MHR • Unit 2 Opticsmisscooper333.weebly.com/uploads/2/4/4/0/24403885/bc8... · 2020-03-01 · 170...

H ave you ever stood between two mirrors that faced each other andnoticed that your reflection repeated over and over? This is called an

infinite regress. Notice that none of the images of the girl in the photographare exactly identical. She appears to be farther away in the last image on theright, as well as darker. This change in appearance is a result of the way lightbehaves when it bounces from mirror to mirror. Light obeys many rules, and when the rules are understood, light can be put to work for us. In thischapter, you can learn some of the rules that govern the interaction of lightwith both mirrors and lenses.

166 MHR • Unit 2 Optics

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Chapter 5 Optical systems make use of mirrors and lenses. • MHR 167

What You Will Learn

In this chapter, you will• describe the behaviour of light using

a ray model• observe how light reflects off

different surfaces• discover how to use the law of reflection

to describe the behaviour of light• investigate ways in which mirrors and

lenses can be used to form images• explain how properties of light rays are

used in designing optical instruments

Why It Is Important

How light reflects off a surface into your eyesdetermines the reflection that you see. Mirrorsenable you to see yourself and objects behindyou, and to reflect beams of light. Lenses areused to focus light and form images.

Skills You Will Use

In this chapter, you will• observe images formed by curved mirrors• measure the angles of incident and

reflected rays• classify objects on their ability to

transmit light• model light using ray diagrams

FOLDABLES TM

Reading & StudySkills

Make the following Foldable to take notes on

what you will learn in Chapter 5.

STEP 1 Collect 3 sheets of paper and layerthem about 2.5 cmapart vertically. (Hint:from the tip of yourindex finger to yourfirst knuckle is about2.5 cm.) Keep theedges level.

STEP 2 Fold up the bottom edges of thepaper to form 6 tabs.

STEP 3 Fold the papersand crease wellto hold the tabsin place. Staplealong the fold.

STEP 4 Label the tabs as shown.(Note: the first tab will be larger than shown here.)

Summarize As you read the chapter,summarize what you learn under theappropriate tabs.

Optical systems make useof mirrors and lenses.

Behaviour of Light Beams

Reflection and Surfaces

Law of Reflection

Mirrors and Lenses

Optical Instruments

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The ray model of light can be used to understand how light moves in straight lines,

reflects off mirrors, and refracts through lenses. Materials can be classified as

opaque, translucent, and transparent depending on their ability to block, obscure,

or transmit light. Mirrors reflect light rays according to the law of reflection, which

states that the angle of incidence equals the angle of reflection. Refraction occurs

when light rays pass between two materials of different density. When this happens,

the direction and speed of a light ray change in a predictable way.

Sir Isaac Newton believed that light is a stream of fast-moving,unimaginably tiny particles. For example, a lantern flame was thoughtto release tiny particles of light, which travelled in a perfectly straightline until they entered an eye, where they were absorbed to make animage. This model came to be called the particle model of light, andparts of the model are still in use today.

However, light also has properties that are best described usingwaves, such as the use of wavelength and frequency to account for the different colours of light. You studied the wave model of light in Chapter 4. The particle model and the wave model correctlydescribe some properties of light, but neither one describes all oflight’s properties.

For the study of optics, especially when looking at the behaviourof light when it reflects off mirrors (see Figure 5.1) and passesthrough lenses, it is very helpful to use a simplified model called theray model of light. In the ray model, light is simply represented as astraight line, or ray, that shows the direction the light wave istravelling (see Figure 5.2).

The Ray Model of Light5.1

Key Termsangle of incidenceangle of reflectionangle of refractionnormalopaquetranslucenttransparent


Figure 5.1 In order for you to see such a clear image in the mirror, reflected light must followa very precise pattern.

Figure 5.2 A ray is an imaginaryline showing the direction in whichlight is travelling.

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Figure 5.3 In order for you to see an object, it must reflect some light back to your eyes.

Absorb, Reflect, Transmit 5-1

When light strikes an object, the light might beabsorbed, reflected, and/or transmitted. In this activity,you will classify a variety of objects based on theirability to transmit light.

Materials• variety of objects, such as a block of wood;

thin and thick blocks of wax; prisms of tinted,frosted, and clear glass or Plexiglas; petri dishes of water; milk

What to Do1. Create a table listing those materials that mostly

absorb light (opaque), mostly transmit light but obscure the image (translucent), or mostly transmitlight and allow the image to pass through(transparent).

2. Place various objects on an overhead projector.Classify the objects based on your observations.

What Did You Find Out?1. Based on the objects you have classified

as “mostly absorb light,” how would you define opaque?

2. Distinguish between the terms “translucent”and “transparent.”

Find Out ACTIVITY

Light and MatterOne use for the ray model is to help in understanding what happenswhen light energy reaches different materials. Imagine you are lookingaround your darkened room at night (see Figure 5.3). After your eyesadjust to the darkness, you begin to recognize some familiar objects.You know that some of the objects are brightly coloured, but theylook grey or black in the dim light. You can no longer tell thedifference between an orange shirt and a green shirt. What you seedepends on the amount of light in the room and the colour of theobjects. The type of matter in an object determines the amount oflight it absorbs, reflects, and transmits.

Did You Know?

Light can bend around corners!When a water wave hits the end of a breakwater, some partof the wave curves aroundbehind it. All waves go aroundedges a little bit, and so doeslight. For this reason no shadowcan be perfectly sharp. Forexample, if a laser light is shoneon a coin, the shadow of thecoin will be visibly fuzzy, as in the picture below.

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Transparent

Some materials will transmit light, which means that light can getthrough them without being completely absorbed. When light passesthrough clear materials, the rays continue along their path. We saythese materials are transparent. A transparent material allows light topass through it freely. Only a small amount of light is absorbed andreflected. Objects can be clearly seen through transparent materials,such as the candle in the transparent candleholder in Figure 5.4A. Air,water, and window glass are all examples of transparent materials.

Translucent

A ray diagram can show the difference between a transparent materialand a translucent material (see Figure 5.5). In a translucent material,such as frosted glass or a lampshade, most light rays get through, butare scattered in all directions. Translucent materials, like thecandleholder in Figure 5.4B, do not allow objects to be seen distinctly.Translucent glass is often used in bathroom windows to let in lightwithout losing privacy.

Opaque

An opaque material prevents any light from passing through it. Forexample, the material in the candleholder in Figure 5.4C only absorbsand reflects light—no light passes through it.

translucentopaquetransparent

Figure 5.4 Thesecandleholders have differentlight-transmitting properties.

Figure 5.5 Light travels in straight lines until it strikes something.

