Cs3-Ch07 Keeping an Eye on Light

How do you know that thespotlights below are not really

sucking in darkness rather than emitting light? Light is certainly

puzzling. You cannot usually see light, yet without it you cannot

see at all.

77KEEPING AN EYE ON LIGHT

K E Y Q U E S T I O N S

Why do your legs look shorter when you are standing in water?

How does a lens bend light?

Why is the lens in your eye more like jelly than glass?

How do glasses help some people see more clearly?

How is it possible to take a photograph inside a living human body?

Why is the sky blue?

Why is the sun so red at sunset?

Why is it so important that you wear sunglasses and choose them wisely?

Distinguish between absorption, reflection, refraction and scattering of light.

Apply the laws of reflection and refraction to everyday situations.

Explain how the structure of the eye allows vision.

Describe light as a form of wave not requiring a medium.

Describe how scientists from many cultures have contributed to our understanding of light, and how this has evolved as a result of available evidence.

K E E P I N G A N E Y E O N L I G H T 153

Thinking aboutThinking aboutLight

1. What is light?Without light you are in the dark. You need light to see your world clearly. Have you stopped to think about what light is and where it comes from?(a) With a partner, make a list of anything you already know about light.

Take time to discuss what you know and explain ideas to each other when necessary.

(b) Write a report on your discussion using the following points.• Group together the words or ideas that are connected and use each

group of words as the basis for a paragraph.• Include a paragraph that covers ideas or words that you do not fully

understand.• Use the table below as a guide in developing your report.

Outline of your report

(c) Once your report is completed, be prepared to share it with your class. When listening to other reports, make a note of new ideas that were not included in your report.

2. Sunglasses(a) Why do you wear sunglasses? Which types are best?(b) What have you learnt from advertisements for sunglasses?

Discuss why you bought the sunglasses you use, and why you wear them.

(c) Make a poster advertising the ultimate pair of sunglasses for maximum protection.

Paragraph 1

Light is . . .

Light has special properties

such as ,

, and

which make it very

useful in our lives.

Start with a general statement that introduces the topic.

Paragraphs 2, 3, 4 etc.

Without light, we would live

in

Describe different aspects of light. Write a paragraph for each group of ideas. Your ideas could focus on some of the following: uses, appearance, behaviour, sources, benefits, effects of no light.

Last paragraph

There is a lot to know about

light. We need to learn more

about

Describe what you do not fully understand.

WORK

3. Light words, heavy meanings

(a) The words in this table have been grouped together. Can you think of a heading for each group? Which words gave you clues for the headings you have chosen?

(b) Under your chosen headings, write out these word lists in your workbook (leave a line after each word). Beside each word that you already know, write a sentence using it. If you are unsure how to pronounce any of the words, check to see if it is in the pronunciation guide in the back of the book.

(c) As you go through this chapter and read these words, check your sentences or add new ones. You may need to rewrite or correct sentences if you notice that you have used a word incorrectly.

4. Why is the sky blue?Why is the sky blue? Why is the sky red at

sunset? How are rainbows formed? These questions are often asked by young children, yet few adults can answer them! Choose one and write the answer that you would give. Make a list of points that explain your answer, and draw a diagram to show what you think is happening.

beamopaquescatteringtranslucentraytransparent

ciliary muscleirisretinaconeslensreceptors

astigmatismhyperopiaphotokeratitiscataractsmyopia

biconcavedivergingbiconvexfocal lengthinversion

WORK

7.17.1

154 C O R E S C I E N C E 3

The ‘ray’ box. It provides a way of tracing the path of light.

You can get it at the flick of aswitch or by striking a match.Without it there would be dark-ness. We call it light. Our mainsource of light energy is the sun.Light from the sun makes life onEarth possible and provides uswith some beautiful images likerainbows and spectacular sun-sets. Artificial light provides uswith brilliant laser shows at rockconcerts and even images ofunborn babies.

Light travels in straight lines asit travels through empty space orthrough a uniform substance likeair or water. The lines that areused to show the path of light arecalled rays. You cannot see asingle light ray. A stream of lightrays is called a beam. You cansee beams of light only whenparticles in substances like airscatter the light as shown in thephotograph below. Some of thescattered light enters your eye,allowing you to see the particleswithin the beam.

A beam of light can be seen if there is smoke or fog in the air. Light is scattered by the tiny particles. Some of the scattered light enters your eye, allowing you to see the particles within the beam.

Tracing the path of lightThe ‘ray’ box shown in the photograph below provides a way oftracing the path of light. It contains a light source and a lenswhich can be moved to produce a wide beam oflight that spreads out, converges or has paralleledges. The light box is placed on a sheet of whitepaper, making the beam visible as some of thelight is reflected from the paper into youreyes.

Black plastic slides can be placedin front of the source to produce asingle thin beam or several thinbeams. The beams are narrowenough to trace with a fine pencilonto the white paper. The fine pencilline can be used to represent a singleray.

Crossing boundariesWhen light meets a boundary between two different substances, anumber of things can happen.

On the reboundThe light may ‘bounce off’ the surface of the substance. This iscalled reflection, and is what allows you to see non-luminousobjects. For example, light from a torch pointed at a door will bereflected from the surface of the door and enter your eye, allowingyou to see the door.

Light can also be reflected from particles suspended in a transparentsubstance. This is called scattering because the light bounces off inso many different directions. Light is scattered by the particles of fog,dust and smoke in the atmosphere. Scattering is also evident in water.A luminous object in very deep or dirty water is not visible from thesurface because all of the light is scattered before it can emerge. Thesame object is more likely to be visible on the surface of shalloweror cleaner water because less light would be scattered.

