Chapter 17: Electromagnetic Waves 573

VOCABULARY

transmission p. 573absorption p. 573scattering p. 575polarization p. 576prism p. 577primary colors p. 578primary pigments p. 579

BEFORE, you learned

• Mechanical waves respond to a change in medium

• Visible light is made up of EM waves

• EM waves interact with a newmedium in the same ways thatmechanical waves do

NOW, you will learn

• How the wave behavior oflight affects what we see

• How light waves interact with materials

• Why objects have color• How different colors

are produced

KEY CONCEPT

Light waves interact with materials.

EXPLORE Light and Matter

How can a change in medium affect light?

PROCEDURE

Fill the container with water.

Add 10 mL (2 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 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?

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MATERIALS• clear plastic

container with lid• water• measuring spoons• milk• flashlight

Light can be reflected, transmitted, or absorbed.

You have read that EM waves can interact with a material medium inthe same ways that mechanical waves do. Three forms of interactionplay an especially important role in how people see light. One form isreflection. Most things are visible because they reflect light. The twoother forms of interaction are transmission and absorption.

(trans-MIHSH-uhn) is the passage of an EM wavethrough a medium. If the light reflected from objects did not passthrough the air, windows, or most of the eye, we could not see theobjects. (uhb-SAWRP-shun) is the disappearance of anEM wave into the medium. Absorption affects how things look, becauseit limits the light available to be reflected or transmitted.

Absorption

TransmissionVOCABULARYDon’t forget to make wordframes for transmissionand absorption.

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This stained-glass windowcontains transparent,translucent, and opaquematerials.

How Materials Transmit LightMaterials can be classified according to the amount and type of lightthey transmit.

Transparent (trans-PAIR-uhnt) materials allow most of the lightthat strikes them to pass through. It is possible to see objectsthrough a transparent material. Air, water, and clear glass are trans-parent. 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, butthey also cause it to spread out in all directions. You can see lightthrough translucent materials, but you cannot see objects clearlythrough them. Some examples are lampshades, frosted light bulbs,frosted windows, sheer fabrics, and notepaper.

Opaque (oh-PAYK) materials do not allow any light to passthrough 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 mainlybecause they absorb light.

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

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translucent

opaque

transparent1

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Chapter 17: Electromagnetic Waves 575

A light filter is a material that is transparent to some kinds of lightand opaque to others. For example, clear red glass transmits red light butabsorbs other wavelengths. Examples of light filters are the colored cov-ers on taillights and traffic lights, infrared lamp bulbs, and UV-protected sunglasses. Filters that transmit only certain colors are called color filters.

ScatteringSometimes fine particles in a material interact with light passingthrough the material to cause scattering. is the spreadingout of light rays in all directions, because particles reflect and absorbthe light. Fog or dust in the air, mud in water, and scratches or smudgeson glass can all cause scattering. Scattering creates glare and makes ithard to see through even a transparent material. Making the lightbrighter causes more scattering, as you might have noticed if you haveever tried to use a flashlight to see through fog.

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 atmospherescatter the blue part of visible light more than they scatter the otherwavelengths. This process makes the sky light and blue. It is too brightto see the faint stars beyond Earth’s atmosphere. At dawn and dusk,light from the Sun must travel farther through the atmosphere before itreaches 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 solittle sunlight, the sky is dark and you can see the stars.

check your reading How does scattering make the sky blue?

Scattering

SUPPORTING MAIN IDEASBe sure to add to yourchart the different ways light interacts with materials.

Fine particles, such asthose in fog, scatter light and reduce visibility.

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A polarizing filter reducesglare, making it possible tosee objects under the water.

Light reflecting off the surfaceof this pond causes glare.

PolarizationPolarizing filters reduce glare and make it easier to see objects.

(POH-luhr-ih-ZAY-shuhn) is a quality of light in whichall of its waves vibrate in the same direction. Remember that EM waves are made of electric and magnetic fields vibrating at rightangles to each other. Polarization describes the electric fields of a lightwave. When all of the electric fields of a group of light waves vibratein 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 electricfield like the bars of a cage. The filter allows through only waveswhose electric fields vibrate in one particular direction. Light thatpasses through the filter is polarized. In the illustration below, thesewaves are shown in darker yellow.

What do you think happens when polarized light passes into a second polarizing filter? If the direction of the second filter is thesame as the first, then all of the light will pass through the second fil-ter. The light will still be polarized. If the second filter is at a rightangle to the first, as in the illustration above, then no light at all willpass through the second filter.

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

Polarization

unpolarized light wavesThe fields of visible light wavesvibrate in all directions.

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polarized light wavesA polarizing filter lets throughonly waves vibrating vertically.

no light wavesA second filter lets through onlywaves vibrating horizontally.

