of refraction indicates how much the speed of light is reduced in the material compared to its speed in empty space. The larger the index of refraction, the more light is slowed down in the material. For example, because glass has a larger index of refraction than air, light moves more slowly in glass than air. The index of refraction is usually largest for solids and smallest for gases. Prisms A sparkling glass prism hangs in a sunny window, refracting the sunlight and projecting a colorful pattern onto the walls of the room. How does the refraction of light create these colors? The spectrum of colors is produced because the speed of light in a material also depends on the wavelength of the light. In a glass prism, light waves with longer wavelengths, such as red light waves, are slowed less than light waves with shorter wavelengths, such as blue light waves. Figure 12 shows what occurs when white light passes through a prism. White light, such as sunlight, is made up of light waves with range of wavelengths from red to blue. The triangular prism refracts the light twice once when it enters the prism and again when it leaves the prism and reenters the air. Because the longer wavelengths of light are slowed less than the shorter wavelengths are, red light is bent the least. As a result of these different amounts of bending, the different colors are separated when they emerge from the prism. Figure 11 The spoon looks broken because light waves are refracted as they change speed when they pass from the water to the air. Figure 12 Refraction causes a prism to separate a beam of white light into different colors. 329

13 Cool air Warm air Mirage Figure 13 Mirages result when air near the ground is much warmer or cooler than the air above. This causes light waves reflected from an object to refract, creating one or more additional images. Mirages When you re traveling in a car, you might see what looks like a pool of water on the road ahead. As you get closer, the water seems to disappear. You ve seen a mirage an image of a distant object produced by the refraction of light through air layers of different densities. Mirages result when the air at ground level is much warmer or cooler than the air above, as shown in Figure 13. The density of air increases as air cools and light waves move slower in cooler air than in warmer air. As a result, light waves are refracted as they pass through air layers with different temperatures. Summary Light and Matter The amount of light that is absorbed, reflected, or transmitted depends on the material making up the object. Reflection of Light Light waves always obey the law of reflection: the angle of incidence equals the angle of reflection. Regular reflection causes parallel light waves to be reflected in only one direction. Diffuse reflection causes parallel light waves to be reflected in many directions. Scattering occurs when light waves are reflected and travel in many different directions. Refraction of Light Refraction occurs when a light wave changes speed in moving from one material to another. The index of refraction of a material indicates how much light slows down in the material. Self Check 1. Compare and contrast opaque, transparent, and translucent materials. Give one example of each. 2. Discuss why you can see your reflection in a smooth piece of aluminum foil but not in a crumpled ball of foil. 3. Infer what happens to light waves that are reflected off dust particles in the air. 4. Explain what happens to white light when it passes through a prism. 5. Think Critically Suppose a material has the same index of refraction as water. How would a light wave change direction as it traveled from water into this material? 6. Find an Angle A light wave strikes a mirror at an angle of 42 from the surface of the mirror. What angle does the reflected wave make with the normal? 7. Find an Angle A light wave reflects from a mirror at 27 from the normal. What was the angle between the mirror and the incident wave? 330 CHAPTER 11 Sound and Light Charles O Rear/CORBIS More Section Review gpescience.com

