mhr • unit 2 optics - wordpress.com · 200 mhr • unit 2 optics ... try the same test with the...

T his remarkable photograph is definitely one for the record books. Theseare the most distant objects ever observed by humans using visible light.

Why is this photograph so amazing? Consider that it is a photograph, not adrawing or a painting or an artist’s imagination of what might be in space.Every swirl and every point of light is a real object. However, none of thepoints of light are individual stars. Every dot and swirl is a complete galaxy,which means that each and every dot is made up of billions of stars. Humansight and the development of optical systems such as the Hubble SpaceTelescope have made it possible for us to see more objects in more detail than ever before. We can peer into distant galaxies, view the inside of abeating heart, and examine our home planet from both close up and far away.

200 MHR • Unit 2 Optics

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Changing Colours 6-1

In this activity, you can observe how your colour visionadapts to changing lighting conditions.

What to Do1. Look at the image of the flag of Canada, which is

printed in a greenish tint. Stare at the image of theflag for 25 s, making sure not to move your eyesaround.

2. Immediately switch your gaze to a white space onthe page, and wait a few seconds. What do yousee? Achieving this effect may take a few tries.

3. Try the same test with the flag of British Columbia.

What Did You Find Out?1. (a) What did you see when you stared at the

white page?

(b) Why do you think you saw this?

2. How might this adaptability of your colour visionhelp you as you walk through a forest in brightsunlight and at twilight?

Find Out ACTIVITY

Flag of Canada Flag of British Columbia

The cornea-lens-retina system focusses light at the back of the eye. Special cells in

the retina called rod cells and cone cells convert light into electrical signals that

are sent to the brain. Light does not always fall on the retina in perfect focus.

Near-sightedness results when the eye cannot form a sharp image of distant objects.

Near-sightedness can be corrected by placing a concave lens in front of the eye.

Far-sightedness results when the lens of the eye cannot form a sharp image of nearby

objects. Far-sightedness can be corrected by placing a convex lens in front of the eye.

Think about the different kinds of objects you see every day. With oneglance you might see the words on this page, the colour illustrations,and a classmate sitting next to you. Human eyes can focus on objectsboth near and far and adapt to both blazing sunlight and the dimmestof moonlight. We have one vision system to see in colour and anotherto see only in shades of grey. How is all of this possible? Much of itcan be understood by taking a close look at the structure of thehuman eye.

Human Vision6.1

Key Termsastigmatismblind spotcorneairisoptic nervepupilretinasclera


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Chapter 6 Human vision can be corrected and extended using optical systems. • MHR 203

How Light Enters the EyeLight enters your eye through the pupil (see Figure 6.1). The pupilis an opening that appears dark because light passes through it without reflecting back. The iris is the coloured circle of musclesurrounding the pupil. The iris is the structure we refer to when we speakabout the colour of someone’s eyes being grey, brown, blue, or hazel.The iris controls the amount of light entering the eye. In dim light,the iris dilates, or expands, the pupil to allow more light to enter (see Figure 6.2A). In bright light, the iris contracts the pupil to reducethe amount of light entering the eye (see Figure 6.2B).

Covering the iris and pupil is a transparent tissue called thecornea. The cornea is made of cells that are transparent enough to let light pass through, yet tough enough to hold the eye together.Surrounding the cornea is an opaque tissue called the sclera. We seethe sclera as the white region surrounding the iris.

Behind the pupil is a flexible convex lens. The light rays passthrough the lens and are focussed on a screen at the back of the eyecalled the retina, where an image is formed. Special light-sensitivecells in the retina detect the image. Other cells in the retina convertthe light rays into electrical signals that are sent to the brain through athick nerve called the optic nerve.

Figure 6.2A A dilated pupil

Figure 6.2B A contracted pupil

Figure 6.1 Light enters the eye through a transparent opening called the pupil.

Did You Know?

The human eye is more sensitiveto green light than to any othercolour. If you look at a greenlight and a red light of the sameintensity, the green light appears to be brighter.

iris

pupil

sclera

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The Cornea-Lens-Retina SystemLight rays pass through a focussing system involving the cornea, thelens, and spaces in the eye filled with a watery fluid (see Figure 6.3).The fluid between the lens and the cornea supports both the corneaand the lens, and provides nutrients to the cornea, which does nothave any blood vessels. The fluid behind the lens gives shape to theeye and supports the lens.

Light rays begin to be focussed as soon as they pass into thecornea. The cornea refracts incoming light rays so that they convergetoward the retina. The cornea provides most of the focussing done byour eyes. The lens does the remaining focussing. This may be asurprise to you because we usually think of the lens as doing thefocussing. Perhaps it is because we do not notice the amount offocussing done by the cornea that we tend not to think about itsfunction in forming the image.

