23
570 Chapter 19 19.1 Mirrors Reading Strategy Comparing and Contrasting After reading this section, compare mirror types by copying and completing the table. Key Concepts What is the law of reflection? What type of image is produced by each of the three types of mirrors? Vocabulary ray diagram angle of incidence angle of reflection plane mirror virtual image concave mirror focal point real image convex mirror It is a bright, sunny day and you are enjoying a peaceful afternoon by a lake. The air is still and the surface of the lake looks just like a sheet of glass. In fact, it is so smooth that you can see your reflection in it. The Law of Reflection Optics includes the study of how mirrors and lenses form images. In your study of optics, assume that light is made up of rays that travel in straight lines. A ray diagram shows how rays change direction when they strike mirrors and pass through lenses. Figure 1 shows a simple ray diagram of the law of reflection. The incoming ray, called the incident ray, approaches the mirror. The angle of incidence is the angle the incident ray makes with a line drawn perpendicular to the surface of the mirror. The mirror reflects the incident ray. The angle of reflection is the angle the reflected ray makes with the perpendicular line. The law of reflection states that the angle of reflection is equal to the angle of incidence. Image (virtual, real, or both) Plane Concave Convex a. ? c. ? Mirror Shape of Surface Flat Virtual b. ? d. ? Angle of incidence Incident ray Reflected ray Angle of reflection Figure 1 The flat (plane) mirror and the mirror-like lake surface both obey the law of reflection. According to the law of reflection, the angle of an incident ray equals the angle of the reflected ray. 570 Chapter 19 FOCUS Objectives 19.1.1 Describe the law of reflection. 19.1.2 Describe how a plane mirror produces an image. 19.1.3 Define real and virtual images and relate them to converging and diverging light rays. 19.1.4 Describe the physical characteristics of plane, concave, and convex mirrors and distinguish between the types of images they form. Build Vocabulary Vocabulary Knowledge Rating Chart Have students construct a chart with four columns: Term, Can Define or Use It, Have Heard or Seen It, Don’t Know. Students should copy the vocabulary terms for this section under column 1. They should then place a checkmark under one of the other columns for each term. Reading Strategy a. Inside curved surface b. Both c. Outside curved surface d. Virtual INSTRUCT The Law of Reflection Build Science Skills Observing Purpose Students compare object and image distance in a plane mirror. Materials graph paper, metric ruler, pencil, plane mirror Class Time 15 minutes Procedure Use lumps of clay to mount a mirror vertically on a sheet of graph paper. Hold a pencil in front of the mirror and observe its image. Place the pencil point down on one of the squares. Look into the mirror to observe the image of the pencil point. Count the number of squares from the object to the mirror and from the image to the mirror. Expected Outcome The image distance behind the mirror appears to be the same as the distance from the mirror to the object. Visual, Kinesthetic L2 2 L2 L2 Reading Focus 1 Section 19.1 Print Reading and Study Workbook With Math Support, Section 19.1 Transparencies, Chapter Pretest and Section 19.1 Technology Interactive Textbook, Section 19.1 Presentation Pro CD-ROM, Section 19.1 Go Online, NSTA SciLinks, Mirrors Section Resources

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Page 1: Section 19.1 19.1 Mirrors - Mr. Baker's Physical …mrbakerphysical2.weebly.com/uploads/3/0/8/0/30809369/ch...mirror. When you look into a plane mirror, you see your reversed reflection—a

570 Chapter 19

19.1 Mirrors

Reading StrategyComparing and Contrasting After readingthis section, compare mirror types by copyingand completing the table.

Key ConceptsWhat is the lawof reflection?

What type of image isproduced by each of thethree types of mirrors?

Vocabulary◆ ray diagram◆ angle of incidence◆ angle of reflection◆ plane mirror◆ virtual image◆ concave mirror◆ focal point◆ real image◆ convex mirror

It is a bright, sunny day and you are enjoying a peaceful afternoon bya lake. The air is still and the surface of the lake looks just like a sheetof glass. In fact, it is so smooth that you can see your reflection in it.

The Law of Reflection Optics includes the study of how mirrors and lenses form images. Inyour study of optics, assume that light is made up of rays that travel instraight lines. A ray diagram shows how rays change direction whenthey strike mirrors and pass through lenses.

Figure 1 shows a simple ray diagram of the law of reflection. Theincoming ray, called the incident ray, approaches the mirror. Theangle of incidence is the angle the incident ray makes with a linedrawn perpendicular to the surface of the mirror. The mirror reflectsthe incident ray. The angle of reflection is the angle the reflected raymakes with the perpendicular line. The law of reflection statesthat the angle of reflection is equal to the angle of incidence.

Image (virtual,real, or both)

Plane

Concave

Convex

a. ?

c. ?

Mirror Shape ofSurface

Flat Virtual

b. ?

d. ?

Angle of incidence

Incident ray

Reflected ray

Angle of reflection

Figure 1 The flat (plane) mirrorand the mirror-like lake surfaceboth obey the law of reflection.According to the law of reflection,the angle of an incident ray equalsthe angle of the reflected ray.

570 Chapter 19

FOCUS

Objectives19.1.1 Describe the law of reflection.19.1.2 Describe how a plane mirror

produces an image.19.1.3 Define real and virtual images

and relate them to convergingand diverging light rays.

19.1.4 Describe the physicalcharacteristics of plane,concave, and convex mirrorsand distinguish between the types of images they form.

Build VocabularyVocabulary Knowledge Rating ChartHave students construct a chart withfour columns: Term, Can Define or UseIt, Have Heard or Seen It, Don’t Know.Students should copy the vocabularyterms for this section under column 1.They should then place a checkmarkunder one of the other columns for each term.

Reading Strategya. Inside curved surface b. Both c. Outside curved surface d. Virtual

INSTRUCT

The Law of ReflectionBuild Science SkillsObserving

Purpose Students compare object and image distance in a plane mirror.

Materials graph paper, metric ruler,pencil, plane mirror

Class Time 15 minutes

Procedure Use lumps of clay to mounta mirror vertically on a sheet of graphpaper. Hold a pencil in front of the mirrorand observe its image. Place the pencilpoint down on one of the squares. Lookinto the mirror to observe the image ofthe pencil point. Count the number ofsquares from the object to the mirror and from the image to the mirror.

Expected Outcome The imagedistance behind the mirror appears tobe the same as the distance from themirror to the object. Visual, Kinesthetic

L2

2

L2

L2

Reading Focus

1

Section 19.1

Print• Reading and Study Workbook With

Math Support, Section 19.1• Transparencies, Chapter Pretest and

Section 19.1

Technology• Interactive Textbook, Section 19.1• Presentation Pro CD-ROM, Section 19.1• Go Online, NSTA SciLinks, Mirrors

Section Resources

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Optics 571

Next time you are in a car behind a large truck, look for a signthat reads,“If you can’t see my mirror, then I can’t see you.” Light trav-els from you to the mirror to the driver’s eyes and also from the driverto the mirror to your eyes. If you do not have a line of vision to thetruck’s side-view mirror, then the truck driver does not have a line ofvision to you. It can be dangerous to drive too close to large trucks!

Plane MirrorsMirrors are usually made of a sheet of glass that is coated with athin layer of shiny metal on one surface. A mirror with a flat surfaceis a plane mirror. The large mirror in your bathroom is a planemirror. When you look into a plane mirror, you see your reversedreflection—a right-left reversed image of yourself. An image is acopy of an object formed by rays of light.

Figure 2 shows how a plane mirror forms an image. To produceyour image in a mirror, rays of light strike you and reflect. Thesereflected rays then strike the mirror and are reflected into your eyes.The dashed lines show how your brain interprets where the rays arecoming from. The rays appear to come from behind the mirror. Yourimage appears the same distance behind the mirror as you are infront, and the image is right side up. If you walk toward the mirror,you’ll see your image also move toward the mirror. A plane mirroralways produces a virtual image. Although you can see a virtualimage, this type of image cannot be projected onto any surface. A vir-tual image is a copy of an object formed at the location from whichthe light rays appear to come. It is important, however, to realize thatthe rays do not really come from behind the mirror.

Figure 2 The girl sees a virtualimage of herself in the planemirror. Virtual images such as thiscannot be projected onto ascreen. Note also how light raysfrom the object (the girl) reflectfrom the mirror’s surface andobey the law of reflection.Interpreting Photos What dothe dashed lines represent?

Measuring the Height of Your Mirror Image

Materialsplane mirror, meter stick, newspaper, masking tape

Procedure1. Have a classmate measure and record your

height in centimeters.

2. Stand directly in front of the mirror, 1 m awayfrom it.

3. Look at your image in the mirror. Have aclassmate tape newspaper over those parts ofthe mirror (above and below your image)where you do not see any part of yourself.

4. Measure and record the height of your imageby measuring the uncovered part of the mirror.

5. Repeat Steps 3 and 4 for distances of 2 m,3 m, and 4 m.

Analyze and Conclude1. Analyzing Data How did the height of your

image change as you moved farther awayfrom the mirror?

2. Drawing Conclusions What is therelationship between your height and theheight of your image in the plane mirror?

Plane Mirrors

Measuring the Height of Your Mirror Image

Objective After completing this lab, students willbe able to • describe how the size of a mirror limits

the size of the area that it reflects.

Completing this lab can help studentsovercome the misconception that it ispossible to see an image of one’s fullheight in a mirror of any size, if themirror is sufficiently far away.

Skills Focus Drawing Conclusions

Prep Time 10 minutes

Advance Prep Full length mirrors canbe framed with black construction paperto provide mirror surfaces of differentheights. One mirror should be longerthan one-half the average height of thestudents. Another should be shorter thanmost of the students. The mirrors shouldbe mounted on the wall and perpen-dicular to the floor. You can mark offdistances of 1 to 4 m from each mirroron the floor with masking tape.

Class Time 20 minutes

Safety Framed mirrors should be used.If the mirrors aren’t framed, cover theedges with duct tape to reduce the risk of breakage and injury. Studentsshould use caution and wear safetygoggles when working with breakableglass mirrors.

Expected Outcome A mirror must be at least half the height of a student to produce a full-length image of the student.