14 Mirrors, Lenses, and the Eye Reading Guide Describe how images are formed by three types of mirrors. Explain how convex and concave lenses form images. Explain how the human eye enables you to see. Describe how lenses are used to correct vision problems. Mirrors and lenses are used in optical instruments such as cameras and telescopes. Review Vocabulary reflection: the process of changing direction after striking a surface New Vocabulary plane mirror concave mirror convex mirror convex lens concave lens Light Rays Light sources send out light waves in all directions. These waves spread out like ripples on the surface of water spread out from the point of impact of a pebble. You can think of the light coming from the source as being many narrow beams of light traveling in all directions. Each narrow light beam is called a light ray. Mirrors A mirror is any surface that produces a regular reflection. A pool of still water, a metal pan, and even the back of a shiny spoon can be mirrors. Mirrors can be flat, curved inward, or curved outward. Figure 14 Seeing an image of yourself in a mirror involves two sets of reflections: light rays are reflected from you and then are reflected by the mirror. Plane Mirrors A flat, smooth mirror is a plane mirror. When you look in a plane mirror, your image appears upright. If you stand 1 m from the mirror, your image appears 1 m behind the mirror, or 2 m from you. In fact, your image is what a friend standing 2 m from you would see. Figure 14 shows how your image is formed. First, light rays from a light source strike you. Every point that is struck by the light rays reflects these rays so they travel outward in all directions. If your friend were looking at you, these reflected light rays coming from you would enter her eyes so she could see you. However, if a mirror is placed between you and your friend, light rays are reflected from the mirror back to your eyes. SECTION 3 Mirrors, Lenses, and the Eye 331 David Young-Wolff/PhotoEdit

15 Virtual and Real Images The formation of an image by a plane mirror is shown in Figure 15. Your brain assumes that light rays travel in a straight line. As a result, rays that enter your eyes seem to come from behind the mirror. This makes the image seem to be behind the mirror. However, no light rays actually pass through the place where the image seems to be located. This type of image is called a virtual image. Plane mirrors always form virtual images. If light rays from an object pass through the location of the image, the image is called a real image. Curved mirrors can form both real and virtual images. Figure 15 Your brain interprets the light rays reflected by the mirror as coming from a point behind the mirror. Infer how the size of your image in a plane mirror depends on your distance from the mirror. Concave Mirrors If the surface of a mirror is curved inward, it is called a concave mirror, as shown in Figure 16. The optical axis is an imaginary straight line drawn perpendicular to the surface of the mirror at its center. Every light ray traveling parallel to the optical axis is reflected through a point on the optical axis called the focal point. The distance from the center of the mirror to the focal point is called the focal length. The image formed by a concave mirror depends on the location of the object relative to the focal point. Figure 16 shows a candle located more than twice the focal length from the mirror. Although many light rays come from each point on the candle, only two rays from the same point are shown. Ray A passes through the focal point and then reflects off the mirror moving parallel to the optical axis. Ray B travels parallel to the optical axis before reflecting off the mirror and passing through the focal point. The point where the two rays meet is the location of the image of the original point on the candle. The image formed is a real image because light rays from the candle pass through the place where the image is located. Ray B Figure 16 Rays A and B start from the same place on the candle, travel in different directions, and meet again on the reflected image. Diagram how two other points on the image of the candle are formed. Ray A Focal point Optical axis 332 CHAPTER 11 Sound and Light

16 Images Produced by a Concave Mirror When an object is between one and two focal lengths from a concave mirror, the image is real, inverted, and larger than the object. An object closer than one focal length from a concave mirror produces a virtual image that is upright and larger than the object. No image is produced if the object is located at the focal point. Ray B Ray A Optical axis Convex Mirrors When you are in a store, look up toward one of the back corners or at the end of an aisle to see if a large, rounded mirror is mounted there. You can see a large area of the store in such a mirror. A mirror that curves outward like the back of a spoon is called a convex mirror. Light rays that hit a convex mirror diverge, or spread apart, after they are reflected. Figure 17 shows how the rays from an object are reflected by a convex mirror to form an image. The reflected rays diverge and never meet, so a convex mirror forms only a virtual image. The image also is upright and smaller than the actual object is. Figure 17 A convex mirror forms a reduced, upright, virtual image. Describe the image formed by a convex mirror. Lenses What do your eyes have in common with cameras, eyeglasses, and microscopes? Each of these contains at least one lens. A lens is a transparent object with at least one curved surface that causes light rays to refract. The image that a lens forms depends on the shape of the lens. Like curved mirrors, lenses can be convex or concave. Convex Lenses A convex lens is thicker in the middle than at the edges. Its optical axis is an imaginary straight line that is perpendicular to the surface of the lens at its thickest point. When light rays approach a convex lens traveling parallel to its optical axis, the rays are refracted toward the center of the lens, as in Figure 18. Light rays traveling along the optical axis are not bent at all. All light rays traveling parallel to the optical axis are refracted so they pass through a single point, which is the focal point of the lens. The less curved the sides of the lens, the less light rays are bent. As a result, lenses with flatter sides have longer focal lengths. Figure 19 on the next page shows that convex lenses can form either real or virtual images. Figure 18 Convex lenses are thicker in the middle than at the edges. Light rays that are parallel to the optical axis are refracted so that they pass through the focal point. A light ray that passes through the center of the lens is not refracted. Focal length Focal point SECTION 3 Mirrors, Lenses, and the Eye 333 David Parker/Photo Researchers