The lens has the ability to fine-tune our focus by automaticallychanging its shape. When certain muscles in the eye contract, there isless tension on the lens, allowing the lens to become thicker. A thickerlens can focus on near objects. When you look at distant objects, thesesame muscles relax, increasing tension on the lens and making itthinner. You can feel your eyes working hard to focus if you hold afinger up very close and try to see it clearly.

Figure 6.3 The eye in cross section

Word Connect

The term “cornea” comesfrom the Latin word for horn,the front part of an animal’shead. The cornea is the mostforward part of the eye.

Did You Know?

In some species of animals, suchas the octopus, the lens and theretina can move closer together.

retina lens

pupil

cornea

optic nerve

focalpoint

muscle

muscle

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An optical or visual illusiontricks the eye and brain intoperceiving something unlikewhat actually exists. Checkout examples of opticalillusions and find out whatthey reveal about the way wesee. Start your search atwww.bcscience8.ca.

internet connect


Forming an image

All the light rays that enter the eye from one spot on the base of anobject come together again in one place at the top of the retina.Similarly, all the light rays that enter the eye from a spot at the top of an object come together at one place at the bottom of the retina.As shown in Figure 6.4 the image formed by the lens is inverted.However, you do not have to stand on your head to see upright. Yourbrain interprets the image as being upright.

Blind SpotThe area where the optic nerve enters the retina does not have anylight-sensing cells. This area is known as the blind spot. You caneasily demonstrate the presence of your blind spot by following thesteps outlined in Figure 6.5. Note that each eye sees what the othermisses because the blind spots are not in the same place.

Figure 6.5 To locate your blind spot, hold this book at arm’s length. Cover your right eye withyour hand. Stare at the X while you move the book slowly toward yourself. The dot shoulddisappear and then reappear as its image moves onto your blind spot and then off again.

Reading Check

1. What happens to light rays after they enter the eye through the pupil?

2. Where does most of the focussing in the eye occur?3. How does the lens change to focus on objects that are close?4. How does the lens change to focus on objects that are distant?5. Why is the image of an object inverted when it strikes the retina?

Figure 6.4 The image formedon the retina is inverted.

Conduct an Investigation 6-4,on page 212

Suggested Activity

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Black-and-White Vision and Colour VisionOnce the light rays are focussed correctly on the retina, the cells thatabsorb the light begin their job. Some cells in your retina specialize indetecting low levels of light. Other cells detect bright light. The cellsin your retina that absorb light come in two basic shapes: longercylindrical ones called rod cells and rounder ones called cone cells (seeFigure 6.6).

Rod cells—shapes, movement, and shades of grey

Rod cells can absorb almost any colour of light, but they absorb green light particularly well. Even so, our brain does not use any ofthe signals from rod cells to determine colour—just shades of light anddark. This is called our black-and-white vision system, and in low-lightconditions it helps us see shapes and movement.

Cone cells—seeing the rainbow

Cone cells allow us to detect colour. We have three kinds of cone cells, each possessing a slightly different kind of pigment. Recall thatby using only red, green, and blue it is possible to see all the coloursof the rainbow. If our brain receives an equal amount of all threecolours, then we see the object as white. The human brain cancombine and balance the different colour signals that it receives. Thisis why the white page of a book can appear white to us under varyingamounts of daylight.

Figure 6.6 An electron micrographof rod cells and cone cells

Did You Know?

We sometimes forget we see inblack and white at nightbecause we know what thecolours should be.

retina

rod

cone

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Correcting Focus ProblemsMost people have trouble focussing clearly at some time in their lives.As children grow, especially in their teen years, the shape of their eyechanges. The change of shape can affect their ability to focus and mayrequire the temporary use of eyeglasses. As adults age, the flexibility of the eyes’ lenses often decreases, making it harder to focus on nearby objects.

Normal vision

When light rays from a distant object enter theeye, the rays are nearly parallel (see Figure 6.7).The lens, which is convex, causes the rays toconverge at the retina, producing a sharp image.Light rays from a nearby object are divergingwhen they enter the eye, so muscles in the eyecause the lens to change shape, making the lensthicker. This gives the lens a greater ability toconverge the light rays to form a clear image.

Correcting near-sighted vision

People who can see nearby objects clearly butwho cannot bring distant objects into focus arenear-sighted (see Figure 6.8). This conditionoccurs because the lens converges the light raysto form an image in front of the retina. By thetime the light rays actually strike the retina theyhave begun to spread out again, causing theperson to see a fuzzy image. A concave lens isused to diverge the parallel rays slightly so thatthe image forms farther back, on the retina.