You may have seen a one-way mirror (sometimes calleda two-way mirror). If youstand on the brightly lit sideof the mirror you see yourown reflection. If you standon the darker side of themirror you can see through it,like a transparent window.Find out how it is possible tosee through one way but not both ways. Go towww.bcscience8.ca.

internet connect

A. Transparent

B. Translucent

C. Opaque

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lightsource

solid objects screen

lightsource

solid objects

screen

bright

bright

bright

shadow

shadow

Reading Check

1. What are three uses for the ray model?2. How is an opaque material different from a translucent material?3. How is a translucent material different from a transparent material?4. Is a glass of water with red food colouring in it translucent or

transparent? Explain.5. What is the relationship between the size of the shadow and the

distance of the object from the light source?

Figure 5.7A A ray diagram shows how the distance from thelight source affects the size of the shadow that an object makes.The smaller object casts the larger shadow because it is closer tothe light source.

Figure 5.7B To make ray diagrams easier to draw and tovisualize, you usually draw them as though you were looking atthe objects from the side. You can represent the light sourcewith a dot.

ShadowsYou can also use the ray model to predictwhere shadows will form and how large theywill be. For example, when you are walkingaway from the Sun during sunset, your shadowbecomes much longer than you are tall (seeFigure 5.6). In the ray diagram, your bodycasts a shadow because it blocks the light raysstriking you. The light rays on either side ofyou continue in a straight line until they hitthe ground. Figure 5.7 shows how a raydiagram can be used to show how the size ofshadows is related to the distance of the objectfrom the light source.

Figure 5.6 Ray diagrams can show how shadows are cast.

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Light Can Be ReflectedThis book uses black letters printed on white paper. The black ink isopaque because all the light falling on the ink is absorbed. But thewhite paper reflects all of the light that falls on it. Does that mean the white paper is a mirror? If so, why can you not see your reflectionin the white parts of the page?

To act as a mirror, the surface needs to be smooth compared tothe wavelength of the light striking the surface (see Figure 5.8A).Even though the page may feel smooth, a photograph taken througha microscope reveals the surface is actually not very smooth at all (seeFigure 5.8B). The ray diagram shows that the light rays bounce offrandomly at all angles, giving the paper the appearance of beingtranslucent (see Figure 5.8C).

(C) Rough surfaces appear to reflect light randomly.

smooth,flat reflectingsurface

roughreflectingsurface

(B) Scanning electron micrograph of the surface of paper

A

B

C

Find Out Activity 5-2 on page 176

Conduct an Investigation 5-5on page 178

Suggested Activities

For more examples of electronmicrographs, see Section 1.1.

Connection

Figure 5.8

(A) Smooth surfaces reflect all light uniformly.

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Neil Armstrong, the firstperson to walk on the Moon,placed a special kind of mirror on the Moon’ssurface. Scientists on Earthregularly shine a laser onthis mirror to measure thedistance from Earth to theMoon. Find out how thisspecial mirror works. Go towww.bcscience8.ca.

internet connect


The Law of ReflectionHow does light reflect off a mirror? It is helpful to think about how alight ray is similar to a water wave bouncing off a solid barrier.Imagine a great rock wall rising high out of the water. If waves strikesuch a barrier head on, the waves will bounce straight back in thereverse direction. However, if a wave strikes the barrier from an angle,then it will also bounce off at an angle—at precisely the same angle asthe incoming wave that struck the barrier.

The incoming ray is called the incident ray. The ray that bounces off the barrier is called the reflected ray. Notice in Figure 5.9 that a dotted line has been drawn at right angles to the solid barrier. This line is called the normal. The normal is an imaginary line that isperpendicular to the boundary between two materials (such as air and glass) and intersects the point at which the incident ray reaches the boundary.

The angle formed by the incident beam and the normal is theangle of incidence, labelled i. The angle formed by the reflectedbeam and the normal is the angle of reflection, labelled r. Noticethat the angle is always measured from the normal line to the ray, notfrom the mirror to the ray. Observations for all types of surfaces haveshown, without exception, that the angle of reflection is the same asthe angle of incidence. Therefore, this observation is considered to bea law. You can state the law of reflection as “the angle of reflectionequals the angle of incidence.” For example, if the angle of incidence,i, is 60º then the angle of reflection, r, will be 60º.

Did You Know?

Objects that bounce off asurface sometimes behave likewaves that are reflected from asurface. For example, suppose you throw a bounce pass whileplaying basketball. The anglebetween the ball’s direction andthe normal to the floor is thesame before and after itbounces.

normal

angle ofreflection

angle ofincidence

r i

incident rayreflected ray

reflecting surface

Figure 5.9 Light reflected from any surface follows the law of reflection.

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05 A

normal normal

Rair

waterR

Section 7.2 has moreinformation about density.

Connection

Word Connect

Density is a measure of howclosely the particles in amaterial are packed together.

Figure 5.11A When light rays travel fromair to water, they slow down and bendtoward normal. R is the angle of refraction.

Figure 5.11B When light rays travel fromwater to air, they speed up and bend awayfrom normal.

Light Can Be RefractedRecall from Chapter 4 that light can be bent, or refracted, if it changesspeed as it travels from one medium into another. You can picture thisprocess by imagining five friends all walking abreast with their elbowslocked (see Figure 5.10). If the people on one end of the line slowdown, but the people on the other end do not, the line will turn.Then, if they all slow to the same speed, they will continue to move inthe new direction.

When light rays move from air into glass, they slow down andchange direction because the glass is denser than air. Once inside theglass, the light rays move in a straight line. But if the light rays leavethe glass and move back into the air, where they can travel faster, theywill change direction again. The angle of refraction (R) is the angle of a ray of light emerging from the boundary between two materials,such as from air into glass, measured between the refracted ray and the normal.

Figure 5.11A shows what happens when a light ray passes into amedium in which it slows down. The light ray is refracted toward thenormal. Figure 5.11B shows what happens when a light ray passes into a medium in which it speeds up. Then the light ray is refracted awayfrom the normal.

Figure 5.10 If only one part ofa line slows down, the linechanges direction.

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A numerical way to measurethe ability of a transparentmaterial to refract light iscalled the index of refraction.Empty space has an index of1.0, and water has an index of about 1.3. Diamond isextremely refractive and hasan index of 2.4. There is a very interesting connectionbetween the speed of light ina material and its refractiveindex. Find out about thisrelationship. Go towww.bcscience8.ca.

Refraction of Light in WaterIf you have ever stood in a pool or water and tried to reach an objecton the bottom, you may have been surprised that the object was notwhere you expected it to be. Figure 5.12 shows how refraction causesthis illusion. The light rays reflected from the fish in Figure 5.12 arerefracted away from the normal as they pass from water to air andenter your eyes. However, your brain assumes that all light rays havetravelled in a straight line. The light rays that enter your eyes seem tohave come from a fish that was higher in the water.

Refraction of Light in AirRefraction can also occur when lighttravels through air at differenttemperatures. Warm air is less densethan cold air. Light bends as it travelsthrough different densities of air. Therefraction of light through air canresult in a mirage, which is amisleading appearance or illusion.