17 VISUALIZING IMAGES FORMED BY A CONVEX LENS Figure 19 Aconvex lens can form images that are real or virtual, enlarged or reduced, and upright or inverted. Just as for a concave mirror, the type of image formed by a convex lens depends on the location of the object relative to the focal point of the lens. If the object is more than two focal lengths from the lens, the image formed is real, inverted, and smaller than the object. As the object moves farther from the lens, the image becomes smaller. Object Ray A Optical axis Ray B Focal point Image Two focal lengths One focal length If the object is between one and two focal lengths from the lens, the image formed is real, inverted, and larger than the object. As the object moves closer to the focal point, the image becomes larger. Object Ray A Focal point Optical axis Two focal lengths One focal length Ray B Image If the object is less than one focal length from the lens, the image is virtual, upright, and larger than the object. The image is virtual because the light rays from the object diverge after they pass through the lens. The image becomes smaller as the object moves closer to the lens. Image Object Ray A Focal point Optical axis One focal length Ray B 334 CHAPTER 11 Sound and Light

18 Concave Lenses A concave lens is thinner in the middle and thicker at the edges. As shown in Figure 20, light rays that pass through a concave lens bend away from the optical axis. The rays spread out and never meet, so a real image is never formed. The image is always virtual, upright, and smaller than the object is. Concave lenses are used in some types of eyeglasses and some telescopes. Concave lenses usually are used in combination with other lenses. Optical axis What type of image is formed by a concave lens? The Human Eye What determines how well you can see the words on this page? If you don t need eyeglasses, the structure of your eye gives you the ability to focus on these words and other objects around you. Look at Figure 21. Light enters your eye through a transparent covering on your eyeball called the cornea. The cornea causes light rays to bend so that they converge. The light then passes through an opening called the pupil. Behind the pupil is a flexible convex lens. Muscles attached to the lens change its shape to help focus light, forming a sharp image on your retina. The retina is the inner lining of your eye. The retina contains light-sensitive cells that convert an image into electrical signals. These signals then are carried along the optic nerve to your brain to be interpreted. With this complex structure, the human eye is capable of seeing clearly in bright and dim conditions, focusing on both near and far objects, and also detecting colors. Figure 20 A concave lens refracts light rays so they spread out away from the optical axis. Classify Is a concave lens most like a concave mirror or a convex mirror? Eye Doctors Optometrists and ophthalmologists treat many different eye problems. They often use advanced technology to diagnose and correct malfunctioning parts of the eye. Research an eye problem and how it is treated. Write a paragraph describing what parts of the eye are affected by the problem and how doctors correct it. Retina Focal point Lens Pupil Cornea Figure 21 The cornea and lens in your eye bend light rays so that a sharp image is formed on the retina. Optic nerve SECTION 3 Mirrors, Lenses, and the Eye 335

