chapter 14: light - skyline high school physical...

Asunrise over the ocean is aspectacle of light and color. Asthe sun tops the horizon, sea

and sky explode into an array of reds,pinks, and oranges. Why does the skyappear red? How are its colorsreflected in the water? In this chapter,you will learn about light—how ittravels, bends, reflects, and enablesyou to see different colors. You willalso learn how different kinds ofbulbs produce light and how light isused by lasers, optical scanners, andoptical fibers.

What do you think?Science Journal Look at the picturebelow with a classmate. Discuss whatthis might be or what is happening.Here’s a hint: It’s full of air and doesn’tlast long. Write your answer or bestguess in your Science Journal.

Light1414

418

419

Light passing through a prism can produce excitingpatterns of color. Imagine what your surroundings

would look like now if humans could see only shadesof gray instead of distinct colors. The ability to seecolor depends on the cells in your eyes that are sensi-

tive to different wavelengths of light. What color is the light produced by aflashlight or the sun?

Make your own rainbow1. In a darkened room, shine a

flashlight through a glass prism.Project the resulting colors onto a white wall or ceiling.

2. In a darkened room, shine a flash-light over the surface of somewater with dishwashing liquidbubbles in it. What do you see?

3. Aim a flashlight at the surface of a compact disc.

ObserveHow did your observations in each case differ? In your Science Journal,explain where you think the colors came from.

EXPLOREACTIVITY

Making a Compare and Contrast Study Fold Make the follow-ing Foldable to compare and contrast the characteristics ofopaque, translucent, and transparent.

1. Place a sheet of paper in front of you so the shortside is at the top. Fold the paper in half from topto bottom.

2. Fold both sides in to divide the paper into equalthirds. Unfold the paper so three sections show.

3. Through one thickness of paper, cut along eachof the fold lines to the topfold, forming three tabs. Label each tab Opaque, Translucent,and Transparent as shown.

4. As you read the chapter, write characteristics of these materials under the tabs, using thewords absorb, reflect, and transmit.

FOLDABLESReading & StudySkills

FOLDABLESReading & Study Skills

420 CHAPTER 14 Light

The Behavior of LightS E C T I O N

� Describe the differences amongopaque, transparent, and translu-cent materials.

� Explain how light is reflected.� Discuss how refraction separates

white light.

Vocabularyopaque index of refractiontranslucent miragetransparent

Knowing how light behaves will helpyou understand various sights, suchas reflections in a store window,rainbows, and mirages.

Figure 1These candleholders have different light-transmittingproperties.

Opaque Translucent Transparent

Light and Matter Look around your room after turning off the lights at night.

At first you can’t see anything, but as your eyes adjust to thedarkness, you begin to recognize some familiar objects. Youknow that some of the objects are brightly colored, but theylook gray or black in the dim light. Turn on the light, and youclearly can see all the objects in the room, including their colors.What you see depends on the amount of light in the room andthe color of the objects. To see an object, it must reflect somelight back to your eyes.

Opaque, Transparent, and Translucent Objects canabsorb light, reflect light, and 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(oh PAYK) material in the candleholder in Figure 1A, onlyabsorbs and reflects light—no light passes through it. As aresult, you cannot see the candle inside.

Other materials allow some light to pass through, but you cannot see clearly through them. These are translucent(trans LEW sunt) materials, like the candleholder in Figure 1B.

Transparent materials like the candleholder in Figure 1Ctransmit almost all of the light that strikes them, so you can seeobjects clearly through them. Only a small amount of light isabsorbed and reflected.

Reflection of LightJust before you left for

school this morning, did youtake one last glance in a mirrorto check your appearance? Tosee your reflection in the mirror,light had to reflect off you, hitthe mirror, and reflect off themirror into your eye. Reflectionoccurs when a light wave strikesan object and bounces off.

The Law of ReflectionBecause light behaves as a wave,it obeys the law of reflection, asshown in Figure 2. According to the law of reflection, the angleat which a light wave strikes a surface is the same as the angle atwhich it is reflected. Light reflected from any surface—a mirroror a sheet of paper—follows this law.

Regular and Diffuse Reflection Why can you see yourreflection in a store window but not in a brick wall? The answerhas to do with the smoothness of the surfaces. A smooth, evensurface like that of a pane of glass produces a sharp image byreflecting parallel light waves in only one direction. Reflection oflight waves from a smooth surface is regular reflection. A brickwall has an uneven surface that causes incoming parallel lightwaves to be reflected in many directions, as shown in Figure 3.Reflection of light from a rough surface is diffuse reflection.

What is diffuse reflection?

Mirror

i = angle of incidence

i

r

i

r

r = angle of reflection

Normal

Normal

Figure 2Light is reflected according to thelaw of reflection so that theangle of incidence always equalsthe angle of reflection.

Figure 3This brick wall has anuneven surface, so itproduces a diffusereflection.

SECTION 1 The Behavior of Light 421

Roughness of Surfaces Even a surface that appears to besmooth can be rough enough to cause diffuse reflection. Forexample, a metal pot might seem smooth, but at high magnifi-cation, the surface shows rough spots, as shown Figure 4. Tocause a regular reflection, the roughness of the surface must beless than the wavelengths it reflects.

Refraction of Light What occurs when a light wave passes from one material to

another—from air to water, for example? Refraction is causedby a change in the speed of a wave when it passes from onematerial to another. If the light wave is traveling at an angle andthe speed that light travels is different in the two materials, thewave will be bent, or refracted.

How does refraction occur?

The Index of Refraction The amount of bending that takesplace depends on the speed of light in both materials. The greaterthe difference is, the more the light will be bent as it passes at anangle from one material to the other. Figure 5 shows an exampleof refraction. Every material has an index of refraction—a prop-erty of the material that indicates how much it reduces the speedof light.

The larger the index of refraction, the more light is sloweddown in the material. For example, because glass has a largerindex of refraction than air, light moves more slowly in glassthan air. Many useful devices like eyeglasses, binoculars, cameras,and microscopes form images using refraction.


Figure 5The spoon looks bent in thewater because the light wavesare refracted as they changespeed when they pass from thewater to the air.