Correcting far-sighted vision

Some people can see distant objects clearly butfind that nearby objects remain fuzzy no matterhow hard they try to focus on them, a conditionknown as far-sighted vision (see Figure 6.9).Light rays from distant objects are nearly parallel,and require less refraction to converge them thanlight from nearby objects. However, light raysfrom nearby objects are diverging as they enterthe eye. A convex lens is needed for the light raysto come into focus exactly on the retina.

Figure 6.7 How the lens in a normal human eye focusses lightrays onto the retina

retina

light fromnearby object

light fromdistant object

light reflected from distant object

Near-sighted vision: image falls short of retina (eye has longer shape than normal eye)

Vision corrected withconcave lens: lens allows image to fall on retina

concave lens

Figure 6.8 How a concave lens in eyeglasses corrects near-sightedness

lens

nearbyobject

Far-sighted vision: image falls behind retina (eye has shorter shape than normal eye)

Vision corrected withconvex lens: lens allows image to fall on retina

convex lens

Figure 6.9 How a convex lens in eyeglasses corrects far-sightedness

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Correcting astigmatism

Some people need vision correction because their cornea has adistorted shape, a condition known as astigmatism. A normal corneais shaped spherically, like a soccer ball, while an astigmatic eye has anirregularly-shaped cornea. This condition causes an image to focus onmore than one point on the retina, resulting in blurred vision (seeFigure 6.10). Astigmatism can be corrected using eyeglasses orcontact lenses (see Figure 6.11) or with laser surgery to reshape thecornea.

BlindnessBlindness can be any vision impairment that keeps people fromdoing important life activities such as riding a bike, reading, orrecognizing their friends through sight. In very rare cases, a blindperson may not be able to detect any light whatsoever. Most peoplewho are legally blind can perceive some light or even have a limitedamount of vision.

In some types of blindness a person can see only a tiny part ofthe middle of a whole scene. Other people who are blind have theopposite situation: they can see on the edges of their vision, but notdirectly ahead. Others can detect light and darkness, but no amountof visual aids can help them to see clearly.

In developing countries, blindness is most often a result ofdisease or malnutrition. Children in poorer communities are morelikely to be affected by blindness caused by disease than are childrenin more affluent communities. Of the approximately 40 millionpeople who are blind in the world today, about 80 percent couldhave some or all of their sight restored through treatment. However,many people in developing countries cannot afford even basic visionaids such as eyeglasses.

Figure 6.10 In astigmatism, the shape of thecornea causes the image to focus on more thanone point of the retina.

Figure 6.11 Contactlenses are small plasticlenses that float on thecornea. Almost anycorrection that can be madeusing prescription eyeglassescan also be made usingcontact lenses.

Think About It 6-3 on page 211

Suggested Activity

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There are many kinds of visionproblems related to focussing,colour perception, and size offield. Other vision problemsinvolve high pressure in theeye, degeneration of parts ofthe eye or optic nerve,detachment of the retina, orhardening of the lens. Find outmore about vision problems.Start your search atwww.bcscience8.ca.


Other Types of BlindnessSnow blindness is a painful condition of temporary partial orcomplete blindness caused by overexposure to the glare of sunlight,such as on snow fields at high altitudes. Treatment includes resting theeyes in a dark room for several days to allow the inflammation todecrease. The Inuit traditionally wore goggles with thin slits to helpprevent snow blindness (see Figure 6.11).

Night blindness is a condition in which it is difficult or impossibleto see in dim light. The most common cause is the rod cells losingtheir ability to respond to light. A person might be born with nightblindness, or it could develop due to injury or malnutrition.

Colour blindness is the ability to see only in shades of grey, andoccurs in about one person in every 40 000. Colour blindness isusually considered a disability, but there are situations in which aperson who is colour-blind has an advantage over a person who seescolour. For example, a person who is colour-blind may find it easier topick out an object from a confusing background.

Although colour blindness is rare, colour vision deficiency is quitecommon, occurring in about 8 percent of males and 1 percent offemales. Colour vision deficiency is an inability to distinguish certaincolours. There are many kinds of colour vision deficiency because one,two, or all three kinds of cone cells may be involved. The mostcommon kind of colour vision deficiency involves the inability to tellred and green apart. For many affected people, both colours appear tobe shades of yellow. A simple test for colour vision deficiency is shownin Figure 6.12.