Just passing throughThe light may travel through the substance. Some light is alwaysreflected when light crosses a boundary between two substances. Ifmost of the light travels through the substance, the surface is calledtransparent because enough light gets through for you to be ableto see objects clearly on the other side. Some materials let just enoughlight through to enable you to detect objects on the other side, butscatter so much light that you can’t see them clearly. The ‘frosty’ glassused in some shower screens is an example. Such materials are saidto be translucent.

Riding on a light beam


(a) Transparent (b) translucent and (c) opaque materials

Lost inside?The light may be absorbed bythe substance, transferring itsenergy to the particles in thesubstance. Substances whichabsorb or reflect all the lightstriking them are said to beopaque. Most objects in yourclassroom are opaque.

(a)

(b)

(c)

YOU WILL NEEDray box kitpower supplyseveral sheets of white paperruler and fine pencil

• Connect the ray box to the power supply. Place a sheet of white paper flat on the bench in front of the ray box. Move the lens backwards and forwards until a beam of light with parallel edges is projected.

• Use one of the black plastic slides to produce a single thin beam of light that is clearly visible on the white paper.

• Trace the path of this single beam of light as it meets the lens, prism and each of the mirrors shown in the diagram on the right. The path can be traced by using pairs of very small crosses along the centre of the beam before and after meeting each ‘obstacle’. Trace and label the shape of each ‘obstacle’ before you trace the light paths.

1. What happens to a beam of light when it meets a block of perspex:(a) ‘head on’?(b) at an angle?

2. What happens to a beam of light when it meets each mirror surface(a) ‘head on’?(b) at an angle?

• Change the slide in the ray box so that you can project several parallel beams towards each of the ‘obstacles’.

3. Use a ruler to draw a small diagram showing the path followed by the parallel beams when they meet each of the ‘obstacles’.

Tracing the path of a beam of light

planemirror

concavemirror

convexmirror lens

ray box

sheet of white paper

SEEING THE LIGHT7.1

Remember1. What is a ray of light?2. You can not usually see light as

it travels through the air. What makes it possible to see a beam of light?

3. What happens to light when it travels through air and meets:(a) a transparent surface?(b) a translucent surface?(c) an opaque surface?

Think1. Explain the difference between a

ray of light and a beam of light.2. Why can’t you see a ray of

light?3. Is the moon a luminous object?

Explain your answer.

4. Distinguish between the scattering of light and the reflection of light by a plane mirror.

5. List one example of each of the following:(a) a transparent object(b) a translucent object(c) an opaque object.

ImagineImagine that the world is plunged into darkness by a mysterious cloud of dust. What problems would be caused by the lack of visible light if the cloud lingered for:(a) one hour?(b) three days?(c) six weeks?

Activities

7.27.2


Where is that toe?

observer

apparent position of toe

real position of toe

InsetLight bendingaway from thenormal

Looks can be deceiving! The person in the photograph belowdoesn’t have unusually short legs. Everything you see is an image.An image of the scene you are looking at forms at the back of your

eye. When light travels in straight lines, the image yousee provides an accurate picture of whatyou are looking at. However, when lightbends on its way to your eye, the imageyou see can be quite different.

A change of direction

When light travels from one substanceinto another substance that is transparentor translucent, it can slow down or speedup. This change in speed as light travelsfrom one substance into another is calledrefraction. Refraction causes light tobend, unless it crosses at right angles tothe boundary between the substances.

The best way to describe which way thelight bends is to draw a line at right anglesto the boundary. This line is called thenormal. When light speeds up, as it doeswhen it passes from water into air, itbends away from the normal. When light

slows down, as it does when itpasses from air into water, itbends towards the normal.

The light coming from theswimmer’s legs in the photo-graph above bends away fromthe normal as it emerges fromthe water into the air. The lightarrives at the eyes of anobserver as if it were comingfrom a different direction. Thediagram on the left shows whathappens to two rays of lightcoming from the swimmer’sright toe. To the observer, therays appear to be coming from apoint higher than the real pos-ition of the toe. It can be seenby looking at the diagrams (leftand right) that the amount ofbending depends on the angleat which the light crosses theboundary.

YOU WILL NEED2 beakers evaporating dishcoin

• Place a coin at the bottom of an empty beaker and look at it from above while your partner slowly adds water from another beaker.

1. How does the position of the coin appear to change while the water is being added?

2. Which other feature of the coin appears to change?

• Place the coin in the centre of an evaporating dish and move back just far enough so you can no longer see the coin. Remain in this position while your partner slowly adds water to the dish.

3. What appears to happen to the coin as water is added to the evaporating dish?

• Make a copy of the diagrams below. Use dotted lines to trace back the rays shown entering the observer’s eye to see where they seem to be coming from. This enables you to locate the centre of the image of the coin.

4. Is the image of the coin above or below the real coin?

beaker

water

coin

water

coin

evaporatingdish

The image of the coin is not in the same place as the real coin.

FLOATING COINS7.2

What happened to my legs?

Short legs? Not really.


HOW MUCH DOES IT BEND?

YOU WILL NEEDray box kit power supply sheet of white paper

• Connect the ray box to the power supply. Place a sheet of white paper flat on the bench in front of the ray box. Project a single thin beam of white light towards a rectangular perspex prism as shown in the diagram below.

1. Does the light bend towards or away from the normal as it enters the perspex? (Remember that the normal is a line that can be drawn at right angles to the boundary of the prism. It is shown as a dotted line in the diagram. If it helps you, draw the line on the paper beneath the prism.)