Figure 4Although the surface of this potmay seem smooth, it produces adiffuse reflection. At high mag-nification, the surface is seen tobe rough.

Water droplet

SunlightIncident ray

Prisms A sparkling glass prismhangs in a sunny window, refractingthe sunlight and projecting a colorfulpattern onto the walls of the room.How does the bending of light createthese colors? It occurs because theamount of bending usually dependson the wavelength of the light. Wave-lengths of visible light range fromthe longer red waves to the shorterviolet waves. White light, such assunlight, is made up of this wholerange of wavelengths.

Figure 6 shows what occurs when white light passes througha prism. The triangular prism refracts the light twice—oncewhen it enters the prism and again when it leaves the prism andreenters the air. Because the longer wavelengths of light arerefracted less than the shorter wavelengths are, red light is bentthe least. As a result of these different amounts of bending, thedifferent colors are separated when they emerge from the prism.Which color of light would you expect to bend the most?

Rainbows Does the light leaving the prism in Figure 6remind you of a rainbow? Like prisms, rain droplets also refractlight. The refraction of the different wavelengths can causewhite light from the Sun to separate into the individual colors ofvisible light, as shown in Figure 7. In a rainbow, the human eyeusually can distinguish only about seven colors clearly. In orderof decreasing wavelength, these colors are red, orange, yellow,green, blue, indigo, and violet.

Figure 6Refraction causes a prism toseparate a beam of white lightinto different colors.

Figure 7As white light passes through the waterdroplet, different wavelengths are refractedby different amounts.This produces theseparate colors seen in a rainbow.

Observing Refractionin Water Procedure 1. Place a penny at the bottom

of a short, opaque cup. Setit on a table in front of you.

2. Have a partner slowly slidethe cup away from you untilyou can’t see the penny.

3. Without disturbing thepenny or the cup and with-out moving your position,have your partner slowlypour water into the cupuntil you can see the penny.

4. Reverse roles and repeat theexperiment.

Analysis 1. What did you observe?

Explain how this is possible.2. In your Science Journal,

sketch the light path fromthe penny to your eye afterthe water was added.

SECTION 1 The Behavior of Light 423

Mirages When you’re riding in a car on a hot day, you mightsee something that looks like a shimmering pool of water on theroad ahead. As you get closer, the water seems to disappear.What you saw was a mirage. A mirage is an image of a distantobject produced by the refraction of light through air layers ofdifferent densities. The density of air increases as the air getscooler. The greater the difference in densities is, the more thelight is refracted. Mirages result when the air at ground level ismuch warmer or much cooler than the layers of air above it, asFigure 8 shows. The image you see is always some distance awayfrom the actual object. For example, the water you think you seeon a hot road surface sometimes is an image of the sky.


Section Assessment

1. Contrast opaque, transparent, andtranslucent materials. Give at least oneexample of each.

2. Explain why you can see your reflectionin a smooth piece of aluminum foil butnot in a crumpled ball of foil.

3. Why are you more likely to see a mirageon a hot day than on a mild day?

4. What happens to white light when itpasses through a prism?

5. Think Critically Consider the followingparts of your body: the lens of your eye,a fingernail, your skin, and a tooth.Decide whether each of these is opaque,transparent, or translucent. Explain.

6. Making and Using Tables Construct a table that shows the light-reflecting or light-absorbing properties of different materials.Use the words opaque, transparent, and translucent. For more help, refer to the Science Skill Handbook.

7. Communicating Walk around your classroomnoting five reflecting objects. Which objects display diffuse reflection and which display regular reflection? How does the surface differ on each? For each object you note,list the colors the object is absorbing. For more help, refer to the Science Skill Handbook.

MirageWarm air

Cool air

Figure 8Mirages result when air near theground is much warmer or coolerthan the air above. This causessome lightwaves reflected fromthe object to refract, creatingone or more additional images.

SECTION 2 Light and Color 425

ColorsWhy do some apples appear red, while others look green or

yellow? An object’s color depends on the wavelength of light itreflects. You know that white light is a blend of all colors of visi-ble 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 9 shows white light striking a green leaf. Only the greenlight is reflected to your eyes.

Although some objects appear to be black, black isn’t a colorthat is present in visible light. Objects that appear black absorball colors of light and reflect little or no light back to your eye.White objects appear to be white because they reflect all colorsof visible light.

Why does a white object appear white?

Colored Filters Wearing tinted glasses changes the color ofalmost everything you look at. If the lenses are yellow, the worldtakes on a golden glow. If they are rose colored, everything looksrosy. Something similar would occur if you placed a colored,clear plastic sheet over this white page. The paper would appearto be the same color as the plastic. The plastic sheet and thetinted lenses are filters. A filter is a transparent material thattransmits one or more colors of light but absorbs all others. Thecolor of a filter is the color of the light that it transmits.

� Explain how you see color.� Describe the difference between

light color and pigment color.� Predict what happens when

different colors are mixed.

Vocabularypigment

From traffic lights to great works ofart, color plays an important role inyour world.

Light and ColorS E C T I O N

Figure 9This green leaf absorbs allwavelengths of visible lightexcept green.

Looking Through Colored Filters Figure 10 shows whathappens when you look at a colored object through various col-ored filters. In the white light in Figure 10A, a blue cooler looksblue because it reflects only the blue light in the white lightstriking it. It absorbs the light of all other colors. If you look atthe cooler through a blue filter as in Figure 10B, the cooler stilllooks blue because the filter transmits the reflected blue light.Figure 10C shows how the cooler looks when you examine itthrough a red filter. Why does it appear to be black?

Seeing ColorAs 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, traf-fic safety depends on your ability to detect immediate colorchanges. How do you see colors?

Light and the Eye In a healthy eye, light enters and isfocused on the retina, an area on the inside of your eyeball, asshown in Figure 11A. The retina is made up of two types of cellsthat absorb light, as shown in Figure 11B. When these cellsabsorb light energy, chemical reactions convert light energy intonerve impulses that are transmitted to the brain. One type ofcell in the retina, called a cone, allows you to distinguish colorsand detailed shapes of objects. Cones are most effective in day-time vision. Why?