Reading Check

1. What can cause focussing problems as children grow? As adultsage?

2. Explain why a person who is near-sighted can see a close objectclearly, but not a distant one.

3. Explain why a person who is far-sighted can see a distant objectclearly, but not a close one.

4. How does an irregularly-shaped cornea cause astigmatism?5. What are three examples of what a person who is blind might be able

to see?6. Why are children in developing countries at a greater risk of

becoming blind?7. How can snow blindness be prevented?

Figure 6.12 Those who do nothave red-green colour visiondeficiency should see the number68 here.

Figure 6.11 Inuit snow goggles ofcaribou antler.

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Checking Concepts 1. Make a table that lists the parts of the eye in

one column and the function of each part inthe other.

2. (a) Which parts of your eye are involved infocussing an image?

(b) What is the role of each part? 3. (a) Describe the vision problem shown by

the illustration below.(b) Why does this vision problem become

more common as people age?

4. Why can the human eye see colours better inbright light than in dim light?

5. Write a definition of blindness in your ownwords.

Understanding Key Ideas6. What would happen to a person’s vision if

the eye’s lens was unable to change shape?7. Why is it necessary to have three kinds of

cone cells operating in order to have fullcolour vision?

8. Describe how the eye adapts to the followingchanges in conditions:(a) sudden increase in brightness(b) gradual dimming of light until it is

almost dark(c) looking at a kite, then down at your

hand to let out string9. Most mammals, including dogs and cats,

cannot see colours. Infer how the retina of acat’s eye might be different from the retinaof a human eye.

After years of work in the field of vision andcommunity service, you have been selected to become the high commissioner for theElimination of Preventable Vision Disabilities.You have a budget of $1 billion. Yourcommission is responsible for defining fourgoals that will improve vision in thedeveloping world. Reflect on this problemand then list your four goals along with theportion of the budget that each shouldreceive. Briefly explain your choices.

Pause and Reflect


lens

retina

optic nerve

pupil

cornea

focalpoint

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Experimenting with a Simple Lens6-5

In this activity, you will observe some properties of a test tube lens.

Materials• glass test tube with stopper• water• paper or note card

What to Do1. Fill a glass test tube with water and seal it with

the stopper.

2. Print the name of your favourite scientist in capitalletters on a piece of paper or a note card.

3. Lay the test tube flat on the note card, running leftto right over the words you have written.

4. Observe whether the letters are magnified , whetherthe letters are in focus, and whether the image isupright or inverted. Record your observations.

5. Hold the tube about 1 cm above the card andobserve the letters. Record your observations.

6. Repeat, holding the tube at several other heightsabove the words.

What Did You Find Out?1. Describe what happens to the images of the letters as

the lens is gradually moved away from the note card.

2. Draw and label a ray diagram showing the situationwhen the image appeared to be inverted andmagnified.

Find Out ACTIVITY

Microscopes and some telescopes use lenses to capture and focus light. A telescope

uses a lens or concave mirror to gather light. Camera design and human vision have

a number of similarities and differences. Lasers produce light of only one wavelength

and have special properties that make them useful in optical communication devices

and in eye surgery. Laser light can be sent through fibre optic cables.

Human knowledge about our planet and the universe was very limiteduntil we developed tools to extend our vision. We now have the abilityto peer into the tiny world of micro-organisms and out into the vastreaches of outer space (see Figure 6.13). The tools we use for theseinquiries may seem quite different from each other, but they are basedon the same understanding of light, mirrors, and lenses.

Extending Human Vision6.2

Key Termslaser lightoptical fibresrefracting telescopereflecting telescopetotal internal reflection


Figure 6.13B A nebula formed froman exploding star

Figure 6.13A A micro-organism

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How to Bring an Image into FocusIn order for the light rays passing through a lens to form a clearimage, the screen that is receiving the image must be the correctdistance from the lens. The screen must be at the place where all thelight rays from a given point on the object converge. If the screen isplaced too close to the lens, then the light rays do not fully convergeby the time they strike the screen. There will be an image formed, butit will appear blurred. On the other hand, if the screen is too far away,then the light rays converge and then begin to diverge before theystrike the screen, resulting in a blurred image. Adjusting the distancebetween the screen and the lens to make a clear image is calledfocussing. Focussing is an important step in using optical devices suchas microscopes, telescopes, binoculars, and cameras.

MicroscopesA compound light microscope uses twoconvex lenses with relatively short focallengths to magnify small, close objects.To magnify means to cause to looklarger than the real size.

Figure 6.14 shows a microscope.The object to be viewed is placed on atransparent slide and illuminated frombelow. The light passes by or throughthe object on the slide and then travelsthrough the objective lens.