Analyze and Conclude 1. The image became smaller. 2. The image is never more than half the student’s height. Logical

L2

Optics 571

Customize for English Language Learners

Cause/Effect ChartUsing a cause/effect chart can ensure thatstudents understand the concepts, as well as theword meanings, in this section. Have studentswork independently or as a group and make atwo-column chart. The left column should belabeled Cause and the right column Effect.Next, have students read the paragraphs about

plane mirrors on this page. Encourage them to write on their charts different cause/effectrelationships about plane mirrors described in the paragraphs. For example, one cause islight reflecting off an object, and the effect is the light striking the mirror. Another cause is walking toward the mirror, and the effect isseeing the image also move toward the mirror.

Answer to . . .

Figure 2 The dashed lines show thepoint of origin of the virtual image that is formed behind the mirror.

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Concave and Convex MirrorsSometimes you see images that are very distorted. Look into both sidesof a polished metal spoon. The images you see are quite different fromthe image formed by a plane mirror. Each side of the spoon producesa different image because each side is curved differently. The curvedsurface of the spoon changes the way light is reflected.

Concave Mirrors When the inside surface of a curved mirror isthe reflecting surface, the mirror is a concave mirror. Figure 3A showshow a concave mirror reflects light rays that are parallel to the opticalaxis. The curvature of the reflecting surface causes the rays to cometogether. The point at which the light rays meet is called the focal point.

Look again at your reflection in the bowl of a spoon. The upside-down image you see is a real image. A real image is a copy of an objectformed at the point where light rays actually meet. Unlike a virtualimage, a real image can be viewed on a surface such as a screen.

Concave mirrors can form either real or virtual images. Thetype of image formed depends upon where the object is in relation tothe mirror. Figure 3B shows how a concave mirror forms a real image.When the object is farther from the mirror than the focal point, thereflected rays meet in front of the mirror. Figure 3C shows how a con-cave mirror forms a virtual image. When the object is closer to themirror than the focal point is the reflected rays spread out and appearto come from behind the mirror.

Concave mirrors are often used in automobile headlights andflashlights to direct the illumination from a single light bulb into abeam. If the bulb is placed at the focal point of a concave mirror, thereflected light rays will be parallel to one another. This results in abrighter beam of light.

At what location does a real image form?

572 Chapter 19

Focalpoint

ObjectVirtualimage

Concave mirror

Focalpoint

Object

Realimage

Opticalaxis

Concave mirror

Focalpoint

Concave mirror

A B C

For: Links on mirrors

Visit: www.SciLinks.org

Web Code: ccn-2191

Figure 3 Concave mirrors canform either real or virtual images.A When parallel incoming raysstrike a concave mirror, they arereflected through the focal point.B Concave mirrors form realimages when the reflected lightrays converge. C Concave mirrorsform virtual images when thereflected rays appear to comefrom a point behind the mirror.Interpreting DiagramsWhat determines the typeof the image formed by aconcave mirror?

572 Chapter 19

Concave and Convex Mirrors

Students may incorrectly think thatconcave mirrors form only real images.Challenge this misconception by havingstudents look carefully at Figure 3C. Point out that when the object is veryclose to the mirror, the curvature of themirror has less effect. The object’s imageis similar to the virtual image producedby a plane mirror. Also point out that the reflected rays in Figure 3C don’tconverge. The rays must converge toform a real image. Finally, explain that areal image produced by a mirror is alwaysinverted. Have students hold a convexmirror very close to their eyes (closer thanfocal point) and then farther away. Visual

Flashlight Mirrors

Purpose Students observe how a small flashlight can focus a beam.

Materials flashlight with a beam that can be focused

Procedure Unscrew the head of theflashlight to show students the smallbulb. Draw a diagram on the boardshowing the placement of the bulb at the focal point of the parabolic lens.Replace the head and show how thebeam changes size as the head movesback and forth. Have students draw ray diagrams showing the pattern of reflection at different positions.

Expected Outcome The concavemirror in the flashlight focuses the lightinto a narrow bright beam when the bulbis placed at the focal point of the concavemirror. When the bulb is in front of orbehind the focal point, the rays diverge. Visual, Logical

L2

L2

Section 19.1 (continued)

Concave Mirror Images There are fivegeneral types of images formed by a concavemirror. Consider the distance from the focalpoint to the mirror as f. The image types are:(1) If the object is between the focal point andthe mirror, the image is virtual, upright, andenlarged. (2) If the object is located exactly at the focal point, no image is produced.

(3) If the object is located between a distance ƒand a distance 2ƒ from the mirror, the image isreal, inverted, and enlarged. (4) If the object islocated at exactly a distance 2ƒ from the mirror,the image is real, inverted, and the same size as the object. (5) If the object is located at adistance greater than 2ƒ, the image is real,inverted, and smaller than the object.

Facts and Figures

Download a worksheet on mirrorsfor students to complete, and find additional teacher supportfrom NSTA SciLinks.

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Section 19.1 Assessment

Reviewing Concepts1. How is the angle of incidence of a light

ray related to the angle of reflection?

2. What type of image does a plane mirror form?

3. What types of image can be produced bya concave mirror? A convex mirror?

4. How are real images different from virtual images?

5. Why can convex mirrors form only one type of image?

Critical Thinking6. Applying Concepts Explain why a plane

mirror cannot form a real image.

7. Inferring If you place an object 10 cm froma particular concave mirror, a virtual imageforms behind the mirror. What can you inferabout the focal point of the mirror?

8. Applying Concepts If you look inside thebowl of a shiny metal spoon, your image isupside down. If you look at the outside of thebowl, your image is right side up. Explain.

Convex Mirrors When the outside surface of a curved mirror isthe reflecting surface, the mirror is a convex mirror. Figure 4A showshow a convex mirror reflects parallel light rays. Note how the curvatureof the convex mirror causes the reflected rays to spread out.

Convex mirrors always cause light rays to spread out and canonly form virtual images. Figure 4B shows how a convex mirror formsa virtual image. As the rays from the object reflect from the mirror, therays spread out. Because they appear to be coming from a point behindthe mirror, that is where the image appears. The image formed by aconvex mirror is always upright and smaller than the object. Thisallows the mirror to show a wide angle of view. Because of their wideangle of view, round, convex mirrors are often used in store aisles, athazardous traffic intersections, and for side mirrors on automobiles.

Optics 573

Compare-Contrast Paragraph Write aparagraph comparing convex mirrors andconcave mirrors. (Hint: Use the informationfrom your completed Reading Strategy onpage 570.)

Object

Focalpoint

Convex mirror

Virtualimage

Opticalaxis

Focalpoint

Convex mirror

BAFigure 4 Convex mirrors canonly form virtual images. A When parallel incoming raysstrike a convex mirror, they arereflected away from oneanother. B Convex mirrorsalways form virtual images thatare upright and smaller thanthe object. Because of theirreduced image size, convexmirrors on automobiles warnthat “Objects are closer thanthey appear.”

For: Activity on mirrors

Visit: PHSchool.com

Web Code: ccp-2191

Use VisualsFigure 4 Have students look carefully atthe incident and reflected rays in Figures4A and 4B. Ask, How can you apply thelaw of reflection to a convex mirror?(Draw a tangent to the mirror at the pointwhere the incident ray strikes and throughthis point draw a normal line, perpendicularto the tangent. The angles from the normalline to the incident and reflected rays areequal.) Visual

Build Reading LiteracyUse Prior Knowledge Refer to page2D in Chapter 1, which provides theguidelines for using prior knowledge.

Ask students to think about times theyhave looked in a store’s convex mirror.Have them explain the advantages and disadvantages a convex mirror hasover a plane mirror. (The convex mirrorallows store workers to monitor a greaterarea of the store. The disadvantage is that the images are smaller and moredifficult to see.) Verbal

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 19.1

ASSESSEvaluate UnderstandingHave students model the law ofreflection by rolling a ball at an angletoward a wall. Encourage them toexplain the angle of incidence and theangle of reflection of the ball’s path.

ReteachHave students make a chart summarizinghow different types of images can bemade with each mirror. They can usetheir charts to review the section.

Student paragraphs should includeseveral similarities and differences.Similarities include each mirror having a curved reflecting surface and the ability to form images.Differences include shape of thereflecting surface and the ability to make light rays converge or diverge.

L1

L2

3

L1

L1

Optics 573

5. Convex lenses always cause light rays to diverge, therefore, they can form onlyvirtual images. 6. For a real image to form, light rays mustconverge. Because a plane mirror cannotcause light rays to converge, it cannot form a real image.7. The focal point of the mirror must befarther than 10 cm from the mirror.8. Light rays from the convex side divergeforming an upright image. Light rays from the concave side converge forming aninverted image.

Section 19.1 Assessment

1. The law of reflection states that the angle ofreflection is equal to the angle of incidence.2. A plane mirror always produces a virtual image.3. Concave mirror: virtual and real; convexmirror: virtual4. Real images can be projected onto a screenand form at the point where light rays actuallyconverge, whereas virtual images cannot beprojected and are formed at the point fromwhich the light rays appear to be coming.

Answer to . . .

Figure 3 The location of the object

At the point where thelight rays meet

PPLS

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19.2 Lenses

Reading StrategyBuilding Vocabulary Copy the table below.As you read the section, define in your ownwords each vocabulary word listed in the table.

Key ConceptsWhat causes light to refract?

What type of images doconcave and convexlenses form?

In what types of materialsis total internal reflectionlikely to occur?

Vocabulary◆ index of refraction ◆ lens◆ concave lens◆ convex lens◆ critical angle◆ total internal

reflection

You may wear eyeglasses or contact lenses and you have probablyused a hand lens like the one shown in Figure 5. If so, you have seenhow the bending, or refracting, of light can change the way you seesomething. The enlarged image seen through the lens in Figure 5 isdue to refraction. The lens material changes the path of the light rayspassing through it. The amount the light rays change direction deter-mines the appearance of the image you see.

Index of Refraction of LightLight usually travels in straight lines. In a vacuum, light travels at aspeed of 3.00 � 108 meters per second. Once light passes from avacuum into any other medium, it slows down. The speed of lightin the new medium depends on the material of the new medium.

Some media, such as air, allow light to pass through almost as fastas it would through a vacuum. In fact, air slows the speed of light onlyby about three ten-thousandths of one percent (0.0003%). Othermedia cause light to slow down much more. For instance, the speedof light in water and in glass slows to 2.25 � 108 meters per second and2.00 � 108 meters per second respectively.

Index of refraction

Critical angle ofrefraction

Total internalreflection

Vocabulary Term Definition

b. ?

a. ?

c. ?

Figure 5 Light rays slow and bend asthey pass through the curved glass lens. Inthis case, the result is a magnified image.