Have you ever been driving alonga highway on a hot summer day andnoticed what looked like pools ofwater lying ahead? However, whenyou got close to the pools, theymysteriously disappeared. You wereseeing a mirage. In this example, theair closer to the ground is hotter andless dense than air higher up. As a result, light from the sky directedat the ground is bent upward as it enters the less dense air. The“pools of water” were actually images of the sky refracted by warm airnear the ground (see Figure 5.13).

Reading Check

1. Why does a white page not reflect like a mirror?2. What is the difference between the incident ray and the

reflected ray?3. What point does the normal intersect?4. What does the angle of incidence always equal?5. What happens when light rays travel from water into air?6. Why do objects underwater seem closer to the surface

than they are?7. Why does the highway ahead of you sometimes look wet when it

is actually dry?

Figure 5.12 Light rays from thefish bend away from the normal asthey pass from water to air. Thismakes the fish seem closer to thesurface than it really is.

apparent position

actual position

Find Out Activity 5-3 on page 177Find Out Activity 5-4 on page 177


Figure 5.13 Refracted light cancreate a mirage.

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When Light Reflects 5-2

In this activity, you will observe whether light reflectsoff liquid surfaces according to the same principles aswhen it reflects off a solid mirror surface.

Materials • clear plastic cup • water• paper• ruler• wooden pencil

What to Do1. Fill the cup about three quarters full of water.

Place the cup on a level surface.

2. Observe the surface of the water. Move your headaround until you can see a reflection of the lightsoverhead, or a reflection of a window.

3. Make a simple ray diagram to record the directionin which light travels before it reaches your eye.Show and label the positions of the light source,the surface of the water, and your eye. This drawingshould show the situation as someone wouldobserve it from the side.

4. Move the cup of water to the edge of the desk ortable. Wait until the water stops jiggling. Crouchdown so that you can look up at the bottom of the water’s surface.

5. Slide a pencil across the desk toward the cup andyour eye. Move the pencil along the desk surface until you can see a reflection of the pencil in thelower surface of the water.

6. Make a simple ray diagram to record the path ofthe light from the pencil to your eye.

7. Look at the reflection of the pencil as you did instep 5, but now gently tap on the rim of the glass.Record your observations.

8. Wipe up any spills. Clean up and put away theequipment you have used.

What Did You Find Out?1. (a) In steps 4 and 5, what happened to some of

the light that struck the lower flat surface between the air and water?

(b) What common device depends on this behaviour of light?

2. (a) In step 7, what change occurred in the surface of the water when you tapped on the glass?

(b) What happened to the reflection of the pencil?

3. During reflection, what happens to the direction inwhich light travels?

4. Does light reflect off liquid surfaces according tothe same principles as when it reflects off a solidmirror surface? Explain your answer.

Find Out ACTIVITY

Slide a pencil across the desk toward the cup.


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Observing Refraction in Water5-4

In this activity, you will observe what happens whenlight rays move from water into air.

Materials • penny• short opaque cup or jar lid• water

What to Do1. Place a penny at the bottom of a short,

opaque cup or jar lid. Set the cup on a tablein front of you.

2. Have a partner slowly slide the cup away from you until you cannot see the penny.

3. Without disturbing the penny or the cup andwithout moving your position, have your partnerslowly pour water into the cup until you can seethe penny.

4. Reverse roles, and repeat the experiment.

5. Clean up any spills.

What Did You Find Out?1. What happens to the path of light from the water

to the air?

2. Sketch the light path from the penny to your eye

(a) before the water was added

(b) after the water was added

Find Out ACTIVITY


Refraction 5-3

In this activity, you will observe what happens whenlight rays strike a transparent object.

Materials • ray box• rectangular block of glass or transparent plastic

• ruler

• protractor

What to Do1. Lay the block of glass flat on the table. Shine the

light from the ray box into the side of the block.Change the angle of the block in relation to theincident beam of light from the ray box. Follow the light ray through the block and then out the far side.

2. Set the glass block in one place. With reference tothe point where the light enters the glass block,draw and label the incident ray, the refracted ray,the normal line, the angle of incidence, and theangle of refraction.

3. Continue the diagram showing the light ray as itpasses out of the glass block. Draw and label theincident ray, the refracted ray, the normal line, theangle of incidence, and the angle of refraction.

What Did You Find Out?1. (a) Does the light ray passing through the glass

block change direction at the surface of the glass or somewhere in the middle?

(b) How do you know?

2. (a) Does the light ray entering the glass block bend toward or away from the normal?

(b) Does the light ray leaving the glass block bend toward or away from the normal?

(c) What can you infer from your answers to (a)and (b) about the speed of light through glassand through air?

Find Out ACTIVITY

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Inferring the Law of Reflection5-5

Skill Check

• Observing

• Measuring

• Classifying

• Evaluating information


Safety • The edges of the mirror

may be sharp. Be carefulnot to cut yourself.

Materials• ray box• small plane mirror (about

5 cm by 15 cm) withsupport stand

• small object with a pointedend such as a short pencilor a nail (the object shouldbe shorter than the mirror)

• protractor• ruler• pencil

• sheet of blank paper (lettersize)

When you look in a mirror, light reflects off your face in all directions. Some of thislight reflects off the mirror into your eyes. This light must follow a consistentpattern because you always see the same image of your face in a mirror.

In this activity, you will be guided through the process of making a raydiagram. When your diagram is complete, you will analyze the relationshipbetween incident and reflected rays. From these data, you will be able to infer thelaw of reflection.

QuestionHow does light behave when it reflects off a flat surface?

HypothesisWhat is the relationship between the angle of incidence and the angle ofreflection? Make a hypothesis and test it.

Procedure1. Near the middle of the blank sheet

of paper, draw a straight line torepresent the reflecting surface ofthe plane mirror. (This is usually theback surface of the mirror becausethe front surface is a sheet ofprotective glass.) Label the line“plane mirror.”

2. Lay the small object on the paper.Place it about 5–10 cm in front ofthe line representing the planemirror. Trace the shape of the object.Label the pointed end “P” and theblunt end “O.”

3. Remove the object. Draw twodifferent straight lines from point Pto the line labelled “plane mirror.”On each line, draw an arrowheadpointing toward the mirror. Theselines represent the paths of twoincident light rays that travel fromthe object to the mirror.

4. Carefully place the mirror in itsstand on the sheet of paper. Makesure the mirror’s reflecting surface isexactly along the line you drew instep 1.

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Conduct an INVESTIGATION

Inquiry Focus

5. Use the ray box to shine a thin beam of lightalong one of the incident rays that you drewfrom point P. Mark the reflected ray with aseries of dots along the path of the reflectedlight.

6. Remove the mirror and the ray box. Locate thereflected ray by drawing a line through the dotsand ending at the mirror. On this line draw anarrowhead pointing away from the mirror toindicate that this is a reflected ray.

7. At the point where the incident ray and itscorresponding reflected ray meet the mirror,draw a line at 90° to the mirror. Label this linethe “normal.”