2. Imagine a normal at the boundary where the light leaves the perspex to go back into the air. Which way does the light bend as it re-enters the air — towards or away from the normal?

3. Does all of the light travelling through theperspex re-enter the air? If not, what happens toit?

4. Look at the light beam as it enters and leaves the perspex. What do you notice about the direction of the incoming and emerging beams?

• Turn the rectangular prism so that the incoming beam enters the perspex at different angles.

5. How can you make the incoming light bend less when it enters the perspex?

6. How can you make the incoming light bend more when it enters the perspex?

• Now concentrate on the light beam inside the perspex. Turn the rectangular prism so that you make the beam inside the perspex bend further away from the normal.

7. What happens to the amount of light reflected atthe perspex–air boundary as the prism is turned?

• See if you can turn the rectangular prism to a position that prevents the light beam from emerging from the perspex into the air. If you succeed, you have observed total internal reflection. The light has bent so much that it can’t get out of the perspex. Draw a diagram to show the path of the light beam through the prism that results in total internal reflection. You can see another example of total internal reflection by looking at the end of a fish tank from some positions. Try it!

thin beam

normal

rectangularperspexprism

to ray box)

sheet ofwhite paper

ray box

7.3

Remember1. What is refraction?2. Does light bend towards or

away from the normal when it slows down while passing from air into water?

Think1. The illustration shows a ray of

light emerging from still water after it has been reflected from a fish. Should the spear be aimed in front of or behind the image of the fish? Use a diagram to explain why.

2. Draw a diagram to show a ray of light travelling through air to a perspex prism under water in a fish tank and back into the air again.

InvestigateFind out about the First Law of Refraction. Use a ray box and prism to carry out some experiments to prove the First Law of Refraction for yourself. Write up a report of your experiments.

ImagineImagine that you are the fish in the illustration below.(a) Will the image of the girl’s

head be higher or lower than her real head?

(b) Draw a sketch of how the girl might appear to you.

normal

Activities

Short legsRefraction of light

7.37.3


Lenses are used in microscopes, telescopes, cameras and otherinstruments that bend light in order to form an image. The mag-

nified picture in the comic on the left is actually an image ofthe real picture on the comic’s page. The light coming from

the page is refracted twice — firstly as it passes fromthe air into the glass and secondly as it emerges

into the air.

Spreading out orclosing in?The diverging lens in the diagram below hasa biconcave shape. It is curved inwards, like acave, on both sides. A beam of light travellingthrough a biconcave lens spreads out (diverges).A magnifying glass is an example of a

converging lens. Its shape is biconvex. Thatmeans it is curved outwards on both sides. Abeam of light travelling through a biconvex lens‘closes in’ (converges) towards a point called thefocus. The distance between the focus and thecentre of the lens is called the focal length.

Lenses are shaped so that light passing through them either spreads out or closes in.

It’s unrealA biconcave lens does not havea real focus. When parallel lightrays emerge from a biconcavelens, they do not convergetowards the same point. How-ever, they do seem to be comingfrom a single point on the otherside of the lens. That point iscalled the virtual focus.

Images formed by glassWhen you look at an objectthrough a lens, what you see isan image of the object. Some-times it is larger than the object,sometimes it is smaller than theobject. It can be upside down(inverted) or the right way up.

biconcave lens

biconvex lens

incoming beam

incoming beam

virtualfocus

focus

focal length

YOU WILL NEEDcandlematchesjar lid to hold candlebiconvex lenslens holderwhite card for screen

• Place the biconvex lens in the lens holder, with the candle about one metre in front of it. Light the candle, and move the white card backwards and forwards on the opposite side of the lens until a clear image of the flame is visible on it.

1. Is the image on the screen upright or inverted?

• Move the candle towards the lens, stopping every 10 cm or so, while you try to locate the image on the white card. Do not move the candle closer than about 10 cm from the lens. Don’t be concerned if you cannot get a clear image on the card when the candle is close to the lens.

2. How does the image change as the candle is moved closer to the lens?

• Place the candle 5 cm from the lens. Attempt to find an image on the screen. If you cannot, look through the lens towards the candle, observing the image in the lens.

3. When the candle is close to the lens, can an image be found on the screen?

4. When you look through the lens at the candle, you see an image. Is it upright or inverted? Is it larger or smaller than the real candle?

SEARCHING FOR AN IMAGE

7.4

Through the looking glass


FOCUSING ON LIGHTYOU WILL NEEDray box kit power supplysheet of white paper ruler and fine pencil

• Connect the ray box to the power supply. Place a sheet of white paper on the bench in front of the ray box. Place the thinner of the two biconvex lenses in the kit on the paper and trace out its shape. Project three thin parallel beams of white light towards the lens.

• Trace the paths of the light beams as they enter and emerge from the lens. Remove the lens from the paper so that you can draw the paths of the light beams through the lens.

1. Why doesn’t the middle beam bend?2. How many times do each of the other beams bend

before arriving at the focus?

• Replace the thin biconvex lens with a thicker one and repeat the previous steps.

3. Which lens bends light more, the thin one or the thicker one?

4. How does the focal length of the thinner lens compare with that of the thicker lens?

• Turn the paper over and place the thinner of the two biconcave lenses on the paper and trace out its shape.

• Trace the path of each of the three thin light beams as they enter and emerge from the lens. Remove the lens from the paper so that you can draw the paths of the light beams through the lens.

5. Do the emerging rays come to a focus?6. Do the emerging rays appear to be coming from the

same direction? Use dotted lines on your diagram to check.