574 Chapter 19

574 Chapter 19

FOCUS

Objectives19.2.1 Explain what causes light

to refract.19.2.2 Define index of refraction.19.2.3 Describe the physical

characteristics of concave and convex lenses anddistinguish between the types of images they form.

19.2.4 Describe total internal reflectionand explain its relationship tothe critical angle.

Build VocabularyConcept Maps Creating conceptmaps can help students understand the convex and concave lenses studiedin this section. Have students includethe following topics: lens shape, imagesformed, applications, diverging rays,and converging rays. Advanced studentscould incorporate sketches showinghow the images are formed.

Reading Strategya. Ratio of the speed of light in a vacuumto the speed of light in the material.b. Angle of incidence that produces anangle of refraction of 90 degrees.c. The complete reflection of a light rayback into its original medium.

INSTRUCT

Index of Refraction of LightBuild Reading LiteracyOutline Refer to page 156D inChapter 6, which provides theguidelines for using an outline.

Have students create an outline of thesection (pp. 574–578). Ask, Based onyour outline, what is an index ofrefraction, and what is total internalreflection? (An index of refraction is ameasure of how much light changes speedas it enters a new medium. Total internalreflection is the complete reflection of alight ray back into its original medium. Theangle depends on the index of refraction.) Verbal, Logical

L1

2

L2

L2

Reading Focus

1

Section 19.2

Print• Laboratory Manual, Investigation 19A• Reading and Study Workbook With

Math Support, Section 19.2 and Math Skill: Calculating Index of Refraction

• Math Skills and Problem SolvingWorkbook, Section 19.2

• Transparencies, Section 19.2

Technology• Interactive Textbook, Section 19.2• Presentation Pro CD-ROM, Section 19.2

Section Resources

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Properties of Gemstones

Gemstones used in jewelry are known for severalof their physical properties—primarily luster andoptical brilliance. Luster is a measure of theamount of light that strikes a gemstone’s surfaceand is reflected. Flat and smooth surfaces increasea gemstone’s luster.

Like luster, the brilliance of a gemstone involvesreflected light. Light that is not reflected by agem’s lustrous surface passes into the stone. Thebrilliance of a gemstone is a measure of the amountof light entering the gem that is reflected back tothe viewer. Precise techniques are used to cutgemstones into shapes that produce maximumbrilliance. The combination of a specialized shapeand the gemstone’s inherent high index ofrefraction gives gems their brilliance.

The table summarizes the index of refractionand luster of several common gemstones. Notethat moissanite is a manufactured material usedto simulate diamond.

1. Interpreting Tables Which material is themost lustrous? The least lustrous?

2. Calculating What percentage of lightstriking a sapphire gemstone enters it?

3. Applying Concepts If a light ray strikes each material at an angle, in which materialwould the light ray bend the most?

4. Applying Concepts The speed of lightthrough an unknown gemstone is 1.69 � 108 m/s. Identify the gemstone.

Optics 575

Air

Water

Glass

Air

Material

Diamond

Moissanite

Ruby

Sapphire

Emerald

Index of Refraction

2.42

2.65

1.77

1.77

1.58

Luster

17.2%

20.4%

7.4%

7.4%

4.8%

Properties of Natural and Synthetic Gemstones

Figure 6 A light ray bends(refracts) as it passes throughmedia with different indicesof refraction. Inferring Based onthe path of the light ray, whichmedium has the greatest index of refraction?

When light enters a new medium at an angle, the change inspeed causes the light to bend, or refract. For example, when lightpasses from air into glass or water, it slows down. When light passesfrom glass or water into air, it speeds up. The amount by which thelight refracts as it passes from one medium to another depends uponthe difference between the speeds of light in the two media.

Figure 6 shows how the path of a light ray changes as it passes fromone medium into another. The incident ray of light, traveling throughair, first strikes the boundary between the air and the water. As the lightray enters the water, it is refracted. You can see in Figure 6 that the lightray is now traveling in a new direction. As the ray enters the glass, it isrefracted even more. Finally, when the ray reenters air, its path is bentagain, but back to its original direction. Note that regardless of therefraction that occurs in the water and glass layers, the ray again trav-els in its original direction when it reenters the air.

How much the speed of a light ray slows as it enters a new materialdepends on the material’s index of refraction. The index of refraction fora material is the ratio of the speed of light in a vacuum to the speed of thelight in the material. A material with a low index of refraction (near 1)causes light to slow and refract very little. Air, with an index of refractionof 1.0003, is such a material. Diamond, however, with an index of refrac-tion of 2.42, causes light to slow and refract significantly.

Use VisualsFigure 6 Have students use a smallruler to compare the directions of theincoming light ray and the outgoinglight ray. Point out that the ray bends toward the normal, or a lineperpendicular to the edge of themedium, when traveling from a lessdense material to a more dense material.It bends away from the normal whentraveling from a more dense material toa less dense material. In this figure, thelight ray changes direction three times.Ask, Which change of direction is thelargest? (From glass into air) Comparethe change from air into water withglass into air. What does this tell youabout the index of refraction of glass?(The index of refraction of glass is higherthan the index of refraction of air.) Visual, Logical

Properties of GemstonesAnswers1. Moissanite is the most lustrous(20.4%); emerald is the least lustrous(4.8%)2. 92.6% of the light striking thesapphire enters the gemstone.3. Light bends the most in moissanitebecause it has the greatest index of refraction.4. Index of refraction � speed of light in vacuum/speed of light in unknowngemstone; Index of refraction �(3.00 � 108 m/s)/(1.69 � 108 m/s) �1.77; the gemstone could be either a ruby or a sapphire.

For Extra HelpHelp students understand luster byshowing them samples of obsidian and galena.Visual

L1

L2

L1

Optics 575

Customize for Inclusion Students

Visually ImpairedOptics may be especially difficult for visuallyimpaired students. Help them understand thebehavior of light rays passing through differentmediums by creating a kinesthetic version of Figure 6. First, glue a length of string onconstruction paper to represent the trajectoryof the original light ray if it continued throughair. Then, glue toothpicks to the side of the

string in the same design shown by the arrowsin Figure 6. Explain that the direction of thetoothpicks shows how much the light is benttoward or away from the normal as it passesfrom air through water, then glass, then air.Students will be able to feel how the directionof the light changes as it moves from onemedium to another.

Answer to . . .

Figure 6 Glass must have the greatestindex of refraction because the lightray is bent farthest away from its initialpath in the glass layer.

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Figure 7 Concave lenses can onlyform virtual images. A Whenparallel incoming rays strike aconcave lens, they are refractedaway from one another. B As thelight rays diverge after passingthrough the concave lens, theyform a virtual image of the object.

Focalpoint

Object

Virtualimage

Concave lens

Focalpoint

Focalpoint

Concave lens

Opticalaxis

Figure 8 A housefly has twolarge compound eyes. Each eye ismade up of thousands of tinyindividual eyes called facets. Theouter surface of each facet isconvex in shape. The eyes givethe fly a nearly 360-degree fieldof view.

A B

576

Concave and Convex LensesLenses are used to change the path of light rays before they enter youreyes. A lens is an object made of transparent material that has one ortwo curved surfaces that can refract light. The curvature and thicknessof a lens affect the way it refracts light.

Concave Lenses A concave lens is curved inward at the centerand is thickest at the outside edges. Figure 7A shows how a concave lensrefracts light rays. As the rays pass through the lens, each one is refracteddue to the change of medium. The rays enter the lens at different anglesand so they emerge from the lens at different angles. Concave lensescause incoming parallel rays to spread out, or diverge. (Concave lensesare a type of diverging lens.) The diverging rays appear to come from asingle point, the focal point, on the same side of the lens as the object.

Concave lenses always cause light rays to spread out and canonly form virtual images. Figure 7B shows how a concave lens formsa virtual image. The image is formed at the point from which therefracted rays appear to come. The image formed by a concave lens isalways smaller than the object.

Concave lenses are often used in the viewfinders of cameras. Thesmall virtual image you see through the viewfinder lens is similar towhat the photograph will show. Concave lenses are also combinedwith mirrors or other lenses to form images in optical instrumentssuch as telescopes.

Convex Lenses Note how the shape of each of the fly’s eyes inFigure 8 resembles the exterior surface of a sphere. A shape like this isknown as convex. Figure 9A shows that a convex lens is curvedoutward at the center and is thinnest at the outer edges. Figure 9A alsoshows how light is refracted by a convex lens. As the rays pass throughthe lens, each one is refracted, and they emerge at different angles.Convex lenses cause incoming parallel rays to come together, orconverge. (Convex lenses are also called converging lens.) The con-verging rays meet at a single point, the focal point, on the side of thelens opposite to the object.

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Concave and Convex LensesUse Community ResourcesInvite a person who repairs cameras tocome to your class and demonstrate howa camera operates and forms images.Encourage students to prepare and askquestions related to optical design. Check the phone book to find camerashops that are authorized repair centers. Interpersonal, Visual

A common misconception is thatblocking part of the surface of a convexlens will block the corresponding part ofthe image. Address this misconceptionby holding a convex lens between alamp and a flat surface (such as a wall or book). Adjust the distances sostudents see an inverted image of thelamp on the surface. Next, cover part of the lens with your hand. Students will see that the lamp’s image on the flat surface is dimmer, but because thelight rays bend, the image is complete.Draw a ray diagram on the board toexplain this effect. Visual

Combining Lenses

Purpose Students see the effects ofcombining lenses.

Materials modeling clay, 2 convexlenses, 2 concave lenses, 2 index cards

Procedure Prior to class, use balls ofmodeling clay to support each lensvertically. Sketch an object, such as atree, on the index cards and use clay tosupport them vertically also. Set up adisplay with an index card about 10 cmbehind a convex lens, and a concavelens about 1 cm in front of a concavelens. Set up a similar display, but nowhave the concave lens about 10 cm infront of the concave lens. Allow studentsto look through both setups.

Expected Outcome Students willobserve that the effect of a concave lenscancels the effect of a convex lens, aslong as the lenses are close together. Visual

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Section 19.2 (continued)

Eyeglass Scientists at Lawrence LivermoreNational Laboratory in California are workingon a new type of space telescope known as“Eyeglass.” An Eyeglass is different from other space telescopes because it does not use mirrors or traditional glass lenses, butdiffractive optics (or Fresnel lenses). Theadvantages of using an Eyeglass telescope

would include its flexibility, lightness, andability to be folded. The Livermore team has currently constructed the world’s largestsuch lens. It has a diameter of 5 meters and is made of 72 glass panels that can be folded.Although the lens is larger than the HubbleSpace telescope, it weighs ten times less.