8. Measure and record the angle of incidence (theangle between the normal and the incident ray).

9. Measure and record the angle of reflection (theangle between the normal and the reflectedray).

10. Repeat steps 4 to 9 for the second incident rayfrom point P.

11. If time permits, repeat steps 3 to 9 for point O.

12. Place the mirror and the object back on thesheet of paper. Observe the image of the objectand the reflected rays that you drew. From whatpoint do the reflected rays seem to come?

Analyze1. You drew two rays from point P to the mirror. If

you had enough time, how many rays could youhave drawn between point P and the mirror? (Youdo not need to draw them all, just think aboutthem and answer the question.)

2. How does the angle of reflection compare to theangle of incidence?

3. Extend each reflected ray behind the mirror, usinga dotted line. Label the point where these twodotted lines meet as P. This is the location of theimage of point P. Measure the perpendiculardistance between:

(a) point P (the object) and the mirror

(b) point P (the image) and the mirror

How do these distances compare?

Conclude and Apply1. From your data, describe the pattern relating the

angle of incidence and the angle of reflection.Does this pattern agree with your hypothesis?Explain.

2. You were able to draw the incident ray, thereflected ray, and the normal all on the surface ofa flat piece of paper. What name is given to a flatsurface? Make up a statement that describes thisrelationship mathematically.

3. Based on your measurements, how does thedistance from the image to the mirror comparewith the distance from the object to the mirror?

P

O

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Sun's rays

Syene

Alexandria

How Big Is Earth?What is the circumference of Earth? Today you might use the Internet to find the answer. But 2250 years ago,you could have asked a man named Eratosthenes ofAlexandria, Egypt. He had just figured it out for himself,and was the first person to do so. Eratosthenes was amathematician, a geographer, and the director of thegreat library of Alexandria, the greatest centre ofknowledge of the ancient world.

How did Eratosthenes measure the circumferenceof Earth? He used a light ray experiment and somegeometry. Eratosthenes knew that if you looked down a well in the southern city of Syene at noon on thelongest day of the year, you could see a reflection of the Sun. This meant that at that moment the Sun wasdirectly overhead, and that flagpoles, for example,did not cast a shadow.

At his more northerly home, in Alexandria, at exactlythe same time, you could not see to the bottom of a well,and flagpoles did cast a shadow. Eratosthenes measuredsome angles and drew a diagram and found somethingstartling. In geometry, it is known that when two parallellines are crossed by a third line, some of the angles thatare formed are equal. In particular, the alternate interiorangles are equal.

How is this information useful? Flagpoles in bothAlexandria and Syene point directly to the centre ofEarth. They meet at an angle: an alternate interior angle.The other alternate interior angle is found by looking atthe light ray that passes the top of the flag pole inAlexandria and forms the shadow on the ground.

This is the angle that Eratosthenes measured. Hefound the angle to be 7.2°. Since a complete circle is360°, the two cities were 7.2 ÷ 360 apart, or about a 50th of the distance around Earth. The distance fromSyene to Alexandria was about 800 km. The circumferenceof Earth must therefore be 50 times longer, or 40 000 km.We know that Earth’s circumference varies betweenabout 40 008 km and 40 075 km depending on where itis measured. Eratosthenes got the right answer, to within1 percent—an amazing feat!

Questions

1. If flagpoles in Syene and Alexandria both pointdirectly to the centre of Earth, why are theflagpoles not parallel?

2. Why is the angle formed by the flagpoles and the centre of Earth the same as the angle formed by a ray of sunlight and the flagpole in Alexandria?

3. If the distance between Syene and Alexandria hadbeen 500 km, with the same alternate interiorangle, what would be Earth's circumference?

The alternate interior angle passes the top of theflag pole and forms the shadow on the ground.

Eratosthenes was a Greek mathematician,born in North Africa in 276 B.C.E.

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Checking Concepts1. Compare and contrast the following terms:

(a) translucent, transparent(b) transmit, absorb(c) reflect, refract

2. The angle of incidence of a light ray is 43°.What is the angle of reflection?

3. Light slows down as it moves from air intowater. Explain how this causes the directionof a light ray to change.

4. Why can you see your reflection in a smoothpiece of aluminum foil, but not in acrumpled ball of foil?

5. A glass window is transparent, but at nightyou can see your reflection in it. Why?

Understanding Key Ideas6. Explain why you are more likely to see a

mirage on a hot day than on a mild day.7. (a) What is meant by the term “normal” in a

ray diagram that represents reflection? (b) Does the meaning of normal change

when representing refraction? Explain.8. Copy the diagram below into your

notebook. Explain, using a light ray diagram,why the reflection of the letter d looks likethe letter b.

9. (a) Draw a line representing a flat mirror.Then add a normal line perpendicular to the mirror. Draw a light rayapproaching and then touching the mirror at the same place as the normal line. Complete the ray diagram showingthe ray’s reflection.

(b) Label the incident ray, normal, reflectedray, angle of incidence, and angle ofreflection.

10. A semi-transparent mirror will both reflectand refract an incident light ray. Draw a straight line representing the surface of a glass mirror. Show a light ray striking the surface of the mirror at a slightly downwardangle. The ray splits into a reflected ray,which bounces back, and a refracted ray,which is transmitted through the glass.When drawing your sketch, make sure touse the laws of reflection. The refracted raywill bend toward the normal since glass isdenser than air.

11. Why is it desirable that the pages of a bookbe rough rather than smooth and glossy?

In Chapter 4, you studied forms ofelectromagnetic radiation, such as X rays and gamma rays. Do you think these invisibleforms of radiation have the property ofreflection? Support your answer.

Pause and Reflect


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Reflections of Reflections 5-6

In this activity, you will find out how many reflectionsyou can see in two plane mirrors.

Materials• 2 plane mirrors • masking tape• protractor • paper clip

Safety

• Handle glass mirrors and bent paper clips carefully.

What to Do1. Create a table to record your data. Give your table

a title.

2. Lay one mirror on top of the other with the mirror surfaces inward. Tape them together so they will open and close. Use tape to label them“L” (left) and “R (right).”

3. Stand the mirrors up on a sheet of paper. Using aprotractor, close the mirrors to an angle of 72°.

4. Bend one leg of a paper clip up 90° and place itclose to the front of the R mirror.

5. Count the number of images of the clip you see inthe R and L mirrors. Record these numbers in yourdata table.

6. Hold the R mirror still. Slowly open the L mirror to90°. Count and record the images of the paper clipin each mirror.

7. Hold the R mirror still. Slowly open the L mirror to120°. Count and record the images of the paperclip in each mirror.

What Did You Find Out?1. What is the relationship between the number of

reflections and the angle between the two mirrors?