The objective lens is a convex lens.Recall that if the distance from anobject to a convex lens is between oneand two focal lengths, it forms anenlarged image of the object. Theeyepiece lens, which is another convexlens, then magnifies the image again.This final image can be hundreds oftimes larger than the actual object,depending on the focal lengths of thetwo lenses.

Figure 6.14 This microscope uses two convex lenses to magnifysmall objects. To focus the image, you have to move the objectyou are studying closer to or farther from the objective lens.

Section 1.1 has moreinformation on microscopes and how to use them.

Connection

eyepiece lens

magnified imageat focal point

objective lens

object

light source

mirror

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TelescopesYou know from experience that it is difficult to see faraway objectsclearly. When you look at an object, only some of the light reflectedfrom its surface enters your eye. As the object moves farther away, theamount of light entering your eye decreases, and so the object appearsto be dimmer.

A telescope uses a lens or a concave mirror that is much larger thanyour eye to gather more of the light from distant objects. The largesttelescopes can gather more than a million times more light than thehuman eye. As a result, objects such as distant galaxies appear muchbrighter. Because the image formed by a telescope is so much brighter,the image can be magnified to a greater extent to reveal more detail.

Refracting telescopes have similarities to microscopes

A telescope, like a microscope, has an objective lens and an eyepiecelens. However, the objective lens in a telescope has a longer focallength than in a microscope because the objects viewed are far fromthe lens. The simplest microscopes and telescopes use only two lenses.The lenses bend the light to focus it, which is why a telescope withthis design is called a refracting telescope (see Figure 6.15).

In both the microscope and the refracting telescope an objectivelens collects light and focusses it into an image (see Figure 6.16). Thisimage is formed inside the microscope or telescope and is never seendirectly. Instead, the image is magnified by the eyepiece lens, anddirected into the eye of the operator or into a camera.

eyepiecelens

focalpoint

objectivelens

light fromdistantobject

light fromlight source

Figure 6.16 In order to focus with a microscope, the objectbeing viewed is moved. In order to focus with a telescope, itseyepiece and the observer are moved.

objective lens light from

distant object image of

distant object

focal point

eyepiece lens

Figure 6.15 Light from a distant object passes though anobjective lens and an eyepiece in a refracting telescope.The two lenses produce a large image.

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Figure 6.18 Reflecting telescopes use two mirrors tocreate an image, which is then magnified by a convexlens. In order to focus an image on a reflecting telescope,the eyepiece (convex lens) is moved.

Figure 6.17 The 102 cmrefracting telescope at the YerkesObservatory in Wisconsin is thelargest refracting telescope everused.

secondary mirror

tertiary mirror

36-segment primary mirror

incoming light

Figure 6.19 The Keck telescope combines light from two mirrorsystems like the one in the diagram to make a single image that ismany times clearer than an image produced by one mirror.

Problems with refracting telescopes

In order to form a detailed image of distant objects, such as planets orgalaxies, the objective lens must be as large as possible (Figure 6.17).A large lens is heavy and can be supported in the telescope tube onlyaround its edge. The lens can sag or flex due to its own weight,distorting the image it forms. Also, heavy glass lenses are costly anddifficult to make, and even when the highest quality of glass is used,the lens absorbs some of the light.

Reflecting telescopes

Due to the problems with making large lenses, most large telescopestoday are reflecting telescopes. A reflecting telescope uses a concavemirror, a plane mirror, and a convex lens to collect and focus lightfrom distant objects. Figure 6.18 shows a reflecting telescope. Lightfrom a distant object enters one end of the telescope and strikes aconcave mirror at the opposite end. The light reflects off this mirrorand converges. Before it converges at a focal point, the light strikes aplane mirror that is placed at an angle within the telescope tube. Thelight is reflected from the plane mirror toward the telescope’seyepiece. The light rays converge at the focal point, creating an imageof the distant object. Just as in a refracting telescope, a convex lens inthe eyepiece then magnifies this image.

Some telescopes used to study distant galaxies collect the light raysfrom several mirrors and then combine the rays into a single image. One such telescope is the Keck telescope located in Hawaii (see Figure 6.19).

eyepiece lens

image ofdistant objectat focal point

concave mirror

plane mirror

light from distant object

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The Hubble Space Telescope

Imagine being at the bottom of a swimming pool and trying to read asign by the pool’s edge. The water in the pool would distort yourview of any object beyond the water’s surface. In a similar way,Earth’s atmosphere blurs the view of objects in space. To overcomethe blurriness of our view into space, the Hubble Space Telescope waslaunched in 1990. The Hubble Space Telescope is a type of reflectingtelescope that uses two mirrors to collect and focus light to form animage. The primary mirror in the telescope is 2.4 m across and cancollect visible light—as well as other types of electromagneticradiation—from planets, stars, and distant galaxies. Freed from thedistortion caused by Earth’s atmosphere, the Hubble Space Telescopehas produced images much sharper and more detailed than the largestground-based telescopes (see Figure 6.20).