19 Experimenting with Focal Lengths Procedure 1. Fill a glass test tube with water and seal it with a lid or stopper. 2. Type or print SULFUR DIOXIDE in capital letters on a piece of paper. 3. Set the test tube horizontally over the words. Record your observations. 4. Hold the tube at several heights above the words. Record your observations. Analysis 1. Describe how the words changed as the height of the test tube increased. 2. Classify the image at each height as real or virtual. Figure 22 Farsightedness can be corrected by convex lenses. Brightness and Intensity The human eye can adjust to the brightness of the light that strikes it. If you step outside on a sunny day, you may have to shade your eyes against the intensity of the sunlight. Light intensity is the amount of light energy that strikes a certain area each second. Brightness is the human perception of light intensity. Sunlight seems bright because a large amount of light energy strikes your retina each second. Your eyes respond to bright light by decreasing the size of your pupil. This reduces the amount of light that enters your eye and strikes the retina. Intensity depends on your distance from a light source. If you move away from a bare lightbulb, you can see how the intensity of the light rapidly decreases. Because light rays spread out from the source, less light energy strikes your retina the farther away you are from the bulb. Correcting Vision Problems If you have good vision, you should be able to see objects clearly when they are 25 cm or farther away from your eyes. A sharp image of an object should be formed on your retina. However, for many people, the image is blurry or formed in the wrong place, causing vision problems. Farsightedness If you can see distant objects clearly but can t bring nearby objects into focus, then you are farsighted. In this case, the eyeball might be too short or the lens isn t curved enough to form a sharp image of nearby objects on the retina, as shown in Figure 22. To correct the problem, convex lenses can be used to bend incoming light rays so they converge before they enter the eye. Focal point Light from nearby object Focal point Light from nearby object Farsighted eye Corrected farsighted eye The focal length of a farsighted eye is too long to form a sharp image of nearby objects on the retina. A convex lens in front of a farsighted eye enables a sharp image of nearby objects to be formed on the retina. 336 CHAPTER 11 Sound and Light

20 Focal point Light from distant object Focal point Light from distant object Nearsighted eye When a nearsighted person looks at distant objects, a sharp image is formed in front of the retina. Corrected nearsighted eye A concave lens in front of a nearsighted eye makes light rays diverge so a sharp image is formed on the retina. Nearsightedness If you have nearsighted friends, you know that they can see only nearby objects clearly. Their eyes cannot form a sharp image on the retina of an object that is far away. Instead, the image is formed in front of the retina, as shown in Figure 23. To correct this problem, a nearsighted person can wear concave lenses. Figure 23 shows how a concave lens causes incoming light rays from distant objects to diverge before they reach the eye. Then the rays can be focused by the eye to form a sharp image on the retina. Figure 23 Nearsightedness can be corrected with concave lenses. Summary Mirrors A plane mirror forms upright, virtual images. The image formed by a concave mirror depends on the location of the object relative to the focal point. Convex mirrors always produce virtual, upright images that are smaller than the object. Lenses The image formed by a convex lens depends on the distance of the object from the lens. Concave lenses always form virtual, upright images that are smaller than the object. The Eye and Vision The eye contains a lens that changes shape to produce sharp images on the retina. Farsightedness occurs when the eye cannot form a sharp image of nearby objects. Nearsightedness occurs when the eye cannot form sharp images of distant objects. Self Check 1. Diagram how light rays from an object are reflected by a convex mirror to form an image. 2. Infer how the size of the image changes if an object that is less than one focal length from a concave mirror moves closer to the mirror. 3. Explain how the focal length of a convex lens changes as the sides of the lens become less curved. 4. Compare the image of an object less than one focal length from a convex lens with the image of an object more than two focal lengths from the lens. 5. Think Critically Describe the image formed by a light source placed at the focal point of a convex lens. 6. Calculate Object Distance If you looked through a convex lens with a focal length of 15 cm and saw a real, inverted, enlarged image, what is the maximum distance between the lens and the object? More Section Review gpescience.com SECTION 3 Mirrors, Lenses, and the Eye 337