2. How could you use two mirrors to see a reflectionof the back of your head?

Find Out ACTIVITY

All mirrors reflect light according to the law of reflection. Plane mirrors form an

image that is upright and appears to be as far behind the mirror as the object is in

front of it. Depending on the distance of the object, a concave mirror can form an

image that is inverted or right side up, and that can be larger or smaller than the

object. Convex mirrors form images that are upright and smaller than the object.

You can see yourself as you glance into a quiet pool of water or walkpast a shop window. You can see unusual reflections of yourself in thewavy mirrors at amusement parks. You can even see reflections ofyourself in a spoon. Most of the time, however, you probably look foryour image in a flat, smooth mirror called a plane mirror.

Using Mirrors to Form Images5.2

Key Termsconcaveconvergingconvexdivergingfocal point


Count the images in each mirror.

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object plane mirror image

Figure 5.14 Only a small fraction of the light reflecting from an object enters the eye of the observer.

Did You Know?

Plane MirrorsLooking at yourself in a plane mirror, you can see that your imageappears to be the same distance behind the mirror as you are in frontof the mirror. How could you test this? Place a ruler between you andthe mirror. Where does the image touch the ruler? You also see thatyour image is oriented as you are and matches your size. This type ofreflection is where the expression “mirror image” comes from. If youmove toward the mirror, your image moves toward the mirror. If youmove away, your image also moves away.

How do reflected rays form an image that we can see in a mirror?Study Figure 5.14 to answer this question. Light from a lamp shines on a blueberry. This light reflects off all points on the blueberry, in all directions. In the figure, only the rays coming from one point areshown. All of the rays from the blueberry that strike the mirror reflectaccording to the law of reflection. The rays that reach your eye appearto be coming from a point behind the mirror. The same process occursfor every point on the blueberry. Your brain “knows” that light travelsin straight lines. Therefore, your brain interprets the pattern of lightthat reaches your eye as an image of a blueberry behind the mirror. In fact, it might even be possible to trick the observer into thinking the blueberry was behind a glass window, rather than in front of a very good mirror. A house of mirrors uses this trick to create a maze.

The mirror on the Hubble Space Telescope is one of thesmoothest mirrors ever made.If the mirror were as large asEarth, the biggest bump on itwould be only 15 cm tall.

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Notice that the points appear to be coming from behind themirror. Each point appears to be coming from a point that is as farbehind the mirror as the real point is in front of the mirror. Alsonotice that the three points are exactly the same distance apart in theimage as they are on the object, the bird. These observations explainwhy an image in a plane mirror is the same size as the object andappears to be the same distance from the mirror as the object.

Image orientation

A plane mirror produces an image withthe same orientation as the object. Ifyou are standing on your feet, a planemirror produces an image of youstanding on your feet. If you are doinga headstand, the mirror shows youdoing a headstand. However, there is adifference between you and theappearance of your image in the mirror.Follow the sight lines in Figure 5.16.The ray that diverges from the righthand of the boy converges at whatappears to be the left hand of hisimage. Left and right appear to bereversed by a plane mirror.

bird image of birdplane mirror

Figure 5.15 We know that whatwe see in a mirror is just animage. However, a pet bird willchatter for hours to a “friend” inthe mirror.

Image size and distance

Another important feature of images in plane mirrors is demonstratedin Figure 5.15. Rays are shown coming from three different points onthe bird. These rays reflect off the mirror and back to the bird’s eye.

e

image object

mirror

Figure 5.16 When the boy blinks his right eye, the left eye of hisimage blinks.

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Concave MirrorsA concave mirror is a mirror that curves inward. Concave mirrors,like plane mirrors, reflect light rays to form images. The difference isthat the curved surface of a concave mirror reflects light in a uniqueway. As shown in Figure 5.17, parallel light rays bounce off thecurved surface of a concave mirror and then meet at a single pointcalled the focal point. Light rays that are coming together at a focalpoint are described as converging.

The image formed by a concave mirror depends on how far theobject is from the focal point of the mirror (see Figure 5.18). If adistant object is reflected in a concave mirror, its image is small andupside down. As the object approaches the focal point, its imageremains inverted but gets ever larger. If the object is between the focalpoint and the mirror, then the image appears to be larger than theobject and is upright.

Concave mirrors have many uses (see Figure 5.19). If a brightlight is placed at the focal point, then all the light rays bounce off themirror and are reflected parallel to each other. This makes an intense,focussed beam of light. Spotlights, flashlights, lighthouses, and carheadlights use this kind of mirror. The largest telescopes all useconcave mirrors to collect light because the mirror concentrates thelight so effectively. Shaving mirrors and make-up mirrors are alsoconcave mirrors. They form an enlarged, upright image of a person’sface so it is easier to see small details.

Figure 5.17 Light rays collectedby a concave mirror converge on a focal point before spreading out again.

focalpoint

Figure 5.19 The boy is between the concave mirror and its focal point.Figure 5.18 The image formed by a concavemirror depends on how far away the object is.

focal point

focal point

focal point(a)

(b)

(c)

object

object

object

A

B

C

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Find out about the centre of curvature and radius ofcurvature for a concave lens.What is the relationshipbetween the radius ofcurvature and the focallength? If a person stands at the centre of curvature infront of a large concavemirror, where will his or herimage form and what will beits size and orientation? Visitwww.bcscience8.ca.


Figure 5.20 The reflected raysfrom a convex mirror diverge anddo not meet.

focalpoint

Figure 5.21 Convex mirrors are used in stores as security mirrors (A), and in cars as rearviewand side-view mirrors (B).

Convex MirrorsA convex mirror is a mirror that curves outwards. Convex mirrors alsoreflect light rays to form an image, but they do so in an opposite wayto concave mirrors. A convex mirror reflects parallel light rays as if theycame from a focal point behind the mirror (see Figure 5.20). Lightrays that spread apart after reflecting are described as diverging. Theimage formed is always upright and smaller than the actual object.

The reflection from a convex mirror has two main characteristics:1. Objects appear to be smaller than they are.2. More objects can be seen in a convex mirror than in a plane

mirror of the same size.Security mirrors, such as those in convenience stores, are large

convex mirrors. Convex mirrors make it possible to monitor a largeregion of the store from a single location. Convex mirrors can alsowiden the view of traffic that can be seen in rearview or side-viewmirrors of automobiles. However, because distances and sizes seen in a convex mirror are not realistic, most convex side-view mirrors carry a printed warning that the objects viewed are closer than they appear to be (see Figure 5.21).

Reading Check

1. What size does the image in a plane mirror appear to be?2. What distance from the mirror does an image in a plane mirror

appear to be?3. How is a concave mirror shaped differently from a plane mirror?4. What are some uses for concave mirrors? 5. How is a convex mirror shaped differently from a plane mirror?6. What are some uses for convex mirrors?

Conduct an Investigation 5-7on page 187

Suggested Activity

A B

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Materials • ray box• convex mirror• concave mirror• ruler• protractor

A ray box can cast several light rays at the same time. This helps to visualizehow an image is changed as light rays are reflected from a curved mirror.