7. Make a prediction about the direction from which the same three incoming rays would emerge from a thicker biconcave lens. Check your prediction with the thicker biconcave lens in the kit.

biconvex lens

three thin parallel beams of light

sheet of white paper

ray box

7.5

Remember1. Name and sketch the shape of

a lens that makes a beam of light converge.

2. Name and sketch the shape of a lens that makes a beam of light diverge.

3. What does the focal length of a converging lens measure?

Think1. List some devices (other

than those listed at the topof page 158) that contain lenses.

2. A diverging lens does not make rays bend towards a single point, yet it has a focal length. Explain why.

3. Explain why the focus of a diverging lens is called a virtual focus.

ImagineYou are given a biconvex lens and asked to make a rough estimate of its focal length. (You are told that it is less than 30 cm.) How could you do this:(a) on a sunny day?(b) inside your classroom on a

cloudy day?

CreateUse two or more lenses and lens holders to make a model telescope on a laboratory workbench.

Investigate1. Find out how lenses are made.2. Find out how a refracting

telescope works. How is it different from a reflecting telescope?

ActivitiesIt’s the position that countsBiconvex lenses produce threetypes of image.• When an object is distant, the

image is inverted and smallerthan the object (e.g. theimage of a person made onphotographic film by acamera).

• When an object is close to thelens, but not closer than thefocal point, the image isinverted and larger than theobject (e.g. the image of asmall slide produced on ascreen by a slide projector).

• When an object is closer tothe lens than the focal point,the image is upright andlarger than the object (e.g. theimage you see in a magni-fying glass).

Focus on lenses

7.47.4


iris

pupil

lens

cornea

retina

opticnerve

to brain

ciliary muscle

suspensoryligaments

Side view of ahuman eye

Everything that you see is an image!A sharp image of what you are

looking at is formed on a‘screen’ at the back of

your eye. Thisscreen, called the

retina, is linedwith sight recep-

tors called rodsand cones. These

light-sensitivecells respond tolight by sendingsignals to yourbrain through

the optic nerve.Some of the light

reflected from your surround-ings, along with light emitted from luminous objects like the sun,enters your eye. It is refracted (bent) as it passes through thecornea, the transparent outer surface of the eye. The cornea iscurved, so that light converges towards the pupil. Light thenpasses through the clear, jelly-like lens and focuses on the retina.Most of the bending of light done by the eye occurs at thecornea.

On its way to the lens, the light travels through a hole in thecoloured iris called the pupil. The iris is a ring of muscle whichcontrols the amount of light entering the lens. In a dark room theiris contracts to allow as much of the available light as possiblethrough the pupil. In bright sunlight the iris relaxes, making thepupil small to prevent too much light from entering. The lensbends the light further, ensuring that the image formed on theretina is sharp. The image formed on the retina is inverted. How-ever, the brain is able to process the signals coming from the retinaso that you see things the right way up.

The image formed on the retina is upside down, but the brain sees it the right way up.

Getting things in focus

Although most of the bending oflight by the eye occurs at thecornea, it is the lens that ensuresthat the image you see is sharp.The shape of the lens is controlledby the ciliary muscles. Whenyou look at distant objects, thesemuscles are relaxed and the lensis thin. When you look at nearbyobjects, the ciliary muscles con-tract. The suspensory ligamentsbecome slack, causing the lens tobulge. This action of the lens inobtaining a sharp image on theretina is called accommodation.

The light coming from a nearby object needs to be bent more than the light coming from a distant object. The lens in your eye becomes thicker when you look at nearby objects.

light from a distant object

light from a nearby object

lens

Each human eye contains just one biconvex lens. Insects have compound eyes. Each eye contains many lenses. Some types of dragonfly have more than 10 000 lenses in each eye. Each eye can focus light coming from only one direction.

It’s the image that counts


YOU WILL NEEDruler

• Look closely at the X printed here from the smallest distance at which you can see it clearly and sharply with comfort. Quickly look away and focus on a distant object for a second or two and then focus on the ‘X’ again from the smaller distance.

• Try to feel the action of the muscles that allow you to see a sharp image of the ‘X’.

• Use the following procedure to estimate the smallest distance at which you can obtain a clear image of a nearby object. (If you are wearing glasses, remove them during this part of the experiment.)

• Hold this book vertically at arm’s length from your eyes and focus on it. Move the book to a position about three or four centimetres from your eyes and then gradually move the book further away until you can see the print clearly and sharply.

• Have a partner use the ruler to estimate the distance between the page and your eyes. The ruler should be placed carefully beside your head for this measurement.

• Record the distance measured.• Collate the results for the whole

class and determine the average smallest distance at which a clear image could be obtained.

1. How does your result compare with the average smallest distance for your class?

2. Write down the highest single result and lowest single result for your class. Comment on the range of results.

GETTING A CLEAR IMAGE

7.6 Too close for comfortAs you get older, the tissues that make up the lens become lessflexible. The lens does not change its shape as easily. Images ofvery close objects (like the words you are reading now) becomeblurred. The lens does not bulge as much as it should and the lightfrom nearby objects converges to a point behind the retina insteadof on the retina. You may have to hold what you are reading fur-ther away in order to obtain a clear image. This means you do nothave 20/20 vision. Having 20/20 vision means you can read letters ofa certain height from a distance of 20 feet (approximately six metres).

This change in accommodation with age is a natural process.Some people are not inconvenienced at all while others need towear reading glasses so that they can read more easily and com-fortably. Optometrists use a retinoscope to measure how the lensin your eye bends light. Then they work out the lens required tocorrect the problem. This information will be written down so thelens maker can make up the correct glasses for your eyes. Thetable below shows how the smallest distance at which a clearimage can be obtained changes with age. The distances shownare averages and there is a lot of variation from person toperson.