Figure 10The color of this cooler seems to change under different lighting conditions.

The cooler appears blackthrough a red filter.

The cooler appears blue whenviewed through a blue filter.

The blue cooler is shown inwhite light.

Cones and Rods Your eyes have three types of cones, eachof which responds to a different range of wavelengths. Redcones respond to mostly red and yellow, green cones respond tomostly yellow and green, and blue cones respond to mostly blueand violet. The second type of cell, called a rod, is sensitive todim light and is useful for night vision.

Interpreting Color Why does abanana look yellow? The lightreflected by the banana causes the

cone cells that are sensitive to red and green light to send signalsto your brain. Your brain would get the same signal if a mixtureof red light and green light reached your eye. Again, your redand green cones would respond, and you would see yellow lightbecause your brain can’t perceive the difference between incom-ing yellow light and yellow light produced by combining redand green light. The next time you are at a play or a concert,look at the lighting above the stage. Watch how the coloredlights combine to produce effects onstage.

Color Blindness If one or more of your sets of conesdid not function properly, you would not be able to distin-guish between certain colors. About eight percent of menand one-half percent of women have a form of colorblindness. Most people who are said to be color blind arenot truly blind to color, but they have difficulty distin-guishing between a few colors, most commonly red andgreen. Figure 12 shows a plate of a color blindness test.Because these two colors are used in traffic signals, driversand pedestrians must be able to identify them.


Retina

Lens

Figure 11Light enters the eye andfocuses on the retina.

Figure 12Color blindness is an inheritedsex-linked condition in which certain cones do not functionproperly. What number do you see

in the dots?

The two types of nerve cells that make up theretina are called rods and cones.The rods are the thinner of the two.

The retina is at the back of your eye.

Mixing Colors If you have ever browsed through a paint store, you have

probably seen displays where customers can select paint samplesof almost every imaginable color. The colors are a result of mix-tures of pigments. For example, you might have mixed blue andyellow paint to produce green paint. A pigment is a coloredmaterial that absorbs some colors and reflects other colors. Thecolor of a pigment results from the different wavelengths oflight that the pigment reflects.

Mixing Colored Lights From the glowing orange of a sun-set to the deep blue of a mountain lake, all the colors you seecan be made by mixing of three colors of light. These three col-ors—red, green, and blue—are the primary colors of light. Theycorrespond to the three different types of cones in the retina ofyour eye. When mixed together in equal amounts, they producewhite light, as Figure 13 shows. Mixing the primary colors indifferent proportions can produce the colors you see.

What are primary colors?

Paint Pigments If you were to mix equal amounts of red,green, and blue paint, would you get white paint? If mixing col-ors of paint were like mixing colors of light, you would, butmixing paint is different. Paints are made with pigments. Paintpigments usually are made of chemical compounds such as tita-nium oxide, a bright white pigment, and lead chromate, whichis used for painting yellow lines on highways.


Figure 13White light is produced whenthe three primary colors oflight are mixed.

Plant pigments allowplants to select the wave-lengths of light they use forphotosynthesis. Leavesusually look green due tothe pigment chlorophyll.Chlorophyll absorbs mostwavelengths of visible lightexcept green, which itreflects. But not all plantsare green. Research differ-ent plant pigments to findhow they allow plantspecies to survive indiverse habitats.

Mixing Pigments You can make any pigment colorby mixing different amounts of the three primary pig-ments—magenta (bluish red), cyan (greenish blue), andyellow. In fact, color printers use those pigments to makefull-color prints like the pages in this book. However,color printers also use black ink to produce a true blackcolor. A primary pigment’s color depends on the color oflight it reflects. Actually, pigments both absorb andreflect a range of colors in sending a single color messageto your eye. For example, in white light, the yellow pig-ment appears yellow because it reflects yellow, red,orange, and green light but absorbs blue and violet light.The color of a mixture of two primary pigments is deter-mined by the primary colors of light that both pigments reflect.

What colors are the three primary pigments?

Look at Figure 14. The area in the center where the colors alloverlap appears to be black because the three blended primarypigments absorb all the primary colors of light. Recall that theprimary colors of light combine to produce white light. They arecalled additive colors. However, the primary pigment colorscombine to produce black. Because black results from theabsence of reflected light, the primary pigments are called sub-tractive colors.


Figure 14The three primary colors of pigment appear to be blackwhen they are mixed.

Section Assessment

1. If a white light shines on a red shirt, whatcolors are reflected and what colors areabsorbed?

2. How do the primary colors of light differfrom the primary pigment colors?

3. Explain why a person with color blindnesscan distinguish among some colors but notothers.

4. If all colors are present in white light, whydoes a white fence appear to be whiteinstead of multicolored?

5. Think Critically If you had only magenta,cyan, and yellow paints, could you paint apicture of a zebra? Explain why this wouldor would not be possible.

6. Concept Mapping Design a concept map toshow the chain of events that must happen foryou to see a blue object. Work with a partner.For more help, refer to the Science Skill Handbook.

7. Researching Information Research the electromagnetic spectrum to find the wave-length and frequency range of visible light.Make a poster showing the wavelengths for the seven main colors in the visible light spectrum. Explain the units used, and the relationship between wavelength and wave frequency. For more help, refer to the Science Skill Handbook.

Gas Bulb

Phosphorescent coating

Electrode


Producing LightS E C T I O N

Incandescent Lights It’s only been 100 years or so since lightbulbs became com-

mon in households. Their shapes, sizes, and varieties today are afar cry from a single lightbulb swinging from an electric cordover a table. Most of the lightbulbs in your house produceincandescent light. Heating a piece of metal until it glows pro-duces incandescent light. If you look into an unlit incandescentlightbulb, you will see a thin wire called the filament. It usuallyis made of the element tungsten. Turn on the light and electric-ity flows through the filament and causes it to heat up. Whenthe tungsten filament gets hot, it gives off light. However, ifyou’ve ever accidentally touched a lit lightbulb, you know that italso produces a great deal of heat. More than 80 percent of theenergy given off by incandescent bulbs is in the form of heat.