QuestionHow is an image affected when light rays from an object bounce off a curved mirror?

Procedure1. Use a ray box to shine several light rays at a concave mirror. Observe how

the rays are affected. Make a diagram of the ray paths.

2. Hold the concave mirror directly in front of you at arm’s length and viewyour own reflection. Bring the mirror gradually closer to one eye andobserve as the image of your eye disappears. Keep moving the mirror closeruntil the image of your eye reappears. Record your observations.

3. Shine the ray box at a convex mirror and observe how the rays are affected.Make a diagram of the ray paths.

4. Hold the convex mirror directly in front of you at arm’s length and view yourown reflection. Bring the mirror gradually closer to one eye. Observe theimage. Keep moving the mirror closer until the image of your eye reappears.Record your observations.

Analyze1. (a) Explain how the orientation of the image of your eye changes as the

concave mirror gets closer to your eye.

(b) Explain why there is a certain point at which the image of your eye disappears completely.

2. (a) Do the light rays reflecting off the convex mirror ever cross each other?Explain.

(b) Explain why your image never disappears and never flips over as youbring the convex mirror close to your eye.

3. Does the angle of incidence equal the angle of reflection in the case ofcurved mirrors?

Conclude and Apply1. Mirrors are placed behind the light in car headlight systems to reflect the

light ahead of the car. Explain why a concave mirror would be more usefulfor this purpose than a convex mirror.

2. Explain why an object appears farther away than it really is when the objectis viewed through a convex mirror.

Observing Images5-7

Skill Check

• Observing

• Communicating

• Explaining systems

• Evaluating information


Inquiry Focus


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Mirrors in Time and SpaceHave you looked at yourself in a mirror today? Ever since the first humans gazed at their images in a pond, mirrors have been used to tell us somethingabout ourselves.

The most ancient manufactured mirrors known are about 8000 years old and were found in Turkey. Thesemirrors were made of obsidian, which is a hard, blackglass produced from molten sand in the fiery heart ofvolcanoes and shot out of the volcano during eruptions.The glass was gathered and polished. In later ages in

the ancient world, copper,bronze, and other metals

were used for mirrors.Because the metalscould be melted andthen poured out, theyformed very flatsurfaces. Metal mirrorswere also resistant tobreakage.

Roman artisans made mirrors by covering one sideof a piece of glass with gold or silver, or mixtures ofmetals such as mercury and tin. Sharp, well-defined,reflected images were not possible until 1857, whenJean Foucault, a French scientist, developed a methodof coating glass with silver. High quality, inexpensivemirrors did not become available until around 1900when it became possible to make large amounts ofextremely flat plate glass.

Modern mirrors are produced by evaporatingaluminum or silver onto highly polished glass. Clearreflections in modern, optical instruments requiresmooth surfaces compared to the wavelengths being reflected.

Mirrors designed to go into space have a specialrestriction: they must be lightweight. If you have evertried to lift a large sheet of glass or a mirror, you willknow that large amounts of glass are very heavy. Thefirst optical space telescope was the Hubble SpaceTelescope, which was launched into space in 1990.Mirrors are now available that are 10 times lighterthan the one used in Hubble.

You probably know that aluminum is much lessdense than iron, which is why aluminum is commonlyused in aircraft. Beryllium is a metal that is even lessdense than aluminum, and research is under way tomake mirrors completely out of this metal.

The mirrors on space telescopes do not only lookout into space. More and more, we turn the mirrorsaround and point them back at Earth. We can usemirrors on space telescopes to measure the growthof cities, the destruction of rain forests, and themelting of glaciers. We can also see the majesty of ourplanet and the potential for preserving and improvingour environment. We humans have come a long wayfrom gazing into a pond. Have you looked at yourEarth in the mirror today?

Questions

1. What are some advantages of a metal mirrorcompared to one made of glass?

2. Mirrors located in orbit are in microgravity,which means they weigh almost nothing. Why,then, is it important to construct mirrors frommaterials like beryllium that are light in weight?

3. List five reasons (other than the ones listed inthe article) why it might be useful to be able tosee images of our Earth from space.

This Roman mirrorwas made over2000 years ago.

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When you look across a lake, you might seethe reflection of the distant mountains andtrees in the water. The image of the treesand mountains appears to be upside down.However, when you look straight down atthe surface of the lake, you see an uprightreflection of yourself. Why would your imagebe upright while the image of the mountainsis upside down?

Pause and Reflect


Checking Concepts1. Describe how your image changes as you

move closer to:(a) a plane mirror(b) a concave mirror(c) a convex mirror

2. One side of a soupspoon is convex and theother is concave. Imagine you are havingsoup and you lift the spoon out of the soupbowl, holding some soup. Is the part of thespoon touching the soup convex or concave?

3. Do convex and concave mirrors obey the lawof reflection? Explain.

4. Explain the difference between divergent andconvergent light rays.

5. Draw and label a mirror that produces: (a) divergent light rays(b) convergent light rays

6. Suppose you find a shiny metal bowl that hasbeen left outside in the sunlight. (a) Are you more likely to see the reflection

of direct sunlight by viewing the outside or the inside of the bowl?

(b) Is it more dangerous to look at theoutside or the inside of the bowl? Explain.

Understanding Key Ideas7. Why is an image in a plane mirror the same

size as the object that is reflected?8. List several uses of:

(a) plane mirrors(b) concave mirrors(c) convex mirrors

9. (a) Draw a ray diagram that shows anarrangement of mirrors that would allowyou to see the back of your own head.Draw the diagram as if looking downfrom above. The rays should leave theback of your head and end in your eye.Show the normal and angles of incidenceand reflection.

(b) Will left and right be reversed in theimage? Explain.

10. Design and label an arrangement of mirrorsto do each of the following: (a) see over the top of a fence without

having to raise your eyes above the top of the fence

(b) read a book by reflected light without having the words backwards in a“mirror image”

(c) collect and concentrate the Sun’s lightinto a small space and then conduct thelight around two corners to a solar panel

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Observing Light Rays 5-8

In this activity, you will observe how light rays refractas they pass through lenses.

Materials• ray box• concave lens• convex lens• printed page

What to Do 1. Shine the ray box at a concave lens. Observe how

the rays are affected. Draw your observations.

2. Look through the concave lens at some printedtext. Observe the appearance of the print. Drawyour observations.

3. Shine the ray box at the convex lens. Observe howthe rays are affected. Draw your observations.

4. Look through the convex lens at some printed text.Observe the appearance of the print. Draw yourobservations.

What Did You Find Out?1. Compare what you observed about the appearance

of the text with each of the two lenses.

2. Which type of lens would be best used as amagnifying glass? Why?

3. What might the other kind of lens be used for?

Find Out ACTIVITY

A lens is a piece of transparent material that can bend, or refract, light rays in useful

ways to help form a well-focussed image. Concave lenses are thinner in the middle

than at the edge. They are used to diverge light rays. Convex lenses are thicker in the

middle than at the edge. They are used to converge light rays.