Facts and Figures

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Object

Realimage

Focalpoint

Focalpoint

Convex lens

Object

Convex lens

Virtualimage

Focalpoint

Focalpoint

Focalpoint

Optical axis

Convex lens

A

C

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Optics 577

Figure 10 In the past, light-houses used a light source placedat the focal point of a convex lensor series of convex lenses to forma beam of parallel light rays.Comparing and ContrastingHow does the setup shown inFigure 9A compare with that usedin old lighthouses?

Convex lenses form either real or virtual images. Whether animage is real or virtual depends upon how far the object is from the lens.Figures 9B and 9C show a convex lens forming real and virtual images.

Convex lenses are used in slide and movie projectors, cameras, andlighthouses like the one in Figure 10. Of course, you don’t see anupside-down real image at the movie theatre because the film is placedupside down in the projector. When the rays of light from the upside-down film pass through the projector’s convex lens, the real image isprojected onto the screen right side up.

How is a convex lens shaped?

Figure 9 Convex lenses can form either real orvirtual images. A When parallel incoming raysstrike a convex lens, they are refracted toward eachother and pass through the focal point. B When anobject is located beyond the focal point of a convexlens, a real image is formed. C A magnified, virtualimage is formed when the object is locatedbetween the focal point and the lens.

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Build Science SkillsObserving

Purpose Students use parallax to find the position of the real image formed by a convex lens.

Materials 5-cm diameter double-convex lens, pencil

Class Time 15 minutes

Procedure Have students hold the lensvertically at arm’s length and observe alight source about 4 meters away. Ask,How would you describe the image ofthe light source? (Smaller and inverted)Have students hold the pencil verticallybetween the lens and their eyes so thatthe image of the light source can’t beseen. If they move their heads slowly left and right with one eye closed, theywill observe parallax. Explain that if anobject and an image are the samedistance away, they will appear to movethe same distance as you move yourhead. Have students try to locate theimage position (of the light) by eliminat-ing the parallax between the image andthe pencil. With students still moving theirheads, have them lower the pencil slightlyand move it closer to their face (about 10cm away) until the pencil appears tomove with the light source. Ask, Where isthe image? (At the same position as thepencil) Is this a real image? (Yes)

Expected Outcome Students willobserve the real image formed by a lens and locate its position. Kinesthetic, Group

Build Math SkillsFormulas and Equations Students canrelate the distances from a lens to thefocal point (f), from the lens to theobject (p), and from the lens to theimage (q) using the lens equation:

� �

Have students practice solving thisequation for each of the three unknowns.Logical

Direct students to the Math Skills in the Skills and Reference Handbookat the end of the student text foradditional help.

1q

1p

1f

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Optics 577

FYIThe distance from a lens or a mirror to its focalpoint is known as the focal length, f. Notice that Figures 6 and 7 show double concave anddouble convex lenses, insofar as both surfaces are concave, or convex, respectively. Note thatthe text uses convex for any converging lens, and concave for any diverging lens. Converginglenses have a positive focal length, and diverginglenses have a negative focal length.

Answer to . . .

Figure 10 The source of light in thelighthouse is located at the focal pointshown in Figure 9A.

A convex lens is curvedoutward at its center

and is thinnest at the outer edges.

IPLS

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Students can interact withsimulations of how mirrors andlenses form images.

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578 Chapter 19

Section 19.2 Assessment

Reviewing Concepts1. What causes light rays to bend?

2. Why can concave lenses form only onetype of image?

3. What type of images are formed byconcave lenses? By convex lenses?

4. Most of the light entering what type ofmaterial is likely to be totally internallyreflected?

Critical Thinking5. Comparing and Contrasting How is a

convex lens different from a concave lens?How are they the same?

6. Applying Concepts Explain how a convexlens is similar to a concave mirror.

Total Internal ReflectionA relatively new and very important application of refraction is fiberoptics. Light rays are generally unable to exit through the sides of thecurving fiber optic strands. Because of this, fiber optics are very usefulfor carrying information in the form of light. Figures 11A through 11Cexplain how fiber optics work.

As shown in Figure 11A, a light ray exiting from glass into air isrefracted. Figure 11B shows that as the angle of incidence of the exitingray increases, an angle known as the critical angle of refraction is reached.The critical angle is the angle of incidence that produces an angle ofrefraction of 90 degrees. At the critical angle the light ray bends so muchthat it takes a path along the glass-air boundary. Figure 11C shows thatat angles larger than the critical angle, the light ray bends so much thatit is reflected back into the glass. This situation is known as total inter-nal reflection. Total internal reflection is the complete reflection of alight ray back into its original medium.

Materials that have small criticalangles are likely to cause most of the lightentering them to be totally internally reflected.Such materials include diamond and the type ofglass used in fiber optic strands. By making useof total internal reflection, fiber optics are ableto transmit data in the form of light pulses overlarge distances with little loss in signal strength.To learn more about fiber optics, see page 586.

Speed of Light In Chapter 18, youlearned that light travels at 3.00 � 108 m/sthrough the air. This speed is the product offrequency and wavelength (Speed �Frequency � Wavelength). Use this equationto show that the wavelength of the lightchanges when it passes from air to water.(Hint: The speed decreases and the fre-quency does not change.)

AirGlass

Criticalangle

Total internalreflection

Ordinaryrefraction CBA

Figure 11 Fiber optics make useof total internal reflection. A When a ray hits the glass-airboundary at an angle less than thecritical angle, it is partly refractedand partly reflected. B At thecritical angle, the angle of refrac-tion is 90 degrees. C When thecritical angle is exceeded, all of thelight is reflected—total internalreflection occurs.

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Total InternalReflectionIntegrate Social StudiesAlhazen, an eleventh-century Arabianscientist, was an early pioneer in thefield of optics. He challenged a commonbelief that rays emanating from the eyesenabled vision. Alhazen’s theory wasthat light originated from the sun andother luminous objects. The eye saw the light that was reflected from objects.Alhazen also studied the refraction oflight and the focusing of light by lenses.He constructed a pinhole camera,magnifying lenses, and parabolicmirrors. Have students research otherearly scientists who studied optics, suchas Archimedes, Galileo, and Kepler. Verbal, Logical

FYINote in Figures 11A and 11B that thereare two rays emanating from the glass-air boundary where the incident raystrikes. The size of these rays areapproximately representative of theamount of light from the incident raythat is reflected and refracted.

ASSESSEvaluate UnderstandingRandomly ask students to list thegeneral properties of a convex orconcave lens. Also, have them name at least one application for the lens.

ReteachHave students draw sketches of thedifferent types of image formation by lenses. They may need to refer to Figures 7 and 9.

Student answers should demonstrate an understanding that, because thefrequency does not change, the onlyway for the speed to decrease is if thewavelength decreases.

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 19.2

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Section 19.2 (continued)

4. Materials that have small critical angles ofrefraction cause most of the light enteringthem to be totally internally reflected.5. Similarities: Both lenses have curvedsurfaces, refract light rays, and can formimages. Differences: Concave lenses makelight rays diverge and can only form virtualimages, whereas convex lens can make lightrays converge or diverge and can form bothreal and virtual images.6. Both cause light rays to diverge and canform virtual images.

Section 19.2 Assessment

1. When light enters a new medium at anangle, the change in speed causes the light tobend, or refract. The part of the wavefront inthe new medium moves at a different speed,causing the wave’s direction to change (bend).2. Because concave lenses always cause light rays to spread out, they can form onlyvirtual images.3. Concave lenses form only virtual images,whereas convex lenses can form either real or virtual images.

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Optics 579

Is Video Surveillancean Invasion of Privacy?

Video Surveillance Is Not an Invasion of PrivacyVideo surveillance is an existing and proventechnology that is very effective for many purposes.Traffic cameras on highways help motorists avoidaccidents and traffic jams. Video cameras in storeshave been used for years to deter and help captureshoplifters. Many major cities have installedcameras at large intersections, to photographdrivers who run red lights. Citations from suchsystems have cut down on traffic violations and onpedestrian accidents. Sophisticated video systems atsome airports can identify known criminals beforethey board an airplane. The common thing in all ofthese examples is that the general public is saferbecause of the use of video surveillance cameras.Having some of our actions recorded on video is asmall price to pay for our greatly increased safety.

Video Surveillance Violatesa Person’s Right to PrivacyVideo surveillance comes at a very high price—the loss of personal privacy. Our society is quicklyheading toward the day when any individual canbe tracked throughout an entire day. Do you wantthe government and private companies to knowevery aspect of your life? The United States SupremeCourt has stated that some acts that violate aperson’s reasonable expectation of privacyconstitute an illegal search. Yet these actions aregoing on every day! Video surveillance, which iscurrently out of control, is just one of many threatsto a person’s right to a private life. Strict lawsdesigned to protect a person’s right to privacyneed to be enacted as soon as possible.

1. Defining the Issue Use your own words todescribe at least two of the major issues involvedin the use of video surveillance cameras.

2. Analyzing the Viewpoints List the argumentsfor and against the use of video surveillancecameras. What are the advantages? What are thedisadvantages or risks?

3. Forming Your Opinion Is video surveillance aninvasion of your right to privacy? If so, in whatsituations, if any, is it acceptable?

4. Going Further Research a recent court caseinvolving video surveillance and privacy. Write ashort report summarizing the case. Explain whyyou agree or disagree with the decision.

Not long ago, video cameras were cumbersome pieces of equipment. Theyrequired long cables and stored images on bulky magnetic tape cassettes.Recent advances in optics, digital imaging, and electronic technology haverevolutionized video equipment. High-quality digital video cameras that areable to store their images in a variety of convenient formats are now veryaffordable. Extremely small, battery-powered video cameras are alsoavailable to the public.

However, not everyone is excited about the spread of affordablevideo technology. The explosion in the number of cameras being usedfor surveillance purposes is a huge concern to many Americans. Is yourright to privacy being violated?

The Viewpoints

Research and Decide

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Is Video Surveillance an Invasion of Privacy?BackgroundAn important issue of video surveillanceis whether it is legal. The courts,including the United States SupremeCourt, have repeatedly ruled that video surveillance is legal under certaincircumstances. A primary requirement is that the surveillance must be donewith high regard for a person’s right to privacy. It should only be done ifother methods for restricting crime areimpossible or ineffective. Surveillancecannot be performed in areas such asrestrooms or locker rooms, where aperson can reasonably expect privacy.