Why does an incandescent lightbulb get hot?

Fluorescent Lights Your house also may have fluorescent (floo RE sunt) lights.

A fluorescent bulb, like the one shown in Figure 15, is filled witha gas at low pressure. The inside of the bulb is coated with phos-phors that emit visible light when they absorb ultraviolet radia-tion. The tube also contains electrodes at each end. Electrons aregiven off when the electrodes are connected in a circuit. Whenthese electrons collide with the gas atoms, ultraviolet radiationis emitted. The phosphors on the inside of the bulb absorb thisradiation and give off visible light.

� Explain how incandescent and fluorescent lightbulbs work.

� Analyze the advantages and disadvantages of different lightingdevices.

� Explain how a laser producescoherent light.

� Describe various uses of lasers.

Vocabularyincandescent lightfluorescent lightcoherent lightincoherent light

Knowing how different lightingdevices work will help you choosethe right one for your needs.

Figure 15Fluorescent lightbulbs donot use filaments. What

property of phosphors makes

them useful in fluorescent

bulbs?

Efficient Lighting A fluorescent light uses phosphors toconvert ultraviolet radiation to visible light. Fluorescent lightsuse as little as one fifth the electrical energy to produce the sameamount of light as incandescent bulbs. Fluorescent bulbs alsolast much longer than incandescent bulbs. This higher efficiencycan mean lower energy costs over the life of the bulb. Reducedenergy usage could reduce the amount of fossil fuels burned togenerate electricity, which also decreases the amount of carbondioxide and pollutants released into Earth’s atmosphere.

Fluorescent bulbs are used widely in hospitals, office build-ings, schools, and factories. Compact fluorescent bulbs, whichcan be screwed into traditional lightbulb sockets, have beendeveloped that are more practical for use in homes.

Neon Lights The vivid, glowing colors of neon lights, such as those

shown in Figure 16, make them a popular choice for signs andeye-catching decorations on buildings. These lighting devicesare glass tubes filled with gas, typically neon, and work similarlyto fluorescent lights. When an electric current flows through thetube, electrons collide with the gas molecules. In this case, how-ever, the collisions produce visible light. If the tube containsonly neon, the light is bright red. Different colors can be pro-duced by adding other gases to the tube.

What causes the color in a neon light?

Discovering EnergyWaste in LightbulbsProcedure 1. Obtain an incandescent

bulb and a fluorescentbulb of identical wattage.

2. Make a heat collector bycovering the top of a foamcup with a piece of plasticfood wrap to make a win-dow. Carefully make a smallhole (diameter less than thethermometer’s) in the sideof the cup. Push a ther-mometer through the hole.

3. Measure the temperature ofthe air inside the cup. Then,hold the window of thetester 1 cm from one of thelights for 2 minutes andmeasure the temperature.

4. Cool the heat collector andthermometer. Repeat step 3 using the second bulb.

Analysis 1. What was the temperature

for each bulb? 2. Which bulb appears to give

off more heat? Explain whythis occurs.

Figure 16This neon light has vivid,glowing colors.

SECTION 3 Producing Light 431

Ne

Ne

Ne

Ne

Electron

Light

Research Visit the Glencoe Science Web site atscience.glencoe.com formore information aboutsodium vapor lights and lightpollution.

Sodium-Vapor Lights Sodium-vapor lights often are used for

streetlights and other outdoor lighting.Inside a sodium-vapor lamp is a tube thatcontains a mixture of neon gas, a smallamount of argon gas, and a small amountof sodium metal. When the lamp is turnedon, the gas mixture becomes hot. The hotgases cause the sodium metal to turn tovapor, and the hot sodium vapor emits ayellow-orange glow, as shown in Figure 17.

Tungsten-Halogen Lights Tungsten-halogen lights sometimes are used to create intensely

bright light. These lights have a tungsten filament inside a quartzbulb or tube. The tube is filled with a gas that contains one of thehalogen elements such as fluorine or chlorine. The presence of thisgas enables the filament to become much hotter than the filamentin an ordinary incandescent bulb. As a result, the light is muchbrighter and also lasts longer. Tungsten-halogen lights sometimesare used on movie sets and in underwater photography.

LasersFrom laser surgery to a laser light show, lasers have become a

large part of the world you live in. A laser’s light begins when anumber of light waves are emitted at the same time. To achievethis, a number of identical atoms each must be given the sameamount of energy. When they release their energy, each atomsends off an identical light wave. This light wave is reflectedbetween two facing mirrors at opposite ends of the laser. One ofthe mirrors is coated only partially with reflective material, so itreflects most light but allows some to get through. Some emittedlight waves travel back and forth between the mirrors manytimes, stimulating other atoms to emit identical light waves also.Figure 18 shows how this process produces a beam of laser light.

How do mirrors help in creating lasers?

Lasers can be made with many different materials, includinggases, liquids, and solids. One of the most common is the helium-neon laser, which produces a beam of red light. A mixture ofhelium and neon gases sealed in a tube with mirrors at both endsis excited by a flashtube. The excited atoms then lose their excessenergy by emitting coherent light waves.


Figure 17Sodium-vapor lights emit mostlyyellow light. Half of this photowas taken under sunlight andhalf was taken under sodium-vapor lighting.

http://www.science.glencoe.com

Figure 18

Lasers produce light waves that have the same wave-length. Almost all of these waves travel in the samedirection and are in phase. As a result, beams of

laser light can be made more intense than ordinary light.In modern eye surgery, shown at the right, lasers are oftenused instead of a traditional scalpel.

VISUALIZING LASERS

When the lamp flashes, energy is absorbed by the atoms in the rod. These atoms then re-emitthat energy as light waves that are in phase andhave the same wavelength.

B

Most of these waves are reflectedbetween the mirrors located at eachend of the laser. One of the mirrors,however, is only partially reflective,allowing one percent of the lightwaves to pass through it and form a beam.

C

As the waves travel back and forth between the mirrors,they stimulate other atoms in the ruby rod to emit lightwaves. In a fraction of a second, billions of identical wavesare bouncing between the mirrors. The waves are emittedfrom the partially reflective mirror in a stream of laser light.

