Light waves interact with materials.

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1 Page of 7 KEY CONCEPT Light waves interact with materials. BEFORE, you learned Mechanical waves respond to a change in medium Visible light is made up of EM waves EM waves interact with a new medium in the same ways that mechanical waves do NOW, you will learn How the wave behavior of light affects what we see How light waves interact with materials Why objects have color How different colors are produced VOCABULARY transmission p. 9 absorption p. 9 scattering p. 95 polarization p. 96 prism p. 97 primary colors p. 98 primary pigments p. 99 EXPLORE Light and Matter How can a change in medium affect light? PROCEDURE Fill the container with water. Add 0 ml ( tsp) of milk to the water. Put on the lid, and gently shake the container until the milk and water are mixed. In a dark room, shine the light at the side of the container from about 5 cm ( in.) away. Observe what happens to the beam of light. MATERIALS clear plastic container with lid water measuring spoons milk flashlight WHAT DO YOU THINK? What happened to the beam of light from the flashlight? Why did the light behave this way? VOCABULARY Don t forget to make word frames for transmission and absorption. Light can be reflected, transmitted, or absorbed. You have read that EM waves can interact with a material medium in the same ways that mechanical waves do. Three forms of interaction play an especially important role in how people see light. One form is reflection. Most things are visible because they reflect light. The two other forms of interaction are transmission and absorption. Transmission (trans-mihsh-uhn) is the passage of an EM wave through the medium. If the light reflected from objects did not pass through the air, windows, or most of the eye, we could not see the objects. Absorption (uhb-sawrp-shun) is the disappearance of an EM wave into the medium. Absorption affects how things look, because it determines what light is available to be reflected or transmitted. Chapter : Electromagnetic Waves 9

2 Page of 7 How Materials Transmit Light Materials can be classified according to the amount and type of light they transmit. Transparent (trans-pair-uhnt) materials allow most of the light that strikes them to pass through. It is possible to see objects through a transparent material. Air, water, and clear glass are transparent. Transparent materials are used for items such as windows, light bulbs, thermometers, sandwich bags, and clock faces. Translucent (trans-loo-suhnt) materials transmit some light, but they also cause it to spread out in all directions. You can see light through translucent materials, but you cannot see objects clearly through them. Some examples are lampshades, frosted light bulbs, frosted windows, fluorescent light coverings, sheer fabrics, and notepaper. Opaque (oh-payk) materials do not allow any light to pass through them, because they reflect light, absorb light, or both. Heavy fabrics, construction paper, and ceramic mugs are opaque. Shiny materials may be opaque mainly because they reflect light. Other materials, such as wood and rock, are opaque mainly because they absorb light. What is the difference between translucent and opaque materials? This stained-glass window contains transparent, translucent, and opaque materials. transparent translucent opaque 94 Unit: Waves, Sound, and Light

3 Page of 7 A light filter is a material that is transparent to some kinds of light and opaque to others. For example, clear red glass transmits red light but absorbs other colors. Examples of light filters are the colored covers on taillights and traffic lights, infrared lamp bulbs, and UV-protected sunglasses. Filters that transmit only certain colors are called color filters. Scattering Sometimes fine particles in a transparent material interact with light passing through the material to cause scattering. Scattering is the spreading out of light rays in all directions, because particles reflect and absorb the light. Fog or dust in the air, mud in water, and scratches or smudges on glass can all cause scattering. Scattering creates glare and makes it hard to see through even a transparent material. Making the light brighter causes more scattering, as you might have noticed if you have ever tried to use a flashlight to see through fog. Fine particles, such as those in fog, scatter light and reduce visibility. Scattering is what makes the sky blue. During the middle of the day, when the Sun is high in the sky, molecules in Earth s atmosphere scatter the blue part of visible light more than they scatter the other wavelengths. This process gives the sky a blue tinge and makes it translucent. Light comes through the sky, but you cannot see through the sky. At dawn and dusk, light from the Sun must travel farther through the atmosphere before it reaches your eyes. By the time you see it, the greens and blues are scattered away and the light appears reddish. At night, because there is so little light, almost no scattering takes place, and you can see through the sky to the stars. SUPPORTING MAIN IDEAS Be sure to add to your chart the different ways light interacts with materials. How does scattering make the sky blue? Chapter : Electromagnetic Waves 95

4 Page 4 of 7 Polarization Polarization of light reduces glare and makes it easier to see objects. Polarization (POH-luhr-ih-ZAY-shuhn) is a way of filtering light so that all of its waves vibrate in the same direction. Remember that EM waves are made of electric and magnetic fields vibrating at right angles to each other. Polarization affects only the electric fields of a light wave. When all of the electric fields of a group of light waves vibrate in the same direction, the light is polarized. Light can be polarized by a particular type of light filter called a polarizing filter. A polarizing filter acts on a light wave s electric field like the bars of a cage. The filter allows through only waves whose electric fields vibrate in one particular direction. Waves that pass through the filter are polarized. In the illustration below, these waves are shown in darker yellow. Light reflecting off the surface of this pond causes glare. unpolarized light waves The fields of visible light waves vibrate in all directions. polarized light waves A polarizing filter stops all waves except those vibrating vertically. A polarizing filter reduces glare, making it possible to see objects under the water. What do you think happens when polarized light passes into a second polarizing filter? If the direction of the bars in the second filter is the same as in the first, then all of the light will pass through the second filter. The light will still be polarized. If the direction of the bars in the second filter is at a right angle to the first, as in the illustration above, then no light at all will pass through the second filter. no light waves A second filter stops all waves except those vibrating horizontally. Wavelength determines color. The section of the EM spectrum called visible light is made up of many different frequencies and wavelengths. When all of these wavelengths are present together, as in light from the Sun or a light bulb, we see ordinary light, which is also called white light. 96 Unit: Waves, Sound, and Light