21 Reflections of Reflections How can you see the back of your head? You can use two mirrors to view a reflection of a reflection of the back of your head. Real-World Problem How many images can you see with two mirrors? Goal Infer how the number of images depends on the angle between two mirrors. Materials plane mirrors (2) masking tape Safety Precautions protractor paper clip Handle glass mirrors and paper clips carefully. Procedure 1. Lay one mirror on top of the other with the mirror surfaces inward. Tape them together so they will open and close. Use tape to label them L and R for left and right. 2. Stand the mirrors on a sheet of paper. Using the protractor, close the mirrors to an angle of 72. Images and Wedges Seen in the Mirrors Angle of Mirrors 72º 90º 120º Number of Paper Clip Images R Do not write in this book. L Number of Wedges 3. Bend one leg of a paper clip up 90 and place it close to the front of the R mirror. 4. Count the number of images of the paper clip you see in the R and L mirrors. Record these numbers in your data table. 5. The mirrors create an image of a circle divided into wedges. Record the number of wedges. 6. Hold the R mirror still and slowly open the L mirror to 90. Record the numbers of images of the clip and wedges in the circle. Repeat, this time opening the mirrors to 120. Conclude and Apply 1. Infer the relationship between the number of wedges and paper clip images. 2. Determine the angle that would divide a circle into six wedges. Hypothesize how many images would be produced. Test your hypothesis. Demonstrate for younger students the relationship between the angle of the mirrors and the number of reflections. 338 CHAPTER 11 Sound and Light Matt Meadows

22 Light and Color Reading Guide Explain how you see color. Describe the difference between light color and pigment color. Predict what happens when different colors are mixed. From traffic lights to great works of art, color is an important part of your world. Review Vocabulary retina: inner layer of the eye containing cells that convert light images into electrical signals New Vocabulary pigment Why Objects Have Color Why do some apples appear red, while others look green or yellow? An object s color depends on the wavelengths of light it reflects. Recall that white light is a blend of all colors of visible light. When a red apple is struck by white light, it reflects red light back to your eyes and absorbs all of the other colors. Figure 24 shows white light striking a green leaf. Only the wavelengths corresponding to green light are reflected to your eyes. Although some objects appear to be black, black isn t a color. Black is the absence of visible light. Objects that appear black absorb all colors of light and reflect little or no light back to your eyes. White objects appear to be white because they reflect all colors of visible light. Why does a white object appear white? Figure 24 This leaf absorbs all wavelengths of visible light except the wavelengths you see as green. Colored Filters Wearing tinted glasses changes the color of almost everything you see. If the lenses are yellow, the world takes on a golden glow. If they are rose colored, everything looks rosy. Something similar would occur if you placed a colored, clear plastic sheet over this white page. The paper would appear to be the same color as the plastic. The plastic sheet and the tinted lenses are filters. A filter is a transparent material that transmits one or more colors of light but absorbs all others. The color of a filter is the color of the light that it transmits. SECTION 4 Light and Color 339

23 Figure 25 The color of this cooler changes when viewed through different color filters. Looking Through Colored Filters Figure 25 shows how the color of an object can change when you look at it through various colored filters. The cooler looks blue under white light because it reflects only the wavelengths of blue light that strike it. If you look at the cooler through a blue filter, the cooler still looks blue because the blue filter transmits the reflected blue light. However, if you look at the cooler through a red filter, the cooler seems black because the red filter blocks the blue light reflected by the cooler. Why does a blue object appear black when viewed through a red filter? Seeing Color As you approach a busy intersection, the color of the traffic light changes from green to yellow to red. On the cross street, the color changes from red to green. At a busy intersection, traffic safety depends on your ability to detect color changes rapidly. How do you see colors? Topic: Color Blindness Visit gpescience.com for Web links to information about the causes of color blindness. Activity Research the causes and types of color blindness. Find out how cones are related to the ability to distinguish colors. Light and the Eye In a healthy eye, light enters the eye through the cornea, is focused by the lens, and finally forms an image on the retina. The retina is made up of two types of cells that absorb light, as shown in Figure 26. When these cells absorb light energy, chemical reactions convert light energy into nerve impulses that are transmitted to the brain. One type of cell in the retina, called a cone, enables you to distinguish colors and detailed shapes of objects. Cones need bright light to generate nerve impulses, so they do not operate in dim light. As a result, even brightly colored objects might look gray or black in dim light. 340 CHAPTER 11 Sound and Light First Image