D

AtomsLight Waves

Light-producingmaterial

Mirror Mirror

Energy source

The key parts of a laser include a mate-rial that can be stimulated to produce light,such as a ruby rod, and an energy source.In this example, the energy source is a lampthat spirals around the ruby rod and emitsan intense light.

A


Coherent Light Lasers produce the narrowbeams of light that zip across the stage andthrough the auditorium during some rock con-certs. Beams of laser light do not spread outbecause laser light is coherent. Coherent light islight of only one wavelength that travels with itscrests and troughs aligned. The beam does notspread out because all the waves travel in the samedirection, as shown in Figure 19A. As a result, theenergy carried by the beam remains concentratedover a small area.

Incoherent Light Light from an ordinary light-bulb is incoherent. Incoherent light can containmore than one wavelength, and its electromag-netic waves are not aligned, as in Figure 19B. Thewaves don’t travel in the same direction, so thebeam spreads out. The energy carried by the lightwaves is spread over a large area, so the intensity ofthe light is much less than that of the laser beam.

Using LasersCompact disc players, surgical tools, and many other useful

devices take advantage of the unique properties of lasers. A laserbeam is narrow and does not spread out as it travels over longdistances. So lasers can apply large amounts of energy to smallareas. In industry, powerful lasers are used for cutting and weld-ing materials. Surveyors and builders use lasers for measuringand leveling. To measure the moon’s orbit with great accuracy,scientists use laser light reflected from mirrors placed on theMoon’s surface. Information also can be coded in pulses of lightfrom lasers. This makes them useful for communications. In tele-phone systems, pulses of laser light transmit conversationsthrough long glass fibers called optical fibers.

Lasers in Medicine Lasers are routinely used to removecataracts, reshape the cornea, and repair the retina. In the eyeand other parts of the body, surgeons can use lasers in place ofscalpels to cut through body tissues. The energy from the laserseals off blood vessels in the incision and reduces bleeding.Because most lasers do not penetrate deeply through the skin,they can be used to remove small tumors or birthmarks on thesurface without damaging deeper tissues. By sending laser lightinto the body through an optical fiber, physicians can also treatconditions such as blocked arteries.


Figure 19Light waves can be eithercoherent or incoherent.

These waves are coherent because they havethe same wavelength and travel with their crestsand troughs aligned.

Incoherent waves such as these can containmore than one wavelength, and do not travelwith their crests and troughs aligned.

A particular helium-neonlaser contains a mixture of15 percent He and 85 per-cent Ne. Where are thesegases located on the peri-odic table? Analyze theirchemical characteristics.Would you be concernedthat a chemical reactionmight occur in the laser?Explain.

Compact Discs Lasers play important roles in producingand using compact discs, which are plastic discs with reflectivesurfaces used to store sound, images, and text in digital form.When a CD is produced, the information is burned into thesurface of the disc with a laser. The laser creates millions of tinypits in a spiral pattern that starts at the center of the disc andmoves out to the edge. A CD player, shown in Figure 20, alsouses a laser to read the disc. The laser’s beam is aimed at thesurface of the disc. As the laser beam strikes a pit or flat spot,different amounts of light are reflected to a light sensor. Thereflected light is converted to an electric signal used by thespeakers to create sound.


Section Assessment

1. Explain how light is produced in an ordinary incandescent bulb.

2. What are the advantages of using afluorescent bulb instead of an incandescentbulb?

3. Describe the difference between coherentand incoherent light.

4. Think Critically Which type of lightingdevice would you use for each of the fol-lowing needs: an economical light sourcein a manufacturing plant, an eye-catchingsign that will be visible at night, and abaseball stadium? Explain.

5. Concept Mapping Make a concept mapshowing the sequence of events that occur in alaser to produce coherent light. Begin with theemission of lightwaves from atoms. For morehelp, refer to the Science Skill Handbook.

6. Using an Electronic Spreadsheet Create a spreadsheet listing the types of lightingdevices described in this chapter. Compare and contrast their features in separate columns.Include a column for how each is used. For more help, refer to the Technology Skill Handbook.

Spinning CD

Laser

Lightsensor

Figure 20The blowup shows the pits (blue)on the bottom surface of a CD. ACD player uses a laser to convertthe information on the CD to anelectric signal.


Using LightS E C T I O N

Polarized Light You may have a pair of sunglasses with a sticker on them

that says polarized. Do you know what makes them differentfrom other sunglasses? The difference has to do with the vibra-tion of light waves that pass through the lenses. You can maketransverse waves in a rope vibrate in any direction—horizontal,vertical, or anywhere in between. Light also is a transverse waveand can vibrate in any direction. In polarized light, however, thewaves vibrate in only one direction.

Polarizing Filters If the light passes through a special polar-izing filter, the light becomes polarized. A polarizing filter actslike a group of parallel slits. Only light waves vibrating in thesame direction as the slits can pass through. If a second polariz-ing filter is lined up with its slits at right angles to those of thefirst filter, no light can pass through, as Figure 21 shows.

Polarized lenses are useful for reducing glare without inter-fering with your ability to see clearly. When light is reflectedfrom a horizontal surface, such as a lake or a shiny car hood, itbecomes partially horizontally polarized. The lenses of polariz-ing sunglasses have vertically polarizing filters that block out thereflected light that has been polarized horizontally, while allow-ing vertically polarized light through.

� Describe polarized light and theuses of polarizing filters.

� Apply the concept of total internalreflection to the uses of opticalfibers.

Vocabularypolarized lightholographytotal internal reflection

Light is used in entertainment,medicine, manufacturing, scientificresearch, communications, and justabout every other facet of life.

Figure 21Slats in a fence behave like apolarizing filter for a transversewave on a rope. If the slats inthe fence are in the same direc-tion, the wave passes through.

If the slats are aligned at rightangles to each other, the wavecan’t pass through. Wave motion transmitted

Wave motion blocked

HolographyScience museums often have exhibits where a three-dimen-

sional image seems to float in space, like the one shown in Figure 22. You can see the image from different angles, just asyou would if you passed the real object. Three-dimensionalimages on credit cards are produced by holography. Holography(hoh LAH gruh fee) is a technique that produces a hologram—acomplete photographic image of a three-dimensional object.