Find out more about theHubble Space Telescope, theimages it has produced, andCanada’s role in servicing thetelescope. Start your searchat www.bcscience8.ca.

internet connect


Figure 6.20 The image from the Hubble Space Telescope is clear (A), not blurred byEarth’s atmosphere (B).

BinocularsBinoculars are actually two refractingtelescopes mounted side by side. Youcan imagine how difficult it would beto hold up two long telescopes. Thetelescopes in binoculars are shortenedby placing prisms inside that serve asplane mirrors. Rather than travellingdown the long tube of a telescope,the prisms reflect the light inbinoculars back and forth inside a shorter tube (see Figure 6.21.

objective lens

eyepiece

prisms

thumbscrew

Figure 6.21The thumbscrew onbinoculars is used tochange the focal lengthin order to focus on theobjects being viewed.

Conduct an Investigation 6-8on page 227

Suggested Activity

A B

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Cameras A digital camera works by gathering and bending light with a convexlens. The lens then projects an image onto a light detector to record adigital image of a scene. When you take a photograph, a shutter opensto allow light to enter the camera for a specific length of time. Thelight reflected off your subject enters the camera through an openingcalled the aperture. The light then passes through the lens, whichfocusses the image on the light detector. Because a convex lens is used,the image is inverted and smaller than the actual object.

Wide-angle lenses

Suppose you and a friend use two different cameras to photograph thesame object at the same distance. If the cameras have different lenses,your pictures might look different. For example, some lenses haveshort focal lengths that produce a relatively small image of the objectbut have a wide field of view (see Figure 6.22). These lenses are calledwide-angle lenses. Wide-angle lenses must be placed close to the lightdetector to form a sharp image with their short focal length.

Telephoto lenses

Telephoto lenses have longer focal lengths. The image through atelephoto lens seems enlarged and closer than it actually is (see Figure6.23). Telephoto lenses are easy to recognize because they usuallyprotrude from the camera to increase the distance between the lensand the light detector (see Figure 6.24).


Figure 6.22 A photograph taken with a wide-angle lens

Figure 6.23 A photograph of the same scene asFigure 6.22 taken with a telephoto lens

Figure 6.24 A telephoto lens

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Cameras Have Similarities to Human Eyes There are many structuralsimilarities between a cameraand the human eye (seeFigure 6.25). For example,compare the lens cap for acamera to the human eyelid.Both reduce the chance ofaccidental damage. An irislimits the amount of lightentering the eye. In cameras,this function is accomplishedusing a device called adiaphragm. The diaphragm is made of a number of opaque circlesthat are arranged in a circle. The circles can be moved to make thecentral hole larger or smaller. Light passes through the lens and formsan inverted image in both the camera and the eye.

Not all structures in a camera work the same way as in the humaneye. For example, changing the distance between the lens and thedetector does the focussing in a camera. Recall that in humans, thelens changes shape, rather than moving closer to the retina.

At the back of the camera is a detector called a charge-coupleddevice (CCD), which absorbs light and provides the electrical signalsneeded to produce a digital image. The CCD has many tiny regions,called pixels, each of which is capable of recording a tiny part of thewhole image. The pixels correspond to the rods and cones that detectlight in our eyes. Research is currently being done to try to connectthe electrical signals from a digital camera directly to human opticnerves. This may one day provide a working vision system that willallow people who are blind to see.


Figure 6.25 A comparison of thecamera and the human eye

Reading Check

1. How does a microscope magnify an image?2. How is a reflecting telescope similar to a microscope?3. How is a reflecting telescope different from a refracting telescope?4. Why is the Hubble Space Telescope able to produce clearer images

than telescopes on Earth?5. What is the function of prisms in binoculars?6. How does the focal length of a telephoto lens compare to the

focal length of a wide-angle lens?7. What are two ways in which a camera is similar to a human eye?8. What are two ways in which a camera is different from a human eye?