24 Figure 26 Light enters the eye and focuses on the retina. The two types of light-detecting cells that make up the retina are called rods and cones. Rod Lens Cone Retina Cones and Rods Your eyes have three types of cones, each of which responds to a different range of wavelengths. Red cones respond to mostly red and yellow light. Green cones respond to mostly yellow and green light. Blue cones respond to mostly blue and violet light. The second type of cell in the retina, called a rod, is sensitive to dim light and enables you to see at night. However, rod cells do not enable you to see colors. Interpreting Color Why does a banana look yellow? The light reflected by the banana causes the cone cells that are sensitive to red and green light to send signals to your brain. Your brain could get the same signal if a mixture of red light and green light reached your eyes. This mixture also would cause your red and green cones to respond, and you would see the color yellow. As a result, light with wavelengths corresponding to yellow light and light that is a mixture of red and green light both cause the color yellow to be seen. What happens when you look at a white shirt? You see white when all wavelengths of visible light enters your eyes. Then all three sets of cones cells send signals to the brain. The combination of these signals cause you to see the shirt as white. Figure 27 Color blindness is an inherited sex-linked condition in which certain sets of cones in the retina do not function properly. Identify the number that you see in the dots. Color Blindness If one or more of your sets of cone cells do not function properly, you might not be able to distinguish certain colors. This condition is called color blindness or color deficiency. About eight percent of men and one-half percent of women have some form of color blindness. The most common form of color blindness makes it difficult to distinguish between red and green. Figure 27 shows an image that is used in a test for red green color blindness. Because red and green are used in traffic signals, drivers and pedestrians must be able to identify them. SECTION 4 Light and Color 341 (t)ralph C. Eagle, Jr./Photo Researchers, (b)diane Hirsch/Fundamental Photographs

25 Color for Photosynthesis Plant pigments determine the wavelengths of light for photosynthesis. Leaves usually look green because of the pigment chlorophyll. Chlorophyll absorbs most wavelengths of visible light except green, which it reflects. However, not all plants are green. Research different plant pigments to find out how they allow plant species to survive in diverse habitats. Mixing Colors If you have ever browsed through a paint store, you have probably seen displays where customers can select paint samples of almost every imaginable color. The colors are a result of mixtures of pigments. For example, you might have mixed blue and yellow paint to produce green paint. A pigment is a colored material that is used to change the color of other substances. The color of a pigment results from the different wavelengths of light that the pigment reflects. Mixing Colored Lights From the glowing orange of a sunset to the deep blue of a mountain lake, all the colors you see can be made by mixing three colors of light. These three colors red, green, and blue are the primary colors of light. They correspond to the three different types of cones in the retina of your eye. When mixed together in equal amounts, they produce white light, as Figure 28 shows. Mixing the primary colors in different proportions can produce the colors you see. What are the primary colors of light? Paint Pigments If you were to mix equal amounts of red, green, and blue paint, would you get white paint? If mixing colors of paint were like mixing colors of light, you would, but mixing paint is different. Paints are made with pigments. Pigments produce color as a result of the wavelengths of light they reflect. Paint pigments usually are made of chemical compounds such as titanium oxide, a bright white pigment, and lead chromate, which is used for painting yellow lines on highways. Figure 28 White light is produced when the three primary colors of light are mixed in equal amounts. 342 CHAPTER 11 Sound and Light Matt Meadows