Making Holograms Illuminating objects with laser lightproduces holograms. Laser light reflects from the object ontophotographic film. At the same time, a second beam split fromthe laser also is directed at the film. The light from the twobeams creates an interference pattern on the film. The patternlooks nothing like the original object, but when laser light shineson the pattern on the film, a holographic image is produced.

Information in Light An ordinary photographic image cap-tures only the brightness or intensity of light reflected from anobject’s surface, but a hologram records the intensity as well asthe direction. As a result, it conveys more information to youreye than a conventional two-dimensional photograph does, butit also is more difficult to copy. Holographic images are used oncredit cards, identification cards, and on the labels of someproducts to help prevent counterfeiting. Using X-ray lasers,scientists can produce holographic images of microscopicobjects. It may be possible to create three-dimensional views ofbiological cells.

How are holographic images produced?

SECTION 4 Using Light 437

Research Visit the Glencoe Science Web site atscience.glencoe.com formore information about holograms. Communicate toyour class what you learned.

Figure 22Lasers can be used to make holograms like this one.


Air

Normal Normal

Total internalreflection

Critical angleWater


Figure 23A light wave is bent away fromthe normal as it passes fromwater to air. At the critical angle,the refracted wave is travelingalong the water surface. Atangles greater than the criticalangle, total internal reflectionoccurs.

Optical Fibers When laser light must travel long distances or be sent into

hard-to-reach places, optical fibers often are used. These trans-parent glass fibers can transmit light from one place to another.A process called total internal reflection makes this possible.

Total Internal Reflection Remember what happens whenlight speeds up as it travels from one medium to another. Forexample, when light travels from water to air the direction ofthe light ray is bent away from the normal, as shown in Figure23. If the underwater light ray makes a larger angle with thenormal, the light ray in the air bends closer the surface of thewater. At a certain angle, called the critical angle, the refractedray has been bent so that it is traveling along the surface of thewater, as shown in Figure 23. For a light ray traveling fromwater into air, the critical angle is about 49º.

Figure 23 shows what happens if the underwater light raystrikes the boundary between the air and water at an anglelarger than the critical angle. There is no longer any refraction,and the light ray does not travel in the air. Instead, the light rayis reflected at the boundary, just as if a mirror were there. Thisbehavior of light is called total internal reflection. Total internalreflection occurs when light traveling from one medium toanother is completely reflected at the boundary between the twomaterials. Then the light ray obeys the law of reflection. Fortotal internal reflection to occur, light must travel slower in thefirst medium, and must strike the boundary at an angle greaterthan the critical angle.

How does total internal reflection occur?

Light

Plastic covering

Glass core

Glass layer

Light rays

SECTION 4 Using Light 439

An optical fiberreflects light so that it ispiped through the fiberwithout leaving it, exceptat the ends.

Figure 24Optical fibers make use of totalinternal reflection.

Light Pipes Total internal reflection makes light transmis-sion in optical fibers possible. As shown in Figure 24A, lightentering one end of the fiber is reflected continuously from thesides of the fiber until it emerges from the other end. Like watermoves through a pipe, almost no light is lost or absorbed inoptical fibers.

Using Optical Fibers Optical fibers are most often used incommunications. Telephone conversations, television programs,and computer data can be coded in light beams. Signals can’tleak from one fiber to another and interfere with other mes-sages, so the signal is transmitted clearly. To send telephone con-versations through an optical fiber, sound is converted intodigital signals consisting of pulses of light by a light-emittingdiode or a laser. Some systems use multiple lasers, each with itsown wavelength to fit multiple signals into the same fiber. Youcould send a million copies of the play Romeo and Juliet in onesecond on a single fiber. Figure 24B shows the size of typicaloptical fibers.

Optical fibers also are used to explore the inside of thehuman body. One bundle of fibers transmits light, whileanother carries the reflected light back to the doctor.

Just one of these optical fiberscan carry thousands of phone conversations at the same time.

Optical ScannersIn supermarkets and many other kinds of

stores, a cashier passes your purchases over a glasswindow in the checkout counter or holds a handheld device up to each item, like the one inFigure 25. In an instant, the optical scanner beepsand the price of the item appears on a screen. Anoptical scanner is a device that reads intensities ofreflected light and converts the information todigital signals. You may have noticed that some-where on each item the cashier scans is a patternof thick and thin stripes called a bar code. Anoptical scanner detects the pattern and translatesit into a digital signal, which goes to a computer.

The computer searches its database for a matching item, finds itsprice, and sends the information to the cash register. The U.S.Postal Service also uses optical scanners to sort mail and keeptrack of mail delivery.

You may have used another type of optical scanner to con-vert pictures or text into forms you can use in computer pro-grams. With a flatbed scanner, for example, you lay a documentor picture facedown on a sheet of glass and close the cover. Anoptical scanner passes underneath the glass and reads the pat-tern of light and dark areas (or colors, if you are scanning acolor picture). Some scanners are used with software that canread text on a page and convert the scanned information into atext file that you can work on. Photocopy machines and facsim-ile (fax) machines also use optical scanners.


Section Assessment

1. What is polarized light?

2. Why is a holographic image three-dimen-sional?

3. What conditions are necessary for totalinternal reflection to occur?

4. Explain how an optical fiber is able totransmit light.

5. Think Critically Geologists and surveyorsoften use lasers for aligning equipment,measuring, and mapping. Explain why.

6. Comparing and Contrasting Make a tablethat compares and contrasts incoherent, coher-ent, and polarized light. For more help, refer tothe Science Skill Handbook.

7. Communicating Many people wear polarizedsunglasses while they are working. Write a list ofjobs or occupations in which wearing polarizedsunglasses is helpful. Explain why. For morehelp, refer to the Science Skill Handbook.

Figure 25Optical scanners like this one uselasers to find the price of variousproducts.

ACTIVITY 441

Make a poster of your diagrams and use it toexplain reflection and refraction of lightwaves to your class. For more help, refer toyour Science Skill Handbook.

