Light Waves Interact with Materials

KEY CONCEPT Light waves interact with materials.

BEFORE, you learned NOW, you will learn • Mechanical waves respond • How the wave behavior of to a change in medium light affects what we see •Visible light is made up of • How light waves interact EM waves with materials • EM waves interact with a new • Why objects have color medium in the same ways that • How different colors mechanical waves do are produced

VOCABULARY EXPLORE Light and Matter transmission p. 573 How can a change in medium affect light? absorption p. 573 scattering p. 575 PROCEDURE MATERIALS polarization p. 576 • clear plastic 1 Fill the container with water. prism p. 577 container with lid primary colors p. 578 2 Add 10 mL (2 tsp) of milk to the water. Put •water primary pigments p. 579 on the lid, and gently shake the container • measuring spoons until the milk and water are mixed. •milk • flashlight 3 In a dark room, shine the light at one side of the container from about 5 cm (2 in.) away. Observe what happens to the beam of light.

WHAT DO YOU THINK? • What happened to the beam of light from the flashlight? • Why did the light behave in this way?

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. VOCABULARY Transmission (trans-MIHSH-uhn) is the passage of an EM wave Don’t forget to make word through a medium. If the light reflected from objects did not pass frames for transmission and absorption. 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 limits the light available to be reflected or transmitted.

Chapter 17: Electromagnetic Waves 573 How Materials Transmit Light Materials can be classified according to the amount and type of light they transmit. 1 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. 2 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, sheer fabrics, and notepaper. 3 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.

check your reading What is the difference between translucent and opaque materials?

This stained-glass window contains transparent, translucent, and opaque materials.

1 translucent transparent 2

opaque 3

574 Unit 4: Waves, Sound, and Light 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 wavelengths. 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 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 SUPPORTING MAIN IDEAS day, when the Sun is high in the sky, molecules in Earth’s atmosphere Be sure to add to your chart the different scatter the blue part of visible light more than they scatter the other ways light interacts wavelengths. This process makes the sky light and blue. It is too bright with materials. to see the faint stars beyond Earth’s atmosphere. 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 scat- tered away and the light appears reddish. At night, because there is so little sunlight, the sky is dark and you can see the stars.

check your reading How does scattering make the sky blue?

Chapter 17: Electromagnetic Waves 575 Polarization Polarizing filters reduce glare and make it easier to see objects. Polarization (POH-luhr-ih-ZAY-shuhn) is a quality of light in which 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 describes 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. Light that passes through the filter is polarized. In the illustration below, these waves are shown in darker yellow.

Light reflecting off the surface of this pond causes glare.

1 unpolarized light waves The fields of visible light waves vibrate in all directions.

2 polarized light waves A polarizing filter lets through only waves vibrating vertically.

3 no light waves A second filter lets through only A polarizing filter reduces waves vibrating horizontally. 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 second filter is the same as the first, then all of the light will pass through the second filter. The light will still be polarized. If 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.

Wavelengths determine color. The section of the EM spectrum called visible light is made up of many different wavelengths. When all of these wavelengths are present together, as in light from the Sun or a light bulb, the light appears white.

576 Unit 4: Waves, Sound, and Light Seen individually, different 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 spread out 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 named color bands. In order of decreasing wavelength, the bands are red, Prisms split light into orange, yellow, green, blue, indigo, and violet. You see a color colors by refracting spectrum whenever you see a rainbow. 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 SUPPORTING MAIN IDEAS Describe the roles of black. A green lime absorbs most wavelengths but reflects green, so reflection and absorption the lime looks green, as shown below. in color.

1 In this simplified 2 The lime absorbs 3 The lime reflects diagram, light of all wavelengths mostly green, so it all colors strikes except green. appears green. the lime. 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.

check your reading What color band or bands does a red apple absorb? a white flower?

Chapter 17: Electromagnetic Waves 577 Primary Colors of Light The human eye can detect only three color bands: red, green, and blue. Your brain per- ceives these three colors and various mixtures of them as all the colors. 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 Primary colors of light when to do so. By causing only some pixels to give off light, the monitor combine to make the can mix the three colors to create an amazing variety of colorful images. secondary colors yellow, cyan (light blue), and magenta (dark pink). check your reading What does an equal mix of all three primary colors produce?

Mixing Colors SKILL FOCUS What is black ink made of? Observing PROCEDURE

1 Trim each of the filter papers to a disk about 10 cm (4 in.) in diameter. Make two parallel cuts about 1 cm (.5 in.) apart and 5 cm (2 in.) long from the edge of each disk toward the center. Fold the paper to make a flap at a right angle. MATERIALS 2 Use a different marker to make a dark spot in the middle of the flap on •3 coffee filters • scissors each disk. •3 brands of 3 Fill each of the cups with water. Set one of the disks on top of each cup so black felt-tip that the water covers the end of the flap but does not reach the ink spot. marker •3 cups 4 After 15 minutes, examine each of the flaps. •water TIME WHAT DO YOU THINK? 30 minutes •What did you observe about the effects of water on the ink spots? • How do the three different samples compare?

CHALLENGE Write a hypothesis to explain what you observed about the colors in a black marker.

578 Unit 4: Waves, Sound, and Light 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 The inks used to make the material absorbs all wavelengths circles on this page are except those of the color it reflects. primary pigments. Yellow paint absorbs all wavelengths They combine to make the secondary pigments except yellow. Because pigments red, blue, and green. 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 reflected light.

check your reading How is mixing pigments different from mixing light?

KEY CONCEPTS CRITICAL THINKING CHALLENGE 1. What are some ways in 4. Apply Imagine that you are a 6. Synthesize If you focus a red which materials affect how firefighter searching a smoke- light, a green light, and a blue light is transmitted? filled apartment. Would using light on the same part of a 2. How does a polarizing filter a stronger light help you see black curtain, what color will reduce glare? better? Explain your answer. the curtain appear to be? Why? 3. In order for an object to 5. Predict Higher-energy EM appear white, which wave- waves penetrate farthest into a lengths must the light contain dense medium. What colors and the object reflect? are more likely to penetrate to the bottom of a lake?

Chapter 17: Electromagnetic Waves 579