Think About It Activity 6-7 onpage 226

Suggested Activity

lens iris

retina

muscle

image of ring

diaphragm shutter

CCD detector

focussingring

diamond ring(object)

opticnerve

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Lasers Recall that white sunlight contains all the colours of the rainbow.Sunlight and light from an incandescent light bulb contain a mixtureof waves of different wavelengths (see Figure 6.26). In laser light, allof the light has the same wavelength, all the light rays are moving inthe same direction, and all of the crests and troughs of the light arelined up (see Figure 6.26). Laser light travels great distances withoutspreading out, and can contain a lot of energy. Because lasers onlycontain one wavelength, the light does not refract into a rainbow, aswould happen with normal light.

Laser Surgery Lasers are routinely used to remove cataracts, re-attach retinas, stopbleeding, and reshape corneas. A cataract is a condition where theeye’s lens has become cloudy. This condition can occur naturally withage. The laser is used to cut through the cornea so that the lens can bereplaced with a synthetic one. If part of the retina has becomedetached from the inside of the eye, a laser can sometimes be used toweld the piece back in place and prevent further detachment fromoccurring. The energy from a laser beam is so intense that it can sealoff blood vessels, which helps reduce bleeding during surgery.

Changing the shape of the cornea can also be accomplished with theuse of laser surgery. It is possible to accurately map the surface of aperson’s cornea and then calculate the changes in shape that are neededto correct the person’s vision. An eye surgeon can use a laser to weakenthe surface of the cornea, allowing it to be folded back. The surgeonchanges the shape of the inside of the cornea, and may use a laser to dothis. The outer surface can then be returned to its original place. Theprocedures are not painful and in some cases the surgery is complete inonly a few minutes. Like all types of medical procedures, there are somerisks involved. Because laser eye surgery is only a few decades old, long-term risks and benefits are not yet known.

A hologram is a photographictechnology in which lasersare used to encode three-dimensional informationabout an object onto a flatsurface. Find out whereholograms are commonlyused and why they work well for this purpose. Startyour search atwww.bcscience8.ca.

internet connect

Word Connect

“Laser” stands for lightamplification by stimulatedemission of radiation.

Figure 6.26A The light waves inlaser light have the samewavelength and travel with theircrests and troughs aligned.

Figure 6.26B The light from an ordinarylight bulb contains more than onewavelength and does not travel with thecrests and troughs aligned.

A

B

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Optical FibresImagine using a flashlight to send a signal down a long hallway. If thehallway is straight, the light will pass down it with no problem. If thehallway has a bend in it, then a well-placed mirror will reflect the lightbeam around the bend. If the hallway has many bends, then manymirrors might be used to reflect the light to do the job. A simple way totransport light in this way is by using an optical fibre (see Figure6.28). Optical fibres are transparent glass fibres that can transmit lightfrom one place to another.

Light entering one end of an optical fibre is reflected continuouslyfrom the sides of the fibre until it emerges from the other end. Just aswater moves through a sealed pipe without leaking away, almost nolight is lost or absorbed in optical fibres. Every time a light ray strikesthe wall of the fibre it is reflected back into it. This is called totalinternal reflection (see Figure 6.29).

Optical fibres are used in medicine to transmit images of the insideof a person’s body from a tiny camera at one end of the fibre opticscable to a monitor at the other end. One bundle of fibres transmitslight, while another carries the reflected light back to the monitor.

In telecommunications, laser light is sent through fibre opticscables to transmit telephone, video, and Internet signals. Just asdifferent colours of two flashlights could be sent down the samehallway without becoming jumbled, laser beams of differentwavelengths can be used to send different messages down the samecable without interfering with each other. This makes fibre opticstechnology useful for broadband transmissions, where thousands ofdifferent signals can be sent at the same time.

Figure 6.29 Optical fibres make useof total internal reflection.

Figure 6.27 Surgeons canuse lasers in place of scalpelsto cut through body tissues.The energy from the laserseals off blood vessels in theincision and reduces bleeding.

Figure 6.28 One optical fibre cancarry thousands of phoneconversations at the same time.

light

glasscore

glass layer

plastic covering

light rays

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In this activity, you will research the advantages anddisadvantages associated with laser eye surgery.

What to Do1. Research information about laser eye surgery by

searching the Internet and other resources. Start yoursearch at www.bcscience8.ca.

With your teacher’s permission, you mightinterview a laser surgeon or person who hasundergone laser eye surgery.

Keep in mind the following skills and attitudesassociated with a scientifically literate person.

• Identify the main points in what you find.Separate the useful information fromunimportant information.

• Be aware of preconceptions and assumptions(your own and the author’s).

• Use criteria for evaluating sources ofinformation. (What is the bias? Is theinformation supported by research? Are youreading/hearing about a personal experience?Does the author have a vested interest in acertain outcome?)

• Recognize that scientific knowledge iscontinually developing, and often builds onprevious experience and theories.