Light rays refract through a piece of glass in a predictable way. Recallfrom Section 5.1 that when a light ray passes from air into a densermaterial, such as glass, it bends toward the normal. When the light raypasses out of the glass, back into the air, it bends away from thenormal. Using these facts about light it is possible to design andconstruct lenses. A lens is a curved piece of transparent material, such as glass or plastic, that refracts light in such a way as to convergeor diverge parallel light rays. The image that a lens forms depends on the shape of the lens. Like curved mirrors, a lens can be convex or concave.

Using Lenses to Form Images5.3

Key Termsconcave lensconvex lensfocal lengthlens


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Concave LensesConcave lenses are lenses that are thinner inthe middle than at the edge. As shown inFigure 5.22, light rays that pass through aconcave lens diverge. The rays are refractedoutward, and never meet at a focal point. The image formed is always upright andsmaller than the actual object (see Figure 5.23and Table 5.1). Concave lenses are used insome types of eyeglasses and some telescopes,and are often used in combination with other lenses.

Distance of Object from Lens

Any location

Type of Image Formed

Smaller, upright

Table 5.1 Images Formed by Concave Lenses

do

di

object

ray 2

ray 1

image F

F

Figure 5.23 Concave lenses produce images that are upright and smallercompared to their objects.

Figure 5.22 Light rays diverge when they pass through aconcave lens.

Did You Know?

Lenses have been made andused for hundreds of years. In1303, French physician Bernardof Gordon wrote of the use of lenses to correct eyesight.Around 1610, Galileo used two convex lenses to make a telescope, with which hediscovered the moons of Jupiter.

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Raindrops take on aspherical shape as they fall,which gives them the shapeof a convex lens. A drop ofwater sitting on a glass slidehas a nearly spherical shape.Investigate whether a waterdroplet or a glass bead ofthe same size would make agood magnifying lens. Startyour search atwww.bcscience8.ca.

internet connect


Figure 5.25 An image formed by a convex lens may be inverted, or flipped upside down.

Convex LensesConvex lenses are lenses that arethicker in the middle than at theedge. As shown in Figure 5.24,light rays that pass through a convexlens come together, or converge.When parallel rays strike a convexlens from one side, they will allcome together at the focal point ofthe lens. Light passing through thethicker, more curved areas of thelens will bend more than lightpassing through the flatter areas. A light ray that passes straightthrough the centre of the lens is not refracted. The image formed by aconvex lens depends on the positions of the lens and the object (see Figure 5.25).

Figure 5.24 Light rays converge whenthey pass through a convex lens.

ray B

ray Aobject

focalpoint

onefocal

length

twofocal

lengths

imageoptical axis

ray Afocalpoint

optical axis

imageray B

onefocal

length

twofocal

lengths

object

ray Afocalpoint

optical axisobject

image

onefocal length

ray B

A

B

C

When the candle ismore than two focallengths away from thelens, its image isreduced and upsidedown.

When the candle isbetween one and twofocal lengths from thelens, its image isenlarged and upsidedown.

When the candle isless than one focallength from the lens,its image is enlargedand upright.

ds

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Distance of Object from Lens

More than two focal lengthsBetween one and two focal lengthsObject at focal pointLess than one focal length

Type of Image Formed

Smaller, invertedLarger, invertedNo imageLarger upright

Table 5.2 Images Formed by Convex Lenses

Reading Check

1. What happens to parallel light rays that strike a concave lens?2. What happens to parallel light rays that strike a convex lens?3. What type of image is formed by a concave lens? 4. What determines the type of image that is formed by a

concave lens?

Focal Length in Convex LensesConvex lenses and concave mirrors share a similar property in that thelight rays converge at the focal point. The distance from the centre ofthe lens or mirror to the focal point is called the focal length (seeFigure 5.26). There is a mathematical relationship linking the distanceof the object in front of the lens to the distance of the image formedby the lens. • If the object is more than two focal lengths in front of the lens, the

image is smaller than the object and inverted. • If the object is moved closer to the lens so that it is one to two focal

lengths away, the image is larger than the object and still inverted. • If the object is very close, less than one focal length away, the

image appears to be located on the other side of the lens and isboth upright and larger than the object.

As summarized in Table 5.2, the type of image a convex lens formsdepends on where the object is relative to the focal point.

Eyeglasses would morecorrectly be called“eyeplastics” these days.Glass refracts well but isheavy and can shatter.The highest quality of plastic in widespread use for glassesis polycarbonate plastic. Findout what properties it has that makes it so useful inlenses. Start your search atwww.bcscience8.ca.

Figure 5.26 The focallength of a convex lens

focallength

focalpoint

Find Out Activity 5-9 on page 194Find Out Activity 5-10 on page 195


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Make a Model of a Projector 5-9

In this activity, you will examine how an image isaffected when seen through a beaker full of water.You will also use a lens to project the image of a lightfilament onto a screen. A light filament is the twistedwire inside a light bulb.

Safety

• Make sure the electrical cord does not get wet.

• Be careful not to burn yourself with the light bulb.

Materials• sheet of paper• felt pen• beaker• water• convex lens • unfrosted light bulb

What to Do1. Draw a series of arrows on a sheet of paper, as

shown, and then view the arrows through a beakerfull of water. Move the paper left and right andthen compare this to the movement of arrows seenthrough the beaker.

2. Darken the room and turn the light bulb on. Holdthe convex lens between the unfrosted light bulband a plain piece of paper.

3. Move the lens back and forth between the lightbulb and the piece of paper. Keep adjusting thedistance until you see a sharp image of thefilament. Note the size of the image compared to the actual size of the filament.

What Did You Find Out?1. In step 1:

(a) How did the orientation of the projected imageof the arrows compare with the actual arrowsside to side, and up and down?

(b) How did the projected image of the arrowscompare with the actual arrows in terms of size?

2. In step 3:

(a) How did the orientation of the projected imageof the filament compare with the actualfilament side to side, and up and down?

(b) How did the projected image of the filamentcompare with the actual filament in terms of size?

3. How is the beaker of water like a double convex lens?

Find Out ACTIVITY


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Safety

• Never look directly at the Sun with any camera,including the oneconstructed in this activity.

Materials • 2 tubes of different

diameters (from wrappingpaper, paper towels,aluminum foil or plasticwrap) or make 2 tubesusing tape and paper

• adhesive tape (with frostyappearance, not clear)

• scissors• aluminum foil• pushpin

A tiny hole can act like a lens.

QuestionCan a pinhole camera be used to make an image of a bright object such as alight filament or a television screen?

Procedure1. Obtain two tubes of different diameters so one can slide inside the other.

2. Completely cover one end of the smaller diameter tube with adhesive tapeby placing overlapping strips of tape together. The tape is the screen thatthe image will be projected on.

3. Completely cover one end of the larger diameter tube with aluminum foiland use tape to hold it in place. Use a pushpin to poke a hole in the foil.The hole in the foil acts like a lens.