Age compared with average smallest distance at which a clear image can be obtained (cm)

Age (years) 10 20 30 40 50 60

Distance (cm) 7.5 9 12 18 50 125

Remember1. How does the eye send

messages to your brain?2. Where in the human eye does

most of the bending of light occur?

3. What is accommodation?4. Sketch the shape of the lens in

the eye when you are viewing:(a) a nearby object(b) a distant object.

5. Explain how the lens in your eye is able to change its shape.

Think1. Does light slow down or speed

up when it passes from the air into the cornea? (Hint: Refer to page 156.)

2. Why does the lens need to be thicker for viewing nearby objects?

InvestigateHow is a camera like an eye? Find out which parts of the camera:(a) bend incoming light(b) control the amount of light

falling on the film(c) act as the screen, like the

retina of the eye.

Using dataUse the data in the table above to draw a line graph to show how the ability to focus on nearby objects changes with age.1. Use your graph to predict the

smallest distance at which a clear image can be obtained by an average person of your age.

2. At what age does the decrease in focusing ability appear to be most rapid?

Activities

Your eye

7.57.5


The eye is truly an amazing optical system: it is able to focus onobjects only centimetres away as well as distant objects many kilo-metres away. However, the ability to obtain sharp images variesfrom person to person.

The most common reasons for being unable to obtain sharpimages are short-sightedness and long-sightedness. Both of theseconditions can be corrected by wearing glasses or contact lenses.

Myopia (short-sightedness)A person who is short-sighted is able to focus clearly on nearbyobjects but is unable to obtain a sharp image of distant objects.Reading the blackboard or whiteboard from the back of the class-room might be difficult for a short-sighted person. However,reading the print on this page as you are now would not be aproblem.

The blurring of the images of distant objects occurs because thelight rays coming from them are bent so much that the image formsin front of the retina. The combined focusing power of the corneaand lens is too strong for the length of the eye.

Myopia can be corrected with glasses with diverging lenses.The diverging lenses used in glasses are called convexo-concavelenses. The concave side is more curved than the convex side,so it diverges parallel rays of light. With correctly prescribedglasses, images of distant objects form on the retina instead of infront of it.

Hyperopia (long-sightedness)A person who is long-sighted is able to focus clearly on distantobjects but is unable to obtain a sharp image of nearby objects.Reading the print on this page might be difficult for a long-sightedperson. However, reading road signs in the distance would not bea problem.

Difficulty in seeing the detail of nearby objects occurs becausethe rays coming from them are not bent enough to form an imageon the retina. The combined focusing power of the cornea and lensis too weak for the length of the eye. The ciliary muscles need towork hard to make the lens bulge enough to make up for the lackof normal focusing power. As a result, long-sighted people oftensuffer from headaches and ‘tired’ eyes. If the lens cannot bulgeenough to bring the rays to a focus, images of the nearby objectsare blurred.

Hyperopia can be corrected with glasses with converginglenses. The converging lenses used in glasses are called concavo-convex lenses. The convex side is more curved than the concaveside, so it converges parallel rays of light. With correctly prescribedglasses, images of nearby objects form on the retina without eyestrain and headaches.

Myopia (short-sightedness): The image of distant objects is blurry because the focusing power of the cornea–lens system is too strong for the length of the eye. The light rays coming from distant objects converge in front of the retina. This problem can be corrected by a diverging lens.

Hyperopia (long-sightedness): The image of nearby objects is blurry because the focusing power of the cornea–lens system is not strong enough for the length of the eye. The light rays coming from nearby objects converge behind the retina. This problem can be corrected by a converging lens.

Two pairs of glasses in oneThe natural decrease in the abilityto accommodate by changing theshape of the lens in the eye isdescribed on the previous page.The problems caused by thisnatural decrease, called presby-opia, can be solved by wearingreading glasses with converginglenses. However, many olderpeople have difficulty formingclear images of distant objects as

light from adistant object


blurry image

sharp imageon retina

convexo-concavelens

light fromnearby object

light from nearby object

blurryimage

sharp imageon retina

concavo-convexlens

Improving your image


well. Instead of having to wearone pair of glasses for readingand another pair for the rest ofthe time, bifocals can be used.The lenses in bifocals are shapedso that they converge light at thebottom to assist close visionwhen reading, and diverge lightat the top to assist distance vision.You can usually tell when peopleare wearing bifocals because theyneed to adjust their head positionwhen they read so that they arelooking through the bottom partof their glasses.

Two pairs of glasses in one. Bifocals assist in the vision of both nearby and distant objects.

AstigmatismApart from short-sightedness andlong-sightedness, the mostcommon eye problem is astig-matism. Astigmatism is acondition where the light comingfrom one direction is bent morethan the light from another direc-tion. It is usually caused by anirregularly shaped cornea orlens. The result is that it is impos-sible to obtain a sharp image ofany object.

Astigmatism can be correctedwith glasses that have a lens espe-cially shaped to compensate forthe irregular shape of the corneaor lens. Hard contact lenses arevery useful in correcting astigma-tism. They are made of plastic andare shaped to fit over the cornea.Soft contact lenses are not aseffective in correcting astigmatismbecause their outside surfacetakes up a shape similar to themisshapen cornea.

CataractsAs the lens becomes less flexible with age, it can become less trans-parent. Small cloudy spots, called cataracts, can develop in parts ofthe lens. Sometimes, they spread through the whole lens causingblurred vision. In severe cases, cataracts cause blindness as the lensbecomes completely opaque.

Cataracts are usually associated with old age. However, they canalso be caused by eye injuries, some drugs and some forms ofradiation. Some babies are born with cataracts.