Answers1. Student answers will vary but shouldinclude the issues of safety and privacy.2. Arguments “for” center aroundincreased safety and the general idea offreedom, whereas “against” argumentscenter around the right to live your life inprivacy. Advantages and disadvantagesare closely related to these arguments.3. Student answers will vary but shouldclearly state a position and detailsituations when video surveillance is acceptable and when it is not.4. Reports will vary, but should contain a concise summary of the key points inthe case and an explanation as to whythe student agrees or disagrees with the decision.

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Have students further research the issues related to this topic.

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580 Chapter 19

19.3 Optical Instruments

Reading StrategyUsing Prior Knowledge Copy the diagrambelow and add to it the names and descrip-tions of other optical instruments you know.Revise the diagram after reading the section.

Key ConceptsWhat are the two maintypes of telescopes?

How does a camera forman image on film?

What type of lenses does acompound microscopeuse to form an image?

Vocabulary◆ telescope◆ reflecting telescope◆ refracting telescope◆ camera ◆ microscope

Your view of the world is not shaped solely by what you can see withthe unaided eye. From the most distant star in the galaxy to the tiniestcell of your skin, optical instruments improve your ability to seeobjects. With telescopes, you can see images of astronomical, far-awayobjects—some of which may no longer exist. With microscopes, youcan see detailed images of objects too tiny to otherwise be seen. Withcameras, you can fill your photo album with images of your family andfriends, or places you have visited.

Telescopes, microscopes, and cameras are all examples ofoptical instruments that enhance your ability to see. All of theseoptical instruments have something in common—they all uselenses or mirrors, or a combination of the two, to reflect andrefract light.

TelescopesThe universe is so vast that the light coming from the fartheststars has traveled billions of years before it reaches Earth. Someof the light takes so long to reach Earth that by the time it getshere, the source of the light—the star—has long since burnedout. With a telescope you can see images of the star even thoughit no longer exists. A telescope is an instrument that uses lensesor mirrors to collect and focus light from distant objects. InGreek, the word teleskopos means “seeing from a distance.”

a. ?

b. ?

OpticalInstruments

Telescopes

Figure 12 Shown below is one ofthe two Keck telescopes locatedon the summit of Hawaii’sdormant Mauna Kea volcano. Thetelescopes, one optical and oneinfrared, are the largest in theUnited States. Inferring Whatmight be a reason for thetelescopes being located on amountain top?

580 Chapter 19

FOCUS

Objectives19.3.1 Distinguish between how

reflecting and refractingtelescopes form images.

19.3.2 Explain how cameras regulateand focus light to form images.

19.3.3 Describe how light travels in a compound microscope toproduce an enlarged image.

Build VocabularyFlowchart As students read aboutreflecting telescopes, have them make a flowchart showing the steps in theimage formation process. Ask studentsto provide definitions for any scientificwords used in their flowcharts.

Reading StrategyAcceptable answers for a. and b.include any optical instruments, such as microscopes and cameras. Aftercompleting the section, reviseddiagrams should include refracting andreflecting telescopes, cameras (pinhole,and modern or SLR), and microscopes.

INSTRUCT

TelescopesBuild Reading LiteracyUsing Context Clues Refer to page568D in this chapter, which providesguidelines for using context clues.

As students read Section 19.3 (pp. 580–585), have them look forunfamiliar words. For example, studentsmay not know definitions for the wordsspecimen, elements, and platform.Encourage them to use surroundingsentences and figures to help themunderstand the word. Demonstrate this procedure with the word shutteron p. 584. Verbal

Use Community ResourcesInvite a member of the local astronomicalsociety to discuss how different types oftelescopes form images and demonstratetheir use. Interpersonal, Visual

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Section 19.3

Print• Laboratory Manual, Investigation 19B• Reading and Study Workbook With

Math Support, Section 19.3• Transparencies, Section 19.3

Technology• Interactive Textbook, Section 19.3• Presentation Pro CD-ROM,

Section 19.3• Go Online, Science News, Light and optics

Section Resources

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Incominglight

Convexobjective

lens

EyepieceViewingdirection

Incominglight

Largeconcavemirror

Eyepiecelens system

Viewingdirection

Plane mirror at45-degree angle

A ReflectingTelescope

B RefractingTelescope

Most historians credit Dutch eyeglass maker Hans Lippershey withinventing the first telescope in 1608. In 1671, Isaac Newton invented atelescope that formed images by reflecting light with a curved mirror.By the end of the 1800s, scientists were looking farther and farther intothe universe. Today’s telescopes map the universe past and present, help-ing astronomers figure out its history and its future. There are twomain types of telescopes, reflecting telescopes and refracting telescopes.

Reflecting Telescopes The reflecting telescope uses mirrorsand convex lenses to collect and focus light. Figure 13A shows the pathof light through a reflecting telescope. Light from a distant objectstrikes a large concave mirror and is brought to a focus. This focusedlight is reflected by an angled mirror and forms a real image. Theconvex lens of the eyepiece then enlarges the image.

Refracting Telescopes The refracting telescope uses convexlenses to collect and focus light. Light from a distant object enters thetelescope by passing through a convex lens called the objective lens.The convex lens forms a real image at its focal point inside the tele-scope. A convex lens in the eyepiece then magnifies this real image. Asyou look through the eyepiece, you see an enlarged, upside-down, vir-tual image of the real image. Figure 13B shows the path of lightthrough a refracting telescope.

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Optics 581

Figure 13 The two main types oftelescopes use combinations ofmirrors and lenses to magnifyimages of distant objects. A Thereflecting telescope uses a largeconcave mirror to focus theincoming light rays. B Therefracting telescope uses a seriesof lenses to focus light fromdistant objects.

Use VisualsFigure 13 Have students trace thepath of the incoming light ray in eachkind of telescope. Ask, What is thepurpose of the objective lens ormirror? (To gather and focus the lightrays) What is the purpose of theconvex eyepiece lens? (To magnify the image produced by the mirror orobjective lens) Visual, Logical

Making a Telescope

Purpose Students observe the basicrequirements for a telescope.

Materials 2 convex lenses (one with a focal length of 10 cm and the otherwith a focal length of 30 cm), 1 stick ofmodeling clay

Procedure Use a lump of clay on adesk to hold the 10-cm focal length lensvertically. Place the other lens verticallyin clay a few centimeters away. Align the lenses so that you can look throughboth, with your eyes a few centimetersfrom the 10-cm focal length lens. Movethe 30-cm focal length lens until you seea sharp image in the lens. This imagewill be upside down and enlarged. Allowstudents to look through the lenses.

Expected Outcome Students willrecognize that one lens gathers lightand the other forms a magnified image. Visual, Logical

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Customize for English Language Learners

Building a Science GlossaryAllow students to work in pairs. Post thesection’s vocabulary terms on the board.Model how to divide each word into parts,determine the meaning of the word,pronounce the word, and use the word in a sentence. Point out that four of the fivevocabulary terms contain the word part -scope. Tell students that -scope means “an

instrument for seeing or observing.” Askstudents how knowing this word part mighthelp them learn the meaning of the vocabularywords. Students can add the words anddefinitions to their science glossaries. Makefrequent references to the words and assignthem in homework or another activity toemphasize their importance.

Science News provides studentswith current information on lightand optics.

Answer to . . .

Figure 12 The high elevation meansthe telescope looks through less ofEarth’s atmosphere, resulting in better images.

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CamerasA camera is an optical instrument that records an image of an object.No matter the type of camera, it uses the same basic principle of focus-ing light rays to form real images. Light rays enter a camerathrough an opening, are focused by the opening or lens, and forman image that is recorded on film or by a sensor.

Pinhole Camera Did you know that the word camera is Latinfor “room?” The earliest cameras were in fact the size of an entireroom, and were known as camera obscura, or “dark room.” One of theearliest uses of a camera obscura is credited to Leonardo da Vinci.

1913 Germaninventor OscarBarnack developsthe UR Leicacamera, the first35 mm camera.It uses the 24 mmx 36 mm frameformat.

1826Nicéphore Niepcemakes the firstpermanentpositivephotograph of a landscape.

PhotographyPhotography has come a long way since the1800s, although the basic apparatus is still thesame—a box with a hole to let in light.

1888 GeorgeEastman launchesthe Kodak camera,the first successfulroll-film camera. It is preloadedwith a 6-meterroll of film atthe factory.

1820 1855 1890

THREE-COLORPROJECTOR

SYSTEM Holders forblack-and-white slides

Holders for red, blue,and green color filters

Simplewooden box

Hole for lens

FIRST CAMERA

1840 William FoxTalbot developsthe first negative-to-positive process,making multipleblack-and-whiteprints possible.

1861 Scottishphysicist James ClerkMaxwell exhibits acolor photographicsystem involvingthree black-and-whitephotos, takenthrough red, blue,and green filters.When the photos areturned into slides,projected on a screenusing the same filters,and superimposed,the original scene isrecreated, in color.

ORIGINALKODAK CAMERA

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CamerasBuild Science SkillsComparing and ContrastingHave students carefully examine the firstcamera (1826) and the modern digitalcamera (2000). Ask, What features are common to both cameras? (Bothcameras are dark containers that gatherlight through a small hole.) What aresome improvements that the moderndigital camera has over the firstcamera? (The digital camera is smaller,lighter, easier to use, has a low-lightindicator, automatic flash, auto-focuszoom lens, and camera dock for imagetransfer to a computer.) Visual

PhotographyDivide the class into groups of two orthree students, and assign each group a date from the time line on p. 583. Have each group discuss how the eventsthey’re investigating helped spread theuse of photography. (Each advancementmade taking photographs easier for theaverage person.) Ask them also to relatethe advancements in materials to changesin camera design. (Lightweight plasticsreplaced metal bodies and glass lenses.) Interpersonal, Verbal

Light enters each camera by passingthrough a lens that focuses the light into an image at the back of the camera.The Kine-Exakta forms an image on film that must be removed from thecamera and developed. The Landcamera records the image on film thatdevelops itself in just a minute or two.The digital camera records the image as an electronic file that is later outputto a computer or a printer.