4. Slide the smaller tube into the larger tube keeping the tape screen and thealuminum foil on the same side. Begin by sliding the tape right up againstthe foil.

5. You have just made a camera! Point your camera at a bright object such asa bare light bulb or a television that is turned on. CAUTION: Never lookdirectly at the Sun through any camera, including this one.

6. Slide the smaller tube away from the foil until the image comes into focus.A darkened room may be helpful for this. Is the image in the sameorientation as the object or is it inverted?

7. Rotate the camera as you view an image. Does the image rotate with the camera?

8. Clean up and put away the equipment you have used.

Analyze1. How would the letter d appear if viewed through your camera?

2. Explain, using a ray diagram, why the image formed in the camera is inverted.

Conclude and Apply1. Passing through a forest on a bright day, you notice that on the ground

right under some leaves there are many tiny images of the Sun. Explain how these images form.

Pinhole Camera5-10

Skill Check

• Observing

• Classifying

• Modelling

• Explaining systems


Inquiry Focus


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Gravitational Lenses Imagine that there is a region deep in space that you would like to explore with your telescope, but thedistance is just too great to see anything. What if youdiscovered that halfway between you and the objectthere was a huge magnifying glass that focussed thelight from the distant object right at Earth?

All objects have mass, and where there is massthere is gravity. Gravity not only holds you to Earthand keeps the Moon from flying out of its orbit, it also attracts light. The effect is small for small objectslike humans, planets, and individual stars. But gravitycan refract light rays passing by a galaxy by a hugeamount. When gravity causes many light rays to come together at one point, then we have a lens—a gravitational lens.

The photograph at the bottom left shows anEinstein ring. The gravitational lens is the bright galaxy in the centre. The blue ring is the distortedimage of another galaxy that is on the far side of the lens. The lens is actually in front of the distantblue-coloured galaxy. Light from the blue galaxypasses on all sides of the lens and is pulled togetheragain as it arrives at Earth.

The photograph above shows what appear to betwo smaller white galaxies on either side of the lens.Actually it is one galaxy that is as far behind the lensas we are in front of it. It may seem strange that weget two images, but some light travels above the lensand other light from the same source travels below thelens. The light from the white galaxy has beentravelling through space for a very long time. It tooktwo billion years to reach the lens, and another two billion years to reach Earth.

An Einstein ring

A white galaxy

What appears to be two galaxies is actually only one galaxy.

lens

we see galaxy here

we also see galaxy here

Earthrealdistantgalaxy

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Checking Concepts1. What is a lens? 2. (a) Make a sketch of three parallel light

rays passing through a concave lens. (b) Make a sketch of three parallel light

rays passing through a convex lens.3. Describe the image formed by a concave

lens.4. As an object comes closer to a convex lens

what happens to: (a) the size of the image?(b) whether the image is upright or upside

down?(c) the location of the image?

5. List two factors that affect the way thatlight is refracted through a lens.

6. List two uses of convex lenses.7. List two uses of concave lenses.

Understanding Key Ideas8. What is the difference between the way

parallel light rays are affected by a concavemirror and a concave lens?

9. Does a concave lens affect light more like aconcave mirror or a convex mirror? Explainyour answer.

10. Explain why a drop of water placed on the page of a book magnifies printingbeneath it.

11. Reading glasses help people to see smallprint. What sort of lens would be used inthem?

The archer fish is a remarkable hunter thatcatches insects that are resting on branchesor reeds up to 2 m above the water. Thearcher fish sights the insect from beneath thewater and then shoots a stream out of its mouth at the insect. Light refracts when it passes from air into water, so the insectappears to be in a different place than it really is. Yet the archer fish is deadlyaccurate. How do you think this is possible?

Pause and Reflect

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Prepare Your Own SummaryIn this chapter, you investigated how opticalsystems make use of mirrors and lenses. Createyour own summary of the key ideas from thischapter. You may include graphic organizers or illustrations with your notes. (See ScienceSkill 10 for help with using graphic organizers.)Use the following headings to organize yournotes:1. The Ray Model of Light2. Convex Mirrors3. Concave Mirrors4. Convex Lenses5. Concave Lenses

Checking Concepts1. State the law of reflection.2. Use a ray diagram to explain why a light

appears dimmer the farther the observer is from it.

3. What is the difference between reflectionand refraction?

4. How is an opaque object different from atranslucent object in terms of its ability totransmit light?

5. How does the direction of a ray of lightchange as it passes from air into water?

6. (a) What are the three basic shapes of mirrors?

(b) With which shape of mirror do light rays converge?

7. How does the reflection from a convexmirror appear to make objects seem smaller?

Understanding Key Ideas8. Draw a diagram of a light ray reflecting

off the surface of a flat mirror. Label the normal, the incident ray, the reflectedray, the angle of incidence, and the angle of reflection.

9. Copy the diagrams below into yournotebook. Complete the missing parts of each diagram.

C h a p t e r

5

r = ?Draw the reflected ray.

Draw the normal.

Draw the two reflected rays. Compare the directions of the light striking and bouncing off the mirror.

r = ?

i = ?

i

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10. Making reference to the normal line,describe the change in the direction of a light ray that travels from: (a) air into water (b) glass into air

11. Draw a diagram to show how an image thesame size as an object can be produced byreflection from a flat mirror.

12. As an object moves closer to a convex lens,what happens to the size and orientation ofthe image?

13. Draw a ray diagram to show what happensto light rays as they pass through:(a) a convex lens(b) a concave lens

14. (a) How does the relative thickness of a convex lens affect its ability to refractlight?

(b) Draw a thin and a thick convex lens.Show how light rays pass through eachof them.

15. Draw ray diagrams to illustrate thedifference between opaque, translucent andtransparent.

16. Copy the following table into yournotebook. For each of the followingexamples, decide what kind of lens orlenses need to be used in the light fixtures.Use a diagram to show how the lens isaffecting the light.

17. A magnifying glass contains a lens that canfocus the light from the Sun at a singlepoint on the ground. (a) What shape of mirror can also do this? (b) Draw a ray diagram to show parallel

light from the Sun striking this mirrorand to show where the light rays converge.

(c) Which is better for focussing the Sun’slight at a point on the ground, a lens or a mirror? Explain.

18. Decide whether each of the following isopaque, translucent, or transparent.Explain your reasons.(a) your tooth(b) your skin(c) your fingernail(d) the lens of your eye

Suppose you have been given a concavemirror and you have been asked to find itsfocal point. Describe a procedure that youcould use to do this.

Pause and Reflect

Light Fixtures

(a) A reading light that lights one spot in the room whileleaving other areas dark

(b) An outdoor light that spreadsan even illumination over awide area

(c) A flashlight that spreads adiffuse, dim light over a widearea, while shining a brightfocussed beam in the middle

Type of Lens How the Lens Affects Light Rays

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