5 Page 5 of 7 Seen individually, the wavelengths appear as different colors of light. This fact can be demonstrated by using a prism. A prism is a tool that uses refraction to separate the different wavelengths that make up white light. The prism bends some of the wavelengths more than others. The lightwaves, bent at slightly different angles, form a color spectrum. The color spectrum could be divided into countless individual wavelengths, each with its own color. However, the color spectrum is usually divided into seven distinct color bands. In order of decreasing wavelength, the bands are red, orange, yellow, green, blue, indigo, and violet. You see a color spectrum whenever you see a rainbow. Prisms split light into colors by refracting wavelengths in different amounts. Color Reflection and Absorption The color of an object or material is determined by the wavelengths it absorbs and those it reflects. An object has the color of the wavelengths it reflects. A material that reflects all wavelengths of visible light appears white. A material that absorbs all wavelengths of visible light appears black. A green lime absorbs all colors except green and reflects green, so the lime looks green, as shown below. SUPPORTING MAIN IDEAS Describe the roles of reflection and absorption in color. Light of all colors strikes the lime. The lime absorbs The lime reflects all colors except green, so it green. appears green. The color that an object appears to the eye depends on another factor besides the wavelengths the object absorbs and reflects. An object can reflect only wavelengths that are in the light that shines on it. In white light, a white object reflects all the wavelengths of visible light and appears white. If you shine only red light on a white piece of paper, however, the paper will appear red, not white, because only red light is available to be reflected. In summary, two factors determine the color of an object: first, the wavelengths that the object itself reflects or absorbs, and second, the wavelengths present in the light that shines on the object. What color wavelengths does a red apple absorb? What color wavelengths does a white flower absorb? Chapter : Electromagnetic Waves 97

6 Page 6 of 7 Primary colors of light combine to make the secondary colors yellow, cyan (light blue), and magenta (dark pink). Primary Colors of Light The human eye can detect only three colors: red, green, and blue. Your brain perceives these three colors and various mixtures of them as all the colors of the rainbow. These three colors of light, which can be mixed to produce all possible colors, are called primary colors. When all three colors are mixed together equally, they appear white, or colorless. Whenever colored light is added to a mixture, specific wavelengths are added. Mixing colors by adding wavelengths is called additive color mixing. An example of the practical use of primary colors is a color television or computer monitor. The screen is divided into thousands of tiny bundles of red, green, and blue dots, or pixels. A television broadcast or DVD sends signals that tell the monitor which pixels to light up and when to do so. By causing only some pixels to give off light, the monitor can mix the three colors to create an amazing variety of colorful images. What does an equal mix of all three primary colors produce? Mixing Colors What is black ink made of? PROCEDURE 4 Trim each of the filter papers to a disk about 0 cm (4 in.) in diameter. Make two parallel cuts about cm (.5 in.) apart and 5 cm ( in.) long from the edge of each disk toward the center. Fold the paper to make a flap at a right angle. Use a different marker to make a dark spot in the middle of the flap on each disk. Fill each of the cups with water. Set one of the disks on top of each cup so that the water covers the end of the flap but does not reach the ink spot. After 5 minutes, examine each of the flaps. WHAT DO YOU THINK? What did you observe about the effects of water on the ink spots? How do the three different samples compare? SKILL FOCUS Observing MATERIALS coffee filters scissors brands of black felt-tip marker cups water TIME 0 minutes CHALLENGE Write a hypothesis to explain what you observed about the colors in a black marker. 98 Unit: Waves, Sound, and Light

7 Page 7 of 7 Primary Pigments Remember that two factors affect an object s color. One is the wavelengths present in the light that shines on the object. The other is the wavelengths that the object s material reflects or absorbs. Materials can be mixed to produce colors just as light can. Materials that are used to produce colors are called pigments. The primary pigments are cyan, yellow, and magenta. You can mix primary pigments just as you can mix primary colors to produce all the colors. The primary pigment colors are the same as the secondary colors of light. The secondary pigment colors are red, blue, and green the same as the primary colors of light. The effect of mixing pigments is different from the effect of mixing light. Remember that a colored material absorbs all wavelengths except those of the color it reflects. Yellow paint absorbs all colors except yellow. Because pigments absorb wavelengths, whenever you mix pigments, you are subtracting wavelengths rather than adding them. Mixing colors by subtracting wavelengths is called subtractive color mixing. When all three primary pigments are mixed together in equal amounts, all wavelengths are subtracted. The result is black the absence of color. The inks used to make the circles on this page are primary pigments. They combine to make the secondary pigments red, blue, and green. How is mixing pigments different from mixing light? KEY CONCEPTS. What are some ways in which materials affect how light is transmitted?. How does a polarizing filter reduce glare?. In order for an object to appear white, which wavelengths must the light contain and the object reflect? CRITICAL THINKING 4. Apply Imagine that you are a firefighter searching a smokefilled apartment. Would using a stronger light help you see better? Explain your answer. 5. Predict Higher-energy EM waves penetrate farthest into a dense medium. What colors are more likely to penetrate to the bottom of a lake? CHALLENGE 6. Synthesize If you focus a red light, a green light, and a blue light on the same part of a black curtain, what color will the curtain appear to be? Why? Chapter : Electromagnetic Waves 99