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Lenses A variety of lenses can be used withcameras. The standard lens used on cameras isa fixed focal length lens. The magnification ofimages taken with this lens cannot be changed.In general, this type of lens is the lightest, most compact, and most useful for everydayphotography. A variable zoom lens permits thephotographer to choose the magnification ofthe image. These lenses are useful for taking up-close pictures of faraway objects. A wide-anglelens captures a wider field of view. You can take

photographs of large objects, such as buildings,with this type of lens. Modern cameras mayhave retractable zoom lenses. These lenses slideback into the camera when not in use. Camerasmay also have interchangeable lenses. Theentire lens may be removed from the cameraand replaced with another type of lens. Pointout to students that, although there are manytypes of camera lenses, they are all converginglenses that focus light to form an image at theback of the camera.

Facts and Figures

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Da Vinci, an Italian scientist, constructed his camera by making apinhole opening in the shutter of a window of a darkened room.Images of the outside scenery were projected onto the wall oppositethe window. Pinhole cameras do not have to be the size of a room. Asimple pinhole camera can consist of a cardboard box with a smallhole in one side. Light rays from the top and bottom of an object passthrough the pinhole and cross paths. The rays form an upside-down,real image on the back wall of the box. For firsthand experience witha pinhole optical device, build and use the pinhole viewer in theQuickLab later in this section.

What type of image does a pinhole camera form?

1947 Americaninventor Edwin H. Landdemonstrates a newone-step photographicsystem. It develops andproduces finished picturesin a few seconds.

1925 1960 1995

2000 Highresolution,compact,multi-featured,affordable digital camerasare availableand popular.

1986 Pentaxproduces the firstfully automaticcompact camerawith a built-inzoom lens.

KINE-EXAKTACAMERA

INSTANT PICTURES

MODERN COMPACTDIGITAL CAMERA

Focusingmagnifier

Standard lens(one of fiveoptions)

Focusingring

Auto-focuszoom lensCamera dock

connects tocomputer forimage transfer.

LED indicator (indicateslow light conditions to

activate flash)

1942 Kodakannounces afilm for colorprints, Koda-color, theworld’s firsttrue colornegative film.

1936 Germancompany Ihageelaunches theKine-Exakta, thefirst single-lensreflex (SLR)camera for 35mm film.

Optics 583

Compare-Contrast ParagraphWrite a paragraph summarizinghow all of the cameras shownare similar. Also describe howthe Kine-Exakta camera (1936),the Land camera (1947), andthe digital camera (2000)differ in the way in which theyrecord images.

Build Science SkillsApplying Concepts Point out tostudents that the first camera shown on the time line had no lens. This is alsotrue for the pinhole camera described in the text. Instead, light enters througha small hole, and the image is formedon the back wall of the camera. Ask,Why is it sufficient to have light enterthrough a hole rather than a lens?(Light rays entering the camera travel in a straight line through the pinhole andtherefore form an image at the back of thecamera.) Why is the image inverted,even without a lens? (Light enteringnear the top of the hole goes downwardthrough the pinhole, and light enteringnear the bottom of the hole goes upwardthrough the pinhole.) What advantagedoes a lens offer over a pinhole? (More light can be collected, while stillproviding a focused image) Verbal, Logical

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Answer to . . .

A pinhole camera formsan upside-down,

real image.

Autofocus Most cameras today contain anelectronic system for autofocusing the image.The camera may do this by emitting an infraredsignal that reflects off the object that is beingphotographed. By measuring the intensity ofthe reflected light, the camera can determinethe object’s distance. It then adjusts the lens to

optimize the focus. Autofocusing may also be done using electronics that examine theamount of contrast in an image. A blurry imagehas less contrast than a well-focused image. Thecamera’s electronics adjust the lens placementto maximize the image contrast.

Facts and Figures

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Eyepiecelens

Objectivelens

Slide withspecimen

Mirror

Modern Film Camera Figure 14 shows a typical modern filmcamera. The lens elements focus the incoming light rays. Thefocused rays then pass through the diaphragm, a device that con-trols the amount of light passing through the lens. When the shutterrelease button is pressed, the mirror flips up and the shutter brieflyopens to let the focused light rays strike the film.

To bring an object into focus, the lens must be moved toward oraway from the film. This focusing, which can be done manually or auto-matically, is needed to form a sharp image. The focused light reacts witha light-sensitive chemical coating on the film that records the real image.The image is upside down and smaller than the object. After the film isdeveloped and printed, you have photographs for your album.

MicroscopesA microscope is an optical instrument that uses lenses to provideenlarged images of very small, near objects. One common type ofmicroscope is called a compound microscope. The compoundmicroscope uses two convex lenses to magnify small objects.

Figure 15 shows the structure of a compound microscope. To viewan enlarged image of an object, you place the object on a glass slide.You then place the slide on a platform located above a light source.Light rays from below pass up through the object and then passthrough a convex lens called the objective. The lens produces anenlarged, upside-down, real image. This image then becomes the“object” for a second convex lens called the eyepiece. The eyepieceenlarges the image. When you look through the eyepiece, you see anenlarged, virtual image of the object. Under the best conditions,modern light microscopes can magnify images more than 1000 times.

Figure 14 Shown hereis a typical single lensreflex (SLR) camera.The mirror reflects theimage through theviewfinder so thephotographer canbring it into focus.When the shutterrelease button ispushed, the mirrorflips up. This allowsthe focused light raysto pass straightthrough the lenssystem and ontothe film.

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Shutter

Viewfinder Prism

Shutterreleasebutton

Diaphragm

Incominglight

Lenselements

Mirror

Film

Figure 15 This compoundmicroscope uses a mirror toreflect light up and intothe microscope.Comparing and ContrastingHow is the way in which afocused image is formed thesame in both a camera anda microscope?

584 Chapter 19

Use VisualsFigure 14 Encourage students todiscuss how the camera in the figureworks. Point out the lenses located both before and after the diaphragm.Explain that together these form the lens elements. Tell students thatadjusting the size of the diaphragm’sopening (the aperture) controls theamount of light. Ask, What is thepurpose of the prism in the camera?(The prism redirects the light from themirror to the viewfinder.) What is thepurpose of the shutter? (The shutter isusually closed to protect the film. It brieflyopens to expose the film to light.) Visual, Logical

Microscopes

Two Types of Microscopes

Purpose Students will learn aboutcompound and stereo microscopes.

Materials compound microscope,stereo microscope

Procedure Show students the twotypes of microscopes. Draw simplesketches on the chalkboard of theiroptical paths. Explain that the compoundand stereo microscopes are the two basic types of microscopes. Point out that a compound microscope has highmagnification (often 10X, 100X, and400X) and gives an inverted image of the object. A stereo microscope has low magnification and an erect, three-dimensional image.

Expected Outcome Students willlearn about compound and stereomicroscopes. Visual, Logical

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Section 19.3 (continued)

Microscope Objectives A typical schoolmicroscope has three objectives. The shortestobjective is low power, with a magnification of 10X. The intermediate objective may have a multiplication of 40X. The longest objectivemay have a magnification of 100X. The convexlens in the eyepiece, however, typically providesan additional multiplication of 10X. This meansthe total magnification of the three objectives

is 100X, 400X, and 1000X. To obtain enoughlight with the 1000X magnification, you mustplace a drop of immersion oil on the lens. Whenthe objective is lowered, the oil eliminates thespace between the lens and the coverslip over the specimen. The oil has the same indexof refraction as glass. Light, therefore, is notrefracted, and you obtain sufficient light to view the object.

Facts and Figures

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Section 19.3 Assessment

Reviewing Concepts1. Name the two main types of telescopes.

2. How does a film camera work?

3. Describe the system of lenses in acompound microscope.

4. What is the purpose of the lens in a camera?

Critical Thinking5. Inferring High-speed film is very sensitive to

light. Explain how this could be useful for dim-light photography.

6. Applying Concepts Can the image seen inthe eyepiece of a compound microscope beprojected on a screen?

Optics 585

Building a Pinhole Viewer

Materials• cardboard tube• black construction paper• aluminum foil• wax paper• 4 rubber bands• pin

Procedure1. Place aluminum foil over one end of the

cardboard tube and use a rubber band to hold the foil in place.

2. Place wax paper over the other end of the tubeand hold the paper in place with a rubberband. There should be no wrinkles in the waxpaper covering the opening of the tube.

3. Roll a piece of black construction paperlengthwise around the tube. The foil endshould be flush with one end of theconstruction paper as shown. Use two morerubber bands to hold the construction paperin place.

4. Verify that the wax paper end is now in themiddle of the black construction paper tube.

5. Use a pin to make a hole in the center of thealuminum foil. You may have to enlarge thehole slightly to see an image clearly.

6. Point the pinhole toward a light source andlook through the other end of the viewer.Observe several objects inside your classroom(chairs, tables, and books) and outside of yourclassroom (houses, trees, and cars). CAUTIONDo not use the viewer to look at the sun, as youmay injure your eyes.

Analyze and Conclude1. Observing Describe the images that

appeared on the wax paper.

2. Relating Cause and Effect What causedthe images to appear on the wax paper theway they did?

Speed of Light From Chapter 18 youknow that light travels at an extremelyhigh speed. Explain how the images ofdeep-space objects you see through tele-scopes are images of the past.

Cardboardtube

Wax paper

Rubberbands

Aluminumfoil

Pinhole

Rolled tubeof blackconstructionpaper

Building a Pinhole ViewerObjective After completing this activity, studentswill be able to • describe the formation of an inverted

image by a pinhole camera.

Skills Focus Observing

Prep Time 20 minutes

Class Time 25 minutes

Safety Caution students not to lookdirectly into the sun.

Expected Outcome Students willobserve an inverted image on the wax paper.

Analyze and Conclude1. The images produced on the waxpaper are reversed left to right, and top to bottom.

2. The rays are reversed left-right and top-bottom as they pass throughthe opening. Logical

ASSESSEvaluate UnderstandingAsk students to name some opticalinstruments they use. Have studentsdescribe what kind of lens or mirror they think is used in the device.

ReteachHave students write several sentencesthat briefly explain how telescopes,cameras, and microscopes form images.

Because deep-space objects are so faraway, it takes thousands of years ormore for light emanating from them toreach Earth. Thus, images we currentlysee in telescopes are actually images ofhow those deep-space objects appearedthousands or more years ago.

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 19.3

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4. Camera lenses focus the incoming light raysto form a sharp image.5. High-speed film is useful in situations where there are very low light levels because it requires less time to form an image. 6. Yes, the light could be projected onto ascreen, but it would not form an image. Theenlarged virtual image formed by a microscopecannot be projected onto a screen.

Section 19.3 Assessment

1. The refracting telescope and the reflectingtelescope2. Light rays enter the camera through anopening, are focused by the lens, and form an image on the film.3. A compound microscope uses two convexlenses to magnify small objects. Answer to . . .