When cataracts are serious enough to blur vision, the affected lensis surgically removed. It is replaced with a plastic lens. Unlike theoriginal lens, it has a fixed shape and cannot accommodate to focuson both distant and nearby objects. People who have had cataractsremoved therefore need glasses or contact lenses to compensate forthe lack of accommodation.

IS IT CLEAR?The astigmatic fan chart on the right can be used to test for astigmatism. Look at the chart from a distance of about 50 cm. If your vision is affected by astigmatism, some of the lines will appear more clearly than others. Test one eye at a time, covering the other eye with your hand. Astigmatic fan chart

Remember1. Which eye condition occurs if

the combined focusing power of the cornea and lens is too strong for the length of the eye?

2. Which type of lens would you expect to find in glasses worn by a person who is long-sighted?

3. What are bifocals and why are they used?

Think1. Which condition of the eye is

most likely to be responsible for each of the following problems?(a) A student who can read the

blackboard from the back of the room has to strain to read the print in a textbook and gets headaches while reading magazines at home.

(b) A science teacher who has never had eye problems before begins to find it easier to read books when they are held further away.

(c) A retired builder who has always had good eyesight begins to experience blurred vision. It gradually gets worse and affects vision of both nearby and distant objects.

(d) A young child cannot see sharp images of nearby or distant objects.

(e) A person who has no problem reading a newspaper can’t read the numbers on the scoreboard at a football match.

2. Explain why it is impossible for a person who has had cataract surgery to accommodate.

Activities


7.6

A L O O K I N S I D E

An endoscope being used to look inside a patient’s stomach. This type of endoscope, called a gastroscope, is passed through the mouth of the patient.

A bundle of optical fibres. The light can be seen through the ends.

The baby in the photograph onthe right is still inside its mother’swomb. It has been photo-graphed through a long, flexibletube called an endoscope.Inside the endoscope are twobundles of narrow glass strandscalled optical fibres. The glassin optical fibres is made so thatlight is unable to emerge fromthe sides of the glass fibres.

A beam of bright light isdirected through one bundle offibres, illuminating the inside ofthe womb. Some of this light isreflected off the womb andtravels through the other bundleof fibres. A lens at the end ofthis bundle allows an image ofthe womb to be viewed, photo-graphed or videotaped.

Different types of endoscopeinclude:• gastroscopes, which are used to

examine the stomach and otherparts of the digestive system

• arthroscopes, which are usedto search for problems in jointslike shoulders and knees

Optical fibres allow us to see inside the human body.

• bronchoscopes, which areused to see inside the lungs.Endoscopes can also be used

in laser surgery. Intense laserbeams can be directed into theoptical fibres. The heat of thelaser beams can be used to sealbroken blood vessels or

destroy abnormal tissue insidethe body.

As light travels from a sub-stance such as glass into air, itbends away from the normal(see page 156). If the lightstrikes the boundary at a smallenough angle, it bends so


much that instead of leavingthe glass, it is reflected backinto it. This process is calledtotal internal reflection. Thediagram below shows howtotal internal reflection occursin an optical fibre.

Communicating with visible lightOptical fibres are used totransmit sound and images overlong distances. They are smaller,lighter, more flexible and moreefficient than the electricalcables previously used for long-distance telephone, radio andTV communication.

Electrical signals from amicrophone, television camera,computer or fax machine areconverted into pulses of lightand transmitted along an opticalfibre. These light pulses arereceived at the other end of thefibre and converted back intoelectrical signals that can be fedinto speakers, a television set,another computer or a faxmachine. The signals can alsobe retransmitted as radio wavesif necessary. Because a laserbeam does not spread out as ittravels through the opticalfibres, the light can travel longdistances through the glass withlittle energy loss.

optical fibre

light beamreflectedoff sides

Total internal reflection in an optical fibre

Even though the first successful glass optical fibres were notmade until 1973, the advantages of optical fibres are so great thatAustralia already has a network of fibre cables between all capitalcities. Optical fibres can be laid under the ground or under water.

KEEPING THE LIGHT INSIDEYOU WILL NEEDray box with triangular prismpower supplysheet of white paper

• Connect the ray box to the power supply. Place a sheet of white paper on the bench in front of the ray box.

• Use a black plastic slide to produce a single thin beam of light that is clearly visible on the white paper.

• Place a perspex triangular prism on the sheet of paper and direct the thin beam of light towards it as shown in the diagram. Observe the beam as it passes through the prism.

• Turn the prism slightly anticlockwise, closely observing the thin beam as it travels from the perspex back into the air. Continue

to turn the prism until the beam no longer emerges from the perspex.

1. What happens to the beam of light when it no longer emerges from the perspex?

2. Draw a diagram showing how the path of the beam of light changed as you turned the prism.

Observe the beam as it passes through the prism.

7.7

Remember1. What is an endoscope?2. List three uses of endoscopes.3. How do optical fibres allow light

to travel along a bent tube?

Think1. Suggest some other methods of

finding out what lies in the wall of a human stomach. What are the advantages of an endoscope over these other methods?

2. Can total internal reflection occur when light travels from air into glass? Explain your answer. Use diagrams if necessary.

Investigate1. Find out how optical fibres

have replaced other methods of long distance communication.

2. When technology reveals a severe problem in a developing fetus, parents may be given the option of an abortion. This is unacceptable to some groups in our society.(a) List some arguments for and

against using abortion as a solution.

(b) Why do some groups have different views on abortion?

(c) Should we use technology to examine a developing fetus? Explain.

(d) What ethical considerations need to be taken into account when using technology that has been developed from new scientific knowledge?