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Chapter 17: Electromagnetic Waves 577

Seen individually, different wavelengths appear as different colors of light. This fact can be demonstrated by using aprism. A is a tool that uses refraction to spread out thedifferent wavelengths that make up white light. The prismbends 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 countlessindividual wavelengths, each with its own color. However, thecolor spectrum is usually divided into seven named colorbands. 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.

Color Reflection and Absorption The color of an object or material is determined by the wavelengths itabsorbs and those it reflects. An object has the color of the wavelengthsit reflects. A material that reflects all wavelengths of visible light appearswhite. A material that absorbs all wavelengths of visible light appearsblack. A green lime absorbs most wavelengths but reflects green, sothe lime looks green, as shown below.

The color that an object appears to the eye depends on anotherfactor besides the wavelengths the object absorbs and reflects. Anobject can reflect only wavelengths that are in the light that shines onit. In white light, a white object reflects all the wavelengths of visiblelight and appears white. If you shine only red light on a white piece ofpaper, however, the paper will appear red, not white, because only redlight is available to be reflected.

In summary, two factors determine the color of an object: first, thewavelengths that the object itself reflects or absorbs, and second, thewavelengths present in the light that shines on the object.

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

prism

The lime absorbsall wavelengthsexcept green.

2 The lime reflectsmostly green, so itappears green.

3In this simplifieddiagram, light ofall colors strikesthe lime.

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Prisms split light into colors by refracting wavelengths in differentamounts.

SUPPORTING MAIN IDEASDescribe the roles ofreflection and absorptionin color.

Primary Colors of LightThe human eye can detect only three colorbands: red, green, and blue. Your brain per-ceives these three colors and various mixturesof them as all the colors. These three colors oflight, which can be mixed to produce all possi-ble colors, are called When allthree colors are mixed together equally, theyappear white, or colorless. Whenever coloredlight is added to a mixture, specific wavelengthsare added. Mixing colors by adding wavelengthsis called additive color mixing.

An example of the practical use of primarycolors is a color television or computer monitor.The screen is divided into thousands of tinybundles of red, green, and blue dots, or pixels.A television broadcast or DVD sends signals thattell the monitor which pixels to light up and

when to do so. By causing only some pixels to give off light, the monitorcan mix the three colors to create an amazing variety of colorful images.

check your reading What does an equal mix of all three primary colors produce?

primary colors.

What is black ink made of?PROCEDURE

Trim each of the filter papers to a disk about 10 cm (4 in.) in diameter. Maketwo parallel cuts about 1 cm (.5 in.) apart and 5 cm (2 in.) long from the edgeof 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 sothat the water covers the end of the flap but does not reach the ink spot.

After 15 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?

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

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Mixing ColorsMixing ColorsSKILL FOCUSObserving

MATERIALS• 3 coffee filters• scissors • 3 brands of

black felt-tipmarker

• 3 cups• water

TIME30 minutes

Primary colors of lightcombine to make thesecondary colors yellow,cyan (light blue), andmagenta (dark pink).

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Primary PigmentsRemember that two factors affect an object’s color. One is the wave-lengths present in the light that shines on the object. The other is thewavelengths that the object’s material reflects or absorbs. Materialscan be mixed to produce colors just as light can. Materials that areused to produce colors are called pigments. The are cyan, yellow, and magenta. Youcan mix primary pigments just asyou can mix primary colors to produce all the colors.

The primary pigment colors arethe same as the secondary colors oflight. The secondary pigment colorsare red, blue, and green—the same as the primary colors of light.

The effect of mixing pigments is different from the effect of mixinglight. Remember that a coloredmaterial absorbs all wavelengthsexcept those of the color it reflects.Yellow paint absorbs all wavelengthsexcept yellow. Because pigmentsabsorb wavelengths, whenever youmix pigments, you are subtracting wavelengths rather than adding them. Mixing colors by subtractingwavelengths is called subtractive color mixing. When all three primarypigments are mixed together in equal amounts, all wavelengths aresubtracted. The result is black—the absence of reflected light.

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

primary pigments

Chapter 17: Electromagnetic Waves 579

KEY CONCEPTS1. What are some ways in

which materials affect howlight is transmitted?

2. How does a polarizing filterreduce glare?

3. In order for an object toappear white, which wave-lengths must the light containand the object reflect?

CRITICAL THINKING4. Apply Imagine that you are a

firefighter searching a smoke-filled apartment. Would usinga stronger light help you seebetter? Explain your answer.

5. Predict Higher-energy EMwaves penetrate farthest into adense medium. What colorsare more likely to penetrate tothe bottom of a lake?

CHALLENGE6. Synthesize If you focus a red

light, a green light, and a bluelight on the same part of ablack curtain, what color willthe curtain appear to be? Why?

The inks used to make thecircles on this page areprimary pigments. They combine to makethe secondary pigmentsred, blue, and green.