Figure 15 Both use a movable lens or series of lenses to focus the image.

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Fiber Optics

Internal reflectionTotal internal reflectioncarries light aroundsharp bends.

Optical fibers have caused a revolution in informationtechnology by making it possible to transmit huge amounts of information alongslender strands of glass.

Opticalfiber

Protectivelayer forcable

Strengtheningsteel core

Inside a fiber optic cable One cable contains severalfibers. Each fiber consists ofa core and an outer layermade from glass of a lowerrefractive index. The light iskept within the fiber bytotal internal reflection.

Fiber opticcable

Regeneratorboosts signal.

Receiverdecodes signal.

TV camera

Television

Transmittersends signal.

Transmitting a TV signalTV signals can be sent alongoptical cables in the form of

light. The electrical signalproduced by the camera isturned into a digital code

by the transmitter, and sentalong the cable as rapid

pulses of light. The receiverthen converts this signal

back into an electricalsignal, which in turn formsa picture on the TV screen.

Protectivesheath

586 Chapter 19

Opticalfiber core

Outerlayer

586 Chapter 19

Fiber OpticsBackgroundOptical fibers are usually made of ahighly transparent glass. The glass mustbe extremely pure. Even small amountsof impurities would absorb too much of the light.

To enable light transmission, therefractive index of the core must begreater than the refractive index of the cladding, or layer just outside thecore. Ideally, the difference in therefractive indices should be as great as possible. However, considerationmust also be given to whether or notthe cladding material is able to adhereto the core fiber.

Light propagates through a fiber by total internal reflection. When light inside the fiber optic reaches theinterface of the core and the outer layer, it reflects off the outer layer backinto the core provided that the angle of incidence is greater than or equal to the critical angle. The light continuesto totally internally reflect down thelength of the fiber.

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� Research how fiber optic strands aremanufactured. Prepare a poster to present your findings to yourclass. Include a detaileddescription of themanufacturing process andthe materials used.

� Take a Discovery Channel VideoField Trip by watching“Traveling Light.”

Going Further

Optical fibers in medicineAs well as carrying digital information,optical fibers are useful in medicine. This isbecause they can bend around corners,allowing otherwise inaccessible parts of thebody to be viewed.

Depth scaleshows howfar theendoscope isinserted intothe body.

Eyepiece

Tip of the endoscope isinserted first into the body.

A light sourceis attachedhere.

Single optical fiber

Eye of a sewing needle

Strand of glassOptical fibers are made

from ultra-pure glass.Each fiber is thinenough to passthrough the eye of a needle. Sometop-quality fiberscan carry a lightbeam with just a10 percent lossof intensity overa kilometer.

ENDOSCOPE

Surgeons usean endoscopeto look atinternal organs.

EndoscopeEndoscopes are used to view the interior of the body. Theyare often used for investigations—such as checking for astomach ulcer—and duringsurgery, to guide the surgeon.

Internal organ is viewedon a computer screen.

Optics 587

Video Field Trip

Build Science SkillsObserving

Purpose Students observe properties of optical fibers.

Materials flashlight, fiber optic bundle

Class Time 10 minutes

Procedure In a darkened room, shinea bright light at the end of a straightbundle of optical fibers. Students willnotice that the light exits the fibers onlyat the ends. Now bend the bundle offibers while continuing to shine the lighton the ends of the fibers. Students willobserve that the light still only exits thefibers at the ends. Ask, Why does thelight travel through the length of thefibers, even though the fibers arebent? (Internal reflection causes the lightto bounce off the outer layer of the fibers.)

Expected Outcome Light enters oneend of the fiber bundle and exits theother end, even if the fibers are bent. Visual, Logical

Going FurtherStudent posters and research findingswill vary but should include adescription of the materials used andthe manufacturing process. Optical fibermanufacturing is basically a two-stepprocess involving the fabrication of aspecially constructed glass rod called a preform, followed by the melting ofthe preform, which is then drawn into a thin fiber. Commercial producers use a variety of processes to fabricatethe preform, all of which are based on a thermal chemical vapor reaction thatforms mixed oxides which are depositedas layers of glass soot onto a rotatinghigh-purity glass rod or tube.Visual

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Optics 587

After students have viewed the Video Field Trip,ask them the following questions: Name oneadvantage of using light signals through glassfibers to transmit information. (The information is transmitted at very high speeds through the fiber.)

Who was the first person to use light totransmit telephone conversations? (AlexanderGraham Bell) Describe the process of making a glass fiber for sending signals using light.(Pure silica is used to make a fine glass rod. The rodis heated. Molten glass from the end of the rod dripsdown to form a long thin fiber.) Name a practicaladvantage that glass fibers have over copperwire. (The glass is less expensive than copper.)

Video Field Trip

Traveling Light

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19.4 The Eye and Vision

Reading StrategyOutlining As you read, make an outline ofthe important ideas in this section. Use thegreen headings as the main topics and theblue headings as subtopics.

Key ConceptsWhat are the main partsof the eye?

What are some commonvision problems?

Vocabulary◆ cornea ◆ pupil ◆ iris◆ retina◆ rods◆ cones◆ nearsightedness ◆ farsightedness◆ astigmatism

Have you ever stopped to appreciate how remarkable your eyes are?They play a very important role in your perception of the world aroundyou.Your eyes, like the one shown in Figure 16, are optical instrumentsthat perform the same tasks of bending and focusing light as tele-scopes, cameras, and microscopes.

Your eyes form images every moment they are exposed to light.They receive and focus visible light from objects near and far. Yourbrain then interprets the images of the objects formed by your eyes.

Structure of the EyeYou can read this page because light reflected fromthe page enters your eye. But how does this light gettransformed into images of objects you recognize?Various parts of the eye, each one having its ownspecific function, work to make your sense of visionpossible. Figure 17 shows the structure of the humaneye. The main parts of the eye are the cornea,the pupil and iris, the lens, and the retina.

Cornea Light rays enter your eyes through thetransparent outer coating of the eye, called thecornea. The cornea’s curved surface helps to focuslight entering your eye.

I. The Eye and Vision A. Structure of the Eye 1. 2. B. and so on . . .

and so on . . .

588 Chapter 19

Figure 16 The lens of the eyefocuses incoming light rays ontolight-sensitive nerve endingslocated inside and at the backof your eye.

588 Chapter 19

FOCUS

Objectives19.4.1 Name the main parts of the eye

and describe their functions.19.4.2 Name common vision

problems, identify their causes, and explain how they can be corrected.

Build VocabularyWord-Part Analysis Tell students thatthe vocabulary terms nearsightednessand farsightedness are compound words with a suffix. Explain that the suffix -ness means “the state of being.” Have students tell each word’s wordparts and tell what the words meanbased on their word parts. (The wordparts of nearsightedness are near,sighted, and -ness. The word parts offarsightedness are far, sighted, and -ness. Nearsightedness means the state of being able to see near. Farsightednessmeans the state of being able to see far.)

Reading StrategyI. The Eye and Vision

A. Structure of the Eye1. Cornea2. Pupil and Iris3. Lens4. Retina5. Rods and Cones

B. Correcting Vision Problems1. Nearsightedness2. Farsightedness3. Astigmatism

INSTRUCT

Structure of the Eye

Many people incorrectly assume that thelens of the eye is the primary focusingelement. Explain to students that lightentering the eye encounters the greatestdifference in optical density (indices of refraction) when it passes from the air to the cornea. Therefore, light isfocused most by the cornea. Visual, Logical

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Reading Focus

1

Section 19.4

Print• Reading and Study Workbook With

Math Support, Section 19.4• Transparencies, Section 19.4

Technology• Interactive Textbook, Section 19.4• Presentation Pro CD-ROM, Section 19.4

Section Resources

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Pupil and Iris After the cornea, light rays pass through the pupil,the part of your eye that looks black. The pupil is the opening thatallows light rays to enter your eye. The colored part of your eye, theiris, contracts and expands to control the amount of light that entersyour eye. The controlled movement of the iris is regulated by signalsfrom your brain.

Lens After passing through the pupil, light enters the convex lens inyour eye. The lens is a sealed capsule containing a clear fluid. This lensfocuses the light onto the light sensor cells at the back of the eye. As youchange your focus from near to distant objects, muscles inside the eyechange the shape of the flexible lens. The muscles relax and the lensbecomes thinner and flatter. If you read for a long time, you’ll noticethat your eyes become tired and feel strained. This is because the mus-cles have been contracting for a long time. The best way to relax youreyes is to look far away.

Retina The focused, refracted light is collected at the retina. Theretina is the inner surface of the eye. Its surface is covered by light-sensitive nerve endings called rods and cones. The rods and conesconvert the light into electrical signals that are sent to the brainthrough the optic nerve. The area of the retina where the nerve end-ings come together to form the optic nerve creates a blind spot. Thisblind spot has no rods or cones and cannot sense light.

Which part of the eye controls how muchlight enters?

Optics 589

Figure 17 The eye is the organthat provides you with sight.Light passes through the cornea,pupil, and lens before striking theretina. Signals from light-sensitivenerves on the retina are sentthrough the optic nerve to thebrain. Predicting What mayresult if the eyeball has anelongated (too long) shape?

Blind spot

Blood vessels

Optic nerve

Retina

Sclera (white of eye)

Cornea

Iris

Pupil

Lens

Aqueous humor(clear fluid thatmaintains shapeof front of eye)

Vitreous humor(gelatinous filling that maintains eye shape)

Choroid(membrane containing

blood supply and a dark pigment that reduces

reflection of light within the eye)

Use VisualsFigure 17 Have students look at thefigure and describe the refraction oflight as it passes through each part. Ask, Are the light rays that enter theeye always parallel? (No, they enter atall angles.) Describe the mediums thatlight encounters as it passes throughthe eye. (Light passes through air, thecornea, aqueous humor, the lens, andvitreous humor before striking the retina at the back of the eye.) What preventsmost of the light that enters the eyefrom being reflected? (The choroid isfilled with blood, which absorbs most of the incoming light.) Visual

Build Science SkillsObserving It is easy for most studentsto detect their “blind spot.” Pass outsome white index cards. Have studentsturn the cards so the long dimension ishorizontal and make an X on the cardabout 3 or 4 cm from the right handedge and a dot 5 or 6 cm further to theleft. Then have students cover their righteyes with one hand and stare at the Xwith their left eye while slowly bringingthe card closer to the eye. At somedistance (around 18 cm) the dot willdisappear. Ask students to explain what happened. (The image of the dotfell on the eye’s blind spot.) Logical, Visual

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Customize for Inclusion Students

GiftedThe process by which a lens changes to alterits depth of focus is called accommodation. In humans, this is done by muscles thatincrease or decrease the roundness of the lens. In animals, accommodation is done in

other ways, such as moving the lens toward or away from the retina. Some animal eyes arenot capable of accommodating. Have studentsresearch the method of accommodation invarious animals. Have students present theirresearch findings to the class.