Activities

7.77.7


Colour your world

Imagine a world without colour.

Imagine how dull life would bewithout colour. Not only doescolour make the world moreinteresting, but it can even affectour moods — or can it? Thinkabout why interior designerschoose different colours for dif-ferent rooms, why some peoplechoose to wear brightly colouredclothes, and why food colouringis added to foods that are alreadycoloured.

It was Sir Isaac Newton, in1666, who discovered that whitelight consisted of different col-ours. This set of colours is calledthe visible spectrum. The col-ours of the visible spectrum areusually described as red, orange,yellow, green, blue and violet.However, there is no sharpboundary between these colours.They merge into each other

because there are very slight dif-ferences in the amount ofbending of the colours as theyrefract. If white light can be bentenough, as it is when it passes inand out of a triangular prism, the

differences in the amount ofbending can be observed and thevisible spectrum is produced.The separation of white light intoits colours as a result of bendingis called dispersion.

WHAT’S IN WHITE LIGHT?

YOU WILL NEEDslide projector2 triangular glass prismssheet of white paper (A4 will do)ray boxpower supply

• Place a triangular glass prism in front of the beam of the slide projector. The prism needs to be about 10 cm in front of the projector.

• Use the sheet of white paper as a screen just behind the prism and move the prism around until you can see a band of different colours on the screen. Once you have found the band, move the screen away from the prism and try to project it onto a wall.

1. What colours could you see when the white light was separated into different colours by the prism?

• Connect a ray box to the power supply. Place a triangular prism on the bench in front of the ray box as shown in the diagram. Project a single thin beam of light towards the triangular prism.

• Use the piece of white paper as a screen to display the beam after it passes through the prism. Move the prism until a band of colour is produced.

2. Which colour is bent the most and which is bent the least by the prism?

3. Suggest how the glass in the prism managed to separate the colours.

• Use a second prism to try to merge the colours into a white light on the screen.

4. Draw a diagram to show how a second prism can be used to merge the colours separated by the first prism.

triangular prism

ray box

7.8


Why is the sky blue?White light is actually made up of six different colours of light, eachwith its own frequency. When light from the sun hits molecules inthe air, it gets scattered in all directions. Blue light has the highestfrequency and is scattered throughout the sky ten times as stronglyas red light. When you look up at the sky, you see blue becausemore of the high frequency blue scattered light is reaching your eye.

Seeing redWhen the sun is low in the sky, it appears to be very red. The pathof the sunlight through the atmosphere is much longer at sunriseand sunset because the sunlight passes through more air. By thetime the light reaches the lower atmosphere, the colours at theblue end of the spectrum have been scattered away even morethan usual. Clouds take on an orange-red colour as the othercolours of the spectrum have been scattered higher in the atmos-phere. When the atmosphere also contains dust particles this alsocreates spectacular sunrises and sunsets.

CAUTION: Never look directly at the sun!

The path of sunlight through the atmosphere is much longer at sunrise and sunset.

sunoverhead

sun atsunset

sun atsunrise

atmosphere

Earth

observer

more blue lightscattered byatmosphere

more blue lightscattered byatmosphere

Remember1. List the six commonly known

colours of the spectrum from the colour that bends most in glass to the one that bends least.

2. Explain why the sky is normally blue.

3. Why does the sun appear to be redder when it is rising and setting than during the middle of the day?

Think1. How do you know that white

light consists of different colours?

2. Which colour of light slows down more when it moves from air into glass, red or blue? Explain how you decided on your answer.

3. Colours can be separated by both dispersion and scattering. Explain the difference between dispersion and scattering.

ImagineImagine what life would be like if you could see things only in ‘black and white’. Think carefully about how that would affect you. Write a short science fiction story or play about a day on Earth when all humans had their colour vision destroyed by a strange type of radiation from outer space.

Investigate1. Try shining a torch through

a glass of water with a few drops of milk added. Look through the glass from the side, perpendicular to the beam of light.

2. Find out how a rainbow is formed and why it is curved.

3. Design an investigation to find out whether colour affects mood.

4. Design an experiment to find out if red is the best colour to represent ‘stop’ in traffic lights.

Activities

Seeing red at sunset

7.87.8


ADDING COLOURS

YOU WILL NEEDray boxpower supplyred, green, blue, yellow, magenta and cyan filterswhite surface to use as a screen

• Connect the ray box to the power supply and darken the room.

• Use the ray box and the coloured filters as shown in the diagram below right to project the combinations of colours in the table onto the white screen. Observe and record the colour produced on the screen by each combination. When only two colours are combined, one of the side mirrors can be closed.

1. Which combinations of coloured light produced white light on the screen?

2. What colour would you expect to see on the screen if you combined:(a) magenta light with green light?(b) yellow light with blue light?Give reasons for your answers and then use the equipment to see if you were correct.

Move the mirrors to project different colours onto the same part of the screen.

Adding colours

Colours combined Colour observed on screen

red + green + blue

red + green

red + blue

green + blue

yellow + magenta + cyan

ray boxmirror

mirrorfilters

7.9

yellowcyan

magentamagenta

white

Although the light coming fromthe sun and most electriclighting is white, your surround-ings are full of colour. Whitelight consists of all the coloursof the visible spectrum. Thecolour of the non-luminousobjects that you see depends onwhich parts of the spectrum arereflected towards your eyes.

When white light falls on anysurface, some colours arereflected while others areabsorbed. A red surfaceabsorbs all of the colours of thespectrum except red. Only redlight is reflected. A green sub-stance absorbs all of the coloursexcept green, and a blue sub-stance absorbs all of the coloursexcept blue.