26 Mixing Pigments You can make any pigment color by mixing different amounts of the three primary pigments magenta (bluish red), cyan (greenish blue), and yellow. In fact, color printers use these pigments, as well as black ink, to make full-color prints like the pages in this book. A primary pigment s color depends on the wavelengths of the light that it reflects. Actually, pigments both absorb and reflect a range of colors in sending a single color message to your eye. For example, in white light, yellow pigment appears yellow because it reflects red and green light but absorbs the other wavelengths of visible light. The color of a mixture of two primary pigments is determined by the primary colors of light that both pigments reflect. Look at Figure 29. The area in the center where the colors all overlap appears to be black because the three blended primary pigments absorb all the primary colors of light. Recall that the primary colors of light combine to produce white light. They are called additive colors. However, when the primary pigment colors are combined, they absorb all wavelengths of visible light and produce black. Because black results from the absence of reflected light, the primary pigments are called subtractive colors. Figure 29 The three primary pigment colors appear to be black when they are mixed. Describe how the primary pigment colors are similar to the primary colors of light. Summary Why Objects Have Color The color of an object is determined by the wavelengths of light it reflects. The color of a filter is the color of the light the filter transmits. Seeing Color Rod and cone cells are light-sensitive cells found in the retina of the human eye. Rod cells are sensitive to dim light. Cone cells enable colors to be seen. There are three types of cone cells. One type responds to red light, another to green light, and another to blue light. Mixing Colors Red, green, and blue are the primary light colors. Any color of light can be created by mixing these primary light colors. Any pigment color can be formed by mixing the primary pigment colors magenta, cyan, and yellow. Self Check 1. Identity what colors are reflected and what colors are absorbed if a white light shines on a red shirt. 2. Discuss how the primary colors of light differ from the primary pigment colors. 3. Explain why a red apple would appear black if you looked at it through a blue filter. 4. Determine why a white fence appears to be white instead of multicolored if all colors are present in white light. 5. Think Critically Light reflected from an object passes through a green filter, then a red filter, and finally a blue filter. What color will the object seem to be? 6. Use Percentages In the human eye there are about 120,000,000 rods. If 90,000,000 rods trigger at once, what percentage of the total number of rods triggered? 7. Convert Units The wavelengths of light are measured in nanometers (nm) which equals mm. Find the wavelength in mm of a light wave that has a wavelength of 690 nm. More Section Review gpescience.com SECTION 4 Light and Color 343

27 Design Your Own Goals Design an experiment that tests the effectiveness of various types of barriers and materials for blocking out noise pollution. Test different types of materials and barriers to determine the best noise blocks. Possible Materials radio, CD player, horn, drum, or other loud noise source shrubs, trees, concrete walls, brick walls, stone walls, wooden fences, parked cars, or hanging laundry sound meter meterstick or metric tape measure Blocking Nfise Pollution Real-World Problem What loud noises do you enjoy, and which ones do you find annoying? Most people enjoy a music concert performed by their favorite artist, booming displays of fireworks on the Fourth of July, and the roar of a crowd when their team scores a goal or makes a touchdown. Although these are loud noises, most people enjoy them for short periods of time. Most people find certain other loud noises, such as traffic, sirens, and loud talking, annoying. Constant, annoying noises are called noise pollution. What can be done to reduce noise pollution? What types of barriers best block out loud noises? What types of barriers will best block out noise pollution? Form a Hypothesis Based on your experiences with loud noises, form a hypothesis that predicts the effectiveness of different types of barriers at blocking out noise pollution. Test Your Hypothesis Make a Plan 1. Decide what type of barriers or materials you will test. 2. Describe exactly how you will use these materials. 344 John Wang/Getty Images