Answer to . . .

Figure 17 The image formed by thelens may not be in focus at the locationof the retina.

The iris controls theamount of light that

enters the eye.

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590 Chapter 19

Figure 18 When the eyeball istoo long, the focused imageforms in front of the retina. Bythe time the image reaches theretina, it is no longer in focus.This common condition, callednearsightedness, can be correctedwith a diverging (concave) lens.

Rods and Cones Low-intensity light is sensed by rods. The brainuses signals from rods to distinguish among white, black, and differ-ent shades of gray. Cones are sensitive to color, but are less sensitivethan rods; that is, they need more light than rods in order to function.The decreased light sensitivity of cones explains why you can’t makeout the colors of objects in very dim light. There are three differenttypes of cones. Each type of cone is able to sense only a single color—red light, green light, or blue light. People who are colorblind havemissing or defective cones of one or more of the three kinds.

Correcting Vision ProblemsYou have probably heard the expression “20/20 vision.” Having 20/20vision means that you can clearly see things of a certain size from20 feet. 20/20 vision is considered normal. Not everybody, however,has 20/20 vision. Several common vision problems are near-sightedness, farsightedness, and astigmatism. They result in peoplehaving vision that is worse than 20/20. In many cases, less-than-perfectvision can be corrected with eyeglasses or contact lenses. Correctiveeyeware is not new—eyeglasses were used in China and Italy as earlyas the 1200s.

Nearsightedness If you have normalvision, the images you see are clear and undis-torted. The light rays that enter your eyes arefocused on your retinas. However, for approxi-mately one out of four people, the rays focusbefore they reach the retina. This condition,called nearsightedness, causes distant objectsto appear blurry. Nearsightedness occurs eitherbecause the cornea is too curved or the eye-ball is too long. In either case, the rays of lightfocus too close to the lens. A nearsighted personcan see nearby objects clearly, but distantobjects seem blurred. Nearsightedness can becorrected by placing a diverging (concave) lensin front of the eye. The lens spreads the rays

out a little before they enter the eye. This causes the image to formfarther back, at the retina, instead of in front of it. Some cases ofnearsightedness can now be treated with surgery. Figure 18 shows adiagram of a nearsighted eye and how it can be corrected with adiverging (concave) lens.

What does it mean to have 20/20 vision?

Image forms in front of retina.

Image forms on retina.

Problem: Nearsightedness (Eyeball is too long.)

Concave lens

Correction: Eyeglasses with concave lenses

590 Chapter 19

Build Reading LiteracyRelate Cause and Effect Refer topage 260D in Chapter 9, whichprovides the guidelines for relatingcause and effect.

Cause-and-effect relationships are thebasis of scientific discovery. However,some students may not fully understandthe nature of the cause-and-effectrelationship. Ask, When you enter a dark theatre from bright sunlight, thepupil opens very quickly. However itmay be a few minutes before your eyefully adjusts to the low light conditions.Why? (Although the iris opens quickly, afew minutes are required for the rods andcones to adjust to the low light conditions.)Visual

Correcting VisionProblemsIntegrate HealthExplain that both optometrists andophthalmologists are trained to diagnosevision defects and prescribe correctivelenses. Tell students that ophthalmologistsalso treat diseases of the eye. Encouragestudents to research eye diseases such asglaucoma, cataracts, and conjunctivitis.Have students describe methods oftreatment used by ophthalmologists for these diseases, and any means of preventing the diseases. Visual

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Section 19.4 (continued)

Interfaces in the Eye Light entering the eyeencounters four interfaces: (1) air to cornea,(2) cornea to aqueous humor, (3) aqueoushumor to lens, and (4) lens to vitreous humor.The indices of refraction are 1.00 for air,approximately 1.376 for the cornea, 1.336 for

aqueous humor, and 1.337 for vitreous humor.The index of refraction for the lens rangesfrom 1.386 to 1.406. Light that enters the eye is bent most when it passes from air intothe cornea, because this is where the largestchange in index of refraction occurs.

Facts and Figures

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Optics 591

Laser Eye SurgeryThe laser has become a sight-saving tool. In laser eyesurgery, a laser beam makes small incisions, half thethickness of a human hair, in the eye. Surgeons correctand control different types of vision problems bycareful positioning of the laser incisions. As shownbelow, a laser can be used to destroy abnormal bloodvessels that cause failing vision in patients withdiabetes. Interpreting Diagrams On what part ofthe eye do the abnormal blood vessels form?

Laser beam By destroyingparts of the outer layer ofthe retina and the abnormalblood vessels, it is possible toprevent further deteriorationof the patient’s vision.

Mirror Viewing lens Thelens neutralizesthe refractivepower of thecornea and focusesthe laser beam.

Abnormalblood

vessels

Opticdisc

Optic disc

BEFORE SURGERY

No abnormalblood vessels

AFTER SURGERYRetina

Laserburn

Laserburn

Lens of eyePupil

Results of surgeryTaken through the pupil ofthe eye, these picturesshow how abnormal bloodvessels on the retina areburned away by lasertreatment. After surgery,the retina appears paler.

Cornea

Abnormal blood vesselsDue to diabetes, abnormalblood vessels form on theretina, impairing vision.

Laser Eye SurgeryMost people who undergo laser eyesurgery have a significant improvementin their vision. This is especially true forthose who previously had moderatevision problems. However, not everyonewith vision problems is eligible for thesurgery. Patients must be over 18 yearsold with stable vision for at least twoyears. Their corneas must be sufficientlythick to allow the physician to safelyperform the incision. People with severe eye problems should not havethe surgery. In addition, certain eyedisorders, such as cataracts andglaucoma, as well as eye injuries mayprevent a person from having laser eye surgery. Patients should realize that the surgery does not guarantee20/20 vision, and, as with all surgery,there are risks involved.

Interpreting Diagrams The abnormalblood vessels form on the retina.Visual

For EnrichmentHave interested students interview a physician who performs laser eyesurgery or a person who has under-gone the procedure. Students might ask about the requirements, risks, andbenefits of the surgery, as well as any drawbacks. Afterwards, students can report to the class about what they learn.Interpersonal

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Answer to . . .

20/20 vision means that you can recognize

objects of a certain size at a distance of 20 feet.

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592 Chapter 19

Section 19.4 Assessment

Reviewing Concepts1. List, in order, the parts of the eye that

light encounters.

2. What are three common problems thataffect vision?

3. What is the eye’s blind spot?

4. Which kind of lens can correct nearsighted-ness? Farsightedness?

Critical Thinking5. Inferring Why is the cornea transparent?

6. Comparing and Contrasting How isthe structure of the eye similar to that of asimple camera?

7. Making Judgments Suppose that whenyou sit at the back of the room, you can’tread the chalkboard. What vision problemmight you have?

8. Connecting Concepts The index ofrefraction of the cornea is about the same asthat of water. Use this fact to explain why youcannot see as well in water as in air.

Farsightedness Blurred images can also bea result of farsightedness. Farsightedness is acondition that causes nearby objects to appearblurry. Common causes of farsightedness areeither a cornea that is not curved enough or aneyeball that is too short. In either case, the rays oflight focus too far from the lens. As a result, theimage focuses beyond the retina. A farsightedperson can see distant objects clearly, but nearbyobjects seem blurred. Farsightedness is usuallycorrected by placing a converging (convex) lensin front of the eye. The lens bends the light raystoward each other before they enter the eye.Because the rays entering the eye are converging,the image is formed on the retina instead ofbehind the retina. Figure 19 shows a diagram ofa farsighted eye and how it can be corrected.

Astigmatism For clear vision, the lens of the eye and the corneamust be properly shaped. When the cornea or lens is misshapen, adefect in vision called astigmatism results. Astigmatism is a conditionin which objects at any distance appear blurry because the cornea orlens is misshapen. Just as with an eyeball that is too long or too short,this irregularity can prevent light from focusing properly on the retina.The result is that the lens has two different focal points, causing dis-tortion or blurring of the image. Specialized eyeglass lens shapes areused to correct astigmatism.

Steps in a Process Write a paragraphdescribing how a light ray passes throughthe eye and results in vision.

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Image forms behind retina.

Problem: Farsightedness (Eyeball is too short.)

Image forms on retina.

Convex lens

Correction: Eyeglasses with convex lenses

Figure 19 Farsightedness occurswhen an image is not focusedbefore it reaches the retina.Farsightedness can be correctedby using a converging (convex)lens in front of the eye. Comparing and ContrastingIn what ways are farsightednessand nearsightedness similar?

592 Chapter 19

ASSESSEvaluate UnderstandingRandomly quiz students about the parts of the eye and their function. Have students draw diagrams showingvision correction for nearsightednessand farsightedness using convex andconcave lenses.

ReteachReview the structure of the eye usingFigure 17. Have students review Figures18 and 19 and discuss how the shape of the eyeball affects the eye’s ability to focus.

Student paragraphs will vary but must include all of the parts of the eye (at least the cornea, iris, pupil, lens,and retina) and their functions. Moredetailed descriptions may include theaqueous humor, vitreous humor, rods,cones, and optic nerve.

If your class subscribesto the Interactive Textbook, use it toreview key concepts in Section 19.4.

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Section 19.4 (continued)

6. Light enters through an opening and isfocused by a lens onto a surface where animage is formed.7. You are probably nearsighted, that is,unable to see objects far away.8. Light passing from water into your eye isnot bent as much as it passes through thecornea as it is when light passes from air intoyour eye. This difference in the bending of the light results in the image not being infocus when it strikes the retina.

Section 19.4 Assessment

1. Cornea, aqueous humor, iris, lens, vitreous humor2. Nearsightedness, farsightedness, astigmatism 3. The eye’s blind spot is the area of the retina where nerve endings come together to form the optic nerve. 4. Nearsightedness: concave lens;farsightedness: convex lens5. The cornea must be transparent in order forlight to pass through it into the rest of the eye.

Answer to . . .

Figure 19 Both conditions result from length of the eyeball or thecurvature of the lens.