Make a Light Bender

4. Looking down through the surface of thewater from the side opposite the pencil,observe the reflection and refraction of the image of the pencil.

5. Draw a diagram of the image and label“reflection” and “refraction.”

6. Repeat steps 4 and 5 two more times butposition the pencil at two different angles.

Conclude and Apply 1. How would the image you see change or be

different if the surface of the water were amirror?

2. Predict how the angles of reflection orrefraction would change if the surface of thecontainer were curved. Explain.

From a hilltop you can see the reflection ofpine trees and a cabin in the calm surface of

a lake. This is possible because some of the lightthat reflects off these objects strikes the water’ssurface and reflects into your eyes. However, youdon’t see a clear, colorful image because much ofthe light enters the water rather than beingreflected.

What You’ll Investigate How does water affect the viewer’s image of anobject that is above the water’s surface?

Materialslight sourceunsharpened pencilclear rectangular containerwaterclay

Goals� Identify reflection of an image in water.� Identify refraction of an image in water.

Safety Precautions

Procedure 1. Fill the container with water.

2. Place the container so that a light source—window or overhead light—reaches it.

3. Stand the pencil on end in the clay and place it by the container as shown in the figure above. The pencil must be taller thanthe level of the water. Also, place the pencilon the same side of the container as the light source.

Goals� Demonstrate when light does and

does not shine through a pair ofpolarizing filters.

� Predict what will happen when youadd a third polarizing filter.

Possible Materialspolarizing filters (3)lamp or flashlight

Safety PrecautionsNever look directly at the sun, evenwith a polarizing filter.

Polarizing filters cause light waves to vibrate only in one direction. Wearing polarized sunglasses can help reduce glare while allowing you to see clearly.

If you have two polarizing filters on top of one another, when will light shine throughand when will it not? What might happen if you added a third filter in between thefirst two?

Recognize the ProblemHow can you demonstrate the effects of polarizing filters?

Form a HypothesisForm a hypothesis about how two polarizing filters that are placed on top of oneanother must be oriented for light to shine through and for no light to shine through.

Polarizing Filters


Test your Hypothesis

Analyze Your Data

Draw Conclusions

5. Using an appropriate light source,test when light does and does notshine through a pair of polarizing filters. Test each of the orientationsyou planned in step 1. Record the results.

6. Test three orientations of the thirdfilter for allowing the maximumamount of light to pass through.Record the results.

7. Repeat step 6 but allow no light toshine through. Record the results.

Do1. Using a pair of polarizing filters,

choose at least three orientations ofthe filters to test your hypothesis.

2. When the two filters are oriented toallow the maximum amount oflight to shine through, predict howa third filter placed between thetwo must be oriented for the sameresults.

3. Repeat step 2 but allow no light toshine through.

4. Make sure that your teacherapproves your plan before you start.

2. In each case, describe what hap-pened when you added a third filterbetween the two. How did thethree orientations of the third filterchange the amount of light thatpassed through? Explain.

1. Describe how the pair of polarizingfilters were oriented when light didand did not shine through. In caseswhere light did shine through, wasit always the same amount of light?Or did the amount of light changein different orientations?

1. Explain why light did or did not shine throughtwo polarizing filters in the various orientations.

2. Was your hypothesis supported? Why or why not?

3. Explain why light did or did not shine throughvarious orientations of three polarizing filters.

4. Were your predictions correct? Why or why not?

5. If a polarizing filter reduces the brightness oflight reflected from the surface of a lake, whatcan you conclude about the polarization ofreflected light?

ACTIVITY 443

The next time you see a family member orfriend wearing sunglasses, explain to themhow polarizing lenses can reduce problemsof glare.

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Science & Language ArtsandandandScience Language Arts

Respond to the ReadingRespond to the Reading

1. What do the wordsone-color world

mean?

2. How do the illustra-tions help the readerbetter understand the poems?

3. What do you think is meant by the word lingering in the Haiku about springsunlight?

A Haiku Garden:The Four Seasons in Poems and Prints

by Stephen Addiss with Fumiko and Akira Yamamoto

Withered by winter

the sound of the wind—

one-color worldBasho

444 SCIENCE AND LANGUAGE ARTS

Lingering

in every pool of water—

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Writing and IllustratingHaiku Write two haikupoems—one about summer andone about fall. In one poem, usecolor to help you describe theseason. In the other, use light orsome property of light to helpdescribe the season. When youhave finished, exchange poemswith one of your classmates.Read your classmate’s haiku andcreate illustrations to accompanythem.

andandandLinking Science Linking Science

Maria Martinez-Cañas uses light in an inventive and exciting way.Martinez-Cañas was born in Havana, Cuba and grew up in PuertoRico. She went to art school in the United States, earning a master offine arts degree from the School of Art Institute in Chicago. Her inno-vative technique involves using a type of photographic material thatblocks light. She cuts the material into artistic shapes, then printsthese shapes on white paper in a manner similar to using stencils.Martinez-Cañas lives in Miami and her photography can be seen inart exhibits all over the world.

To learn more about careers in photography,visit the Glencoe Science Web site at science.glencoe.com.

Photographer �

CareerCareer ConnectionConnection

SCIENCE AND LANGUAGE ARTS 445

Understanding LiteratureJapanese Haiku You have just read English trans-lations of two poems from a book that combines Jap-anese haiku and art. A haiku is a verse that consists ofthree lines and 17 syllables in the Japanese language.The first and third lines have five syllables each, andthe middle line has seven syllables. Using few words, ahaiku allows the reader’s imagination to complete thepicture. For instance, the sound of the wind mightmake you think of the sound of winter wind whippingaround the house while you sit snuggly inside.

Science Connection Research has determined thatthere is a connection between color and mood. Forexample, the color of a room can affect a person’s feel-ings and behavior. Warm colors have longer wave-lengths, and can be more stimulating. Cool colors,which have shorter wavelengths, tend to have a calm-ing or soothing effect on people. Light and color havelong been used as literary symbols. When the haiku iscombined with Japanese prints, do you read it differ-ently? Does the use of color change what you imaginewhen you read the haiku?