28 3. Identify the controls and variables you will use in your experiment. 4. List the steps you will use and describe each step precisely. 5. Prepare a data table in your Science Journal to record your measurements. 6. Organize the steps of your experiment in logical order. Follow Your Plan 1. Ask your teacher to approve your plan and data table before you start. 2. Conduct your experiment as planned. 3. Test each barrier two or three times. 4. Record the results from each test in your data table in your Science Journal. Analyze Your Data 1. Identify the barriers that most effectively reduced noise pollution. 2. Identify the barriers that least effectively reduced noise pollution. 3. Compare the effective barriers and identify common characteristics that might explain why they reduced noise pollution. 4. Compare the natural barriers you tested with the artificial barriers. Which type of barrier best reduced noise pollution? 5. Compare the different types of materials the barriers were made of. Which type of material best reduced noise pollution? Conclude and Apply 1. Evaluate whether your results support your hypothesis. 2. Predict how your results would differ if you used a louder source of noise such as a siren. 3. Infer from your results how people living near a busy street could reduce noise pollution. 4. Identify major sources of noise pollution in or near your home. How could they be reduced? 5. Research how noise pollution can be unhealthy. Draw a poster illustrating how builders and landscapers could use certain materials to better insulate a home or office from excess noise pollution. LAB 345 David Young-Wolff/PhotoEdit

29 A Haiku Garden: The Four Seasons in Poems and Prints by Stephen Addiss with Fumiko and Akira Yamamoto Understanding Literature Japanese Haiku A haiku is a verse that consists of three lines and 17 syllables in the Japanese language. The first and third lines have five syllables each, and the middle line has seven syllables. Why is imagination important in reading haiku? Withered by winter the sound of the wind one-color world Basho Lingering in every pool of water spring sunlight Issa Respond to the Reading 1. How do the illustrations help the reader better understand the poems? 2. What do you think is meant by the word lingering in the Haiku about spring sunlight? 3. Linking Science and Writing Write one haiku about summer and another about fall. In one poem, use color to help you describe the season. In the other, use light or some property of light to help describe the season. Research has determined that there is a connection between color and mood. Warm colors have longer wavelengths and can be more stimulating. Cool colors, which have shorter wavelengths, tend to have a calming or soothing effect on people. Light and color have long been used as literary symbols. Does the use of color change what you imagine when you read the haiku? 346 CHAPTER 11 Sound and Light (t)archivo Iconografico, S.A./CORBIS, (b)ashmolean Museum, Oxford, UK/Bridgeman Art Library, London/New York

30 Sound 1. Sound waves are compressional waves that travel only in matter. 2. The speed of sound in a material depends on the type of material as well as its temperature. 3. Loudness depends on a sound wave s intensity. Pitch depends on a sound wave s frequency. 4. Sound waves are used in echolocation, sonar, and medical imaging. Reflection and Refraction of Light 1. Light can be absorbed, reflected, or transmitted by a material. 2. When light waves are reflected, they obey the law of reflection the angle of incidence equals the angle of reflection. Mirrors, Lenses, and the Eye 1. Plane mirrors form upright, virtual images. 2. Images formed by concave mirrors and convex lenses depend on the location of the object relative to the focal point. 3. Convex mirrors and concave lenses form virtual, upright images that are smaller than the object. 4. The lens in the human eye changes shape to produce a sharp image on the retina. Light and Color 1. You see color when light is reflected off objects and into your eyes. 2. Cone cells in the retina are light-sensitive cells that enable you to distinguish colors. 3. Red, blue, and green are the three primary colors of light and can be mixed to form all other colors. 4. The primary pigment colors are magenta, cyan, and yellow. 3. A light wave is refracted, or bent, when it changes speed as it travels at an angle from one material to another. Use the Foldable you made at the beginning of the chapter to help you review sound and light. Interactive Tutor gpescience.com CHAPTER STUDY GUIDE 347 (t)jeff Greenberg/Visuals Unlimited, (b)dick Poe/Visuals Unlimited