2. Summarize your research by compiling three lists:

(a) advantages or uses of laser eye surgery

(b) pre-existing conditions that make it inadvisableto have laser eye surgery performed

(c) risks of laser eye surgery

3. Compare your lists to other students’ lists andgenerate a single list together.

4. Select one particular point of view, such as that of asurgeon, a person trying to decide whether to havelaser eye surgery, a satisfied patient, a dissatisfiedpatient, the chair of a regulatory body such as theCollege of Physicians and Surgeons of BC, arepresentative of a support group for persons withpoor outcomes of laser eye surgery, or an advocacygroup promoting the use of laser eye surgery.

5. Research enough about your chosen role toparticipate in a panel discussion, where each personcontributes to a conversation on the topic: “What an informed person needs to know about laser eye surgery.”

What Did You Find Out?1. (a) Did your opinion of laser eye surgery change as

a result of your research? If so, why? If not,how was your opinion strengthened by whatyou found?

(b) Did your opinion of laser eye surgery change asa result of the panel discussion? If so, why? Ifnot, how was your opinion strengthened bywhat you heard?

The Pros and Cons ofLaser Eye Surgery

6-7 Think About It


Heads-up displays (HUDs) are optical devices that canproject information from an instrument panel onto ascreen in front of a viewer. Tofind out more about HUDs visitwww.bcscience8.ca.

Reading Check

1. What is the difference between laser light and ordinary light froma light bulb?

2. How can laser light be used to correct vision problems?3. What is total internal reflection?4. What are three uses for light sent through a fibre optics cable?

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Checking Concepts 1. Make a table that lists the parts of a camera

in one column and the function of each partin the other.

2. (a) Make a labelled diagram to show thearrangement of lenses in a refractingtelescope.

(b) Show how light rays pass from a distantobject to the eye of a person lookingthrough the eyepiece.

3. (a) Which lens in a microscope is responsiblefor producing a magnified image on theinside of the microscope that is not seendirectly by the person using themicroscope?

(b) Why is this image produced?4. List three features of laser light that make it

special compared to light from a regular light bulb.

5. List three technological applications forlasers.

Understanding Key Ideas6. (a) Explain why telescopes used to study

distant galaxies need to have such largemirrors.

(b) Give two reasons why a large lens isinferior to a large mirror in a telescope.

7. How is it possible for a fibre optics cable tocarry many different signals at one timewithout the signals becoming scrambled?

8. How is the property of total internalreflection used in the operation of opticalfibres?

9. How can a laser be used to perform surgeryon the inside of an eye without having to cutopen the eye first?

Pause and Reflect


Below are two photographs, one takenthrough a telescope and the other through amicroscope. One is of Earth, taken fromabove the Moon. The other is of commonbacteria called E. coli, which are found in themouth of every healthy person on Earth.These two viewpoints were unknown tohumans only three generations ago. Reflecton each, select one, and explain why beingable to see this scene might be important tohow humans see the world.

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Prepare Your Own SummaryIn this chapter, you investigated how we seeand how human vision can be extended usingoptical systems. Create your own summary ofthe key ideas from this chapter. You may includegraphic organizers or illustrations with yournotes. (See Science Skill 10 for help with usinggraphic organizers.) Use the following headingsto organize your notes:1. The Structure of the Human Eye2. How We See3. Correcting Focus Problems4. Using Optical Systems to Magnify Close

Objects5. Using Optical Systems to See Distant

Objects

Checking Concepts 1. List the parts of the eye that are used to

refract light. 2. Write a sentence that connects the given

words in a meaningful way:(a) cornea/sclera(b) iris/pupil(c) rod cells/cone cells/retina

3. What does the diagram below reveal aboutyour eye?

4. (a) Which type of lens, convex or concave,should a person who is near-sighted use?Explain.

(b) Which type of lens, convex or concave,should a person who is far-sighted use?Explain.

5. What is astigmatism?6. Draw a diagram showing the mirror and

lens arrangement in a reflecting telescope.7. (a) Explain how focussing occurs in a

microscope. (b) Explain why this type of focussing

would not work for a telescope.8. Binoculars are similar to two telescopes

mounted in parallel, except that they arenot very long. How is this shorteningaccomplished?

Understanding Key Ideas9. Explain how a white sheet of paper can

continue to look white even thoughlighting conditions can gradually changethroughout the day.

10. Explain why it takes a few minutes to beable to see when you walk from fulldaylight into a darkened room. List theadaptations that happen in the eye toadjust to low-light vision.

11. What does wearing glasses have incommon with laser surgery as a method forcorrecting vision problems?

C h a p t e r

6

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