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446 CHAPTER STUDY GUIDE

2. Fluorescent bulbs give off light when ultra-violet radiation produced inside the bulbcauses the phosphor coating inside the bulbto glow.

3. Neon lights contain a gas that glows whenelectric current passes through it.

4. A laser produces coherent light by emittinga beam of light waves that have only onewavelength, with their crests and troughsaligned, and moving in a single direction.

Section 4 Using Light1. Polarized light con-

sists of transversewaves that vibratealong only oneplane. What could bedone to reduce theglare in this photo?

2. Total internal reflec-tion occurs when alight wave strikesthe boundarybetween two materi-als at an anglegreater than thecritical angle.

3. Optical scanners sense reflected light and convert the information to digital signals.

Section 1 The Behavior of Light1. You can’t see through opaque materials. You

can see hazily through translucent materialsand clearly through transparent materials.

2. Light behaves as a wave, so it can bereflected. Is this reflection from the lake regu-lar or diffuse?

3. Light waves can be refracted, or bent, whena wave changes speed as it travels from onematerial to another.

Section 2 Light and Color1. You see color when light is reflected off

objects and into your eyes.

2. Specialized cells in your eyes called conesallow you to distinguish colors and shapesof objects. Other cells, called rods, allow youto see in dim light.

3. The three primary colors of light can bemixed to form all other colors.

4. The colors of pigments are determined bythe colors they reflect.

Section 3 Producing Light1. Incandescent bulbs produce light by heating

a tungsten filament until it glows brightly.

Study GuideChapter 1414

List examples of commonmaterials that are opaque,transparent, and translucent

on the front of your Foldable.

After You ReadFOLDABLESReading & StudySkills

FOLDABLESReading & Study Skills

CHAPTER STUDY GUIDE 447

Vocabulary Wordsa. coherent light i. pigmentb. fluorescent light j. polarized lightc. holography k. total internal d. incandescent light reflectione. incoherent light l. translucentf. index of refraction m. transparentg. mirageh. opaque

Using VocabularyAnswer the following questions using

complete sentences.

1. What type of light does heating a filamentuntil it glows produce?

2. What process would you use to produce acomplete three-dimensional image of anobject?

3. How would you describe an object that youcan see through?

4. What process makes it possible for opticalfibers to transmit telephone conversationsover long distances?

5. What is a false image of a distant object?

Study GuideChapter 1414

Get together with a friend. Quiz each other fromtextbook and class material.

Study Tip

Complete the following concept map about light.

Light

is reflectedaccording to

is bentaccording to

seen by eyes with

all shades aremixtures of

by heatedmetals

by glowinggases

used in

by coherentlight waves

used in

AssessmentChapter 1414

Choose the word or phrase that best answersthe question.

1. Which word describes materials that absorbor reflect all light?A) translucent C) ultravioletB) opaque D) diffuse

2. What is the term for the property of a material that indicates how much lightslows down when traveling in the material? A) pigment C) index of refractionB) filter D) mirage

3. Which of the following explains why aprism separates white light into the colorsof the rainbow?A) interference C) diffractionB) fluorescence D) refraction

4. What do you see when noting the color ofan object?A) the light it reflectsB) the light it absorbsC) polarizationD) diffuse reflection

5. What do the phosphors inside fluorescentbulbs absorb to create a glow?A) incandescent lightB) ultraviolet radiationC) halogensD) argon

6. What term describes objects that allowsome light, but not a clear image to passthrough?A) translucent C) transparentB) reflective D) opaque

7. Which light waves are bent most whenpassing through a prism?A) red waves C) blue wavesB) yellow waves D) violet waves

8. Which type of cells in your eyes allows youto see the color violet?A) red cones C) blue conesB) green cones D) rods

9. What color of light is produced when thethree primary colors of light are combinedin equal amounts?A) black C) whiteB) yellow D) cyan

10. Which term describes laser light?A) incoherent C) incandescentB) coherent D) fluorescent

11. Explain howlight is pro-duced by anincandescentbulb. What isa disadvan-tage of thesebulbs?

12. How is thereflection oflight from awhite wall similar to the reflection of lightfrom a mirror? How is it different?

13. What color would a blue shirt appear to beif a blue filter were placed in front of it? A red filter? A green filter?

14. Which color of light changes speed themost when it passes through a prism?Explain.

15. Drawing Conclusions Most mammals, suchas dogs and cats, can’t see colors. Infer howa cat’s eye might be different from your eye.

448 CHAPTER ASSESSMENT

CHAPTER ASSESSMENT 449

16. Concept Mapping Use this blank conceptmap to show the steps in the production offluorescent light.

17. Interpreting Scientific Illustrations Make adrawing that shows how a fluorescent bulbproduces light.

18. Drawing Conclusions Why is the inside of acamera painted black?

19. Making and Using Tables Construct a tableto show the properties and applications ofincandescent, fluorescent, neon, sodium-vapor, and tungsten-halogen lightingdevices. For each type of device, includeinformation on how light is produced, typi-cal uses, and its advantages or disadvantages.

20. Poster Make a poster to show how thethree primary pigments are combined toproduce common colors such as blue, red,yellow, green, purple, brown, and black.

Judy and Markus are studying light.They have just read about an experimentthat shows how light has certain propertiesand follows certain rules.

Study the pictures above and answer the following questions.

1. Angle i measures 20°. Using this information, what is angle x?A) 50°B) 60°C) 70°D) 80°

2. Which of the following is taking place in both of the experimentsabove?F) refractionG) reflectionH) diffusionJ) fluorescence

3. Which of the following statements is true about reflection?A) ∠ i � ∠ x � 100°B) ∠ i � ∠ x � 60°C) ∠ i � ∠ x � 80°D) ∠ i � ∠ x � 90°

Test Practice

i

x y

i

AssessmentChapter 1414

Initiating step

Go to the Glencoe Science Web site at science.glencoe.com or use the Glencoe Science CD-ROM for additionalchapter assessment.

TECHNOLOGY

Final outcome


chapter 14: light - skyline high school physical...

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