What’s so special about red, green and blue?

Red, green and blue light can becombined to produce white light.Different combinations of thesethree colours can also be used toproduce all other colours. Forthis reason they are known asprimary colours. Colours madeby mixing primary colours arecalled secondary colours.

The coloured images that yousee on TV or computer screensare produced by hundreds ofthousands of narrow red, greenand blue beams. The strength ofeach of the beams is controlled bythe television or video signal toproduce a wide range of colours.

Blue, green and red light can be combined to produce white light.

Seeing in colour


Colour in printThe print and colour photographs in this book consist of thousandsof tiny yellow, magenta, cyan and black dots made of pigments. Thedots are so close together your eyes and brain are not able to seethem separately. Instead, you see the combined effect. The blackdots add sharpness to the image. Pigment colours work by absorbinglight.• Magenta pigments absorb green light and reflect a mixture of

blue and red light.• Yellow pigments absorb blue light and reflect a mixture of red

and green light.• Cyan pigments absorb red light and reflect a mixture of blue and

green light.By varying the number of dots, all colours can be produced. The

light reflected from this book, therefore, consists of yellow,magenta and cyan. The photograph below shows how the colourphotograph on page 164 was created colour by colour. It alsoshows a magnified view of a small area of the photograph.

Subtracting colours

The filters used in the experi-ment on page 168 absorb all ofthe colours in the spectrumexcept the individual filter’scolour. In other words, they‘subtract’ colours from whitelight. A red filter absorbs all ofthe colours except red. Whenyou hold a red filter in front ofyou it appears red because onlyred light passes through it. Amagenta filter allows both redand blue light to pass through,allowing you to see the colourmagenta when you hold thefilter up to the light.

Most colour photographs in books are printed separately in four different colours.

Yellow, magenta, cyan and black platesYellow, magenta and cyan plates

Yellow and magenta platesYellow plate

Magnified area 5×


SUBTRACTING COLOURS WITH FILTERS

YOU WILL NEEDray boxpower supplyred, green, blue, yellow, magenta

and cyan filterswhite surface to use as a screen

• Make a copy of the table below in which you can record your predictions and observations.

• Connect the ray box to the power supply and darken the room.

• Project a beam towards the screen and place the magenta filter in the ray box. Predict the colour that you would expect to observe on the screen if you place a red filter in front of the magenta filter. Test your prediction and record the colour observed in the table.

• Replace the red filter with a green filter and again predict and observe the colour seen on the screen.

Subtracting colours with filters

Filter in ray box

Filter placed in front

Predicted colour on

screen

Observed colour on

screen

magenta red

green

blue

cyan and yellow

cyan red

green

blue

yellow red

green

blue

• Replace the green filter with a blue filter and yet again predict and observe the colour seen on the screen.

• Remove the blue filter and place both the cyan and yellow filters directly in front of the magenta filter. Make and record your prediction about what you will observe on the screen before you make your observation.

• Use the filters that you have available to complete the table. Add lines to the table if you would like to test other combinations.

1. Which primary colours (red, green or blue) are transmitted by:(a) the magenta filter?(b) the cyan filter?(c) the yellow filter?

2. Which primary colour is subtracted by:(a) the magenta filter?(b) the cyan filter?(c) the yellow filter?

3. What colour was produced when the magenta, cyan and yellow filters were all placed in front of the white beam?

7.10

Paints and dyesThe colours used in mixingpaints and dyes are yellow,magenta and cyan. These col-ours can be mixed in differentproportions to produce a widerange of colours. Adding colourswith paints and dyes is very dif-ferent from adding colouredlight beams. Paints and dyes aremade to subtract colours. Forexample, green paint is made bymixing yellow paint with cyanpaint. The yellow paint absorbs(or subtracts) blue light. It looksyellow because red and greenlight are reflected from it. Thecyan paint absorbs (or subtracts)

red light. Its cyan colour is theresult of reflected green andblue light. The diagram below

shows that the light reflectedfrom the mixture of yellow andcyan paint is mostly green.

Green paint is a mixture of yellow and cyan paint.

blue light absorbed

white light

yellow paint

red light absorbed

white light

cyan paint

some blue and red light absorbed

white light

green paint


Colour in the eye of the beholder

The retina contains two different types of receptor cell. Thesereceptor cells, called rods and cones, respond to light by sendingsmall electrical signals to the brain, which then interprets the imagethat you see. The retina contains about 100 million rod cells andabout six million cone cells.

The receptor cells in the retina respond to brightness and colour.

The rods contain a single pigment which detects the brightness ofthe light falling on the retina. The rods are unable to detect colour.

There are three types of cone cell. Each type contains a differentcoloured pigment which corresponds to one of the primary colours:red, green and blue. The huge range of colours that you see are allcombinations of different proportions of these three primary colours.

The cone cells are most closely packed in the centre of theretina. This is where the image of what you are focusing on isformed. The cone cells are spread more thinly in other parts of theretina, so it is harder to distinguish the colour of objects in yourperipheral (side) vision.

The cone cells are more sensitive than rod cells in bright light.Because of this, it is more difficult to distinguish between colours indim light.

Colour confusionMost colour-blind people are unable to distinguish between redand green because of abnormalities in the red and green pigmentsin the cones. Almost all colour-blindness is inherited and cannotbe cured. About eight per cent of men are colour blind, while only0.5 per cent of women are colour blind.

ganglioncell layer

bipolar layer

receptor layer

pigment cell layer

nerve fibres

rod

cone

Remember1. Why does a red surface look

red when white light is falling on it?

2. Why are red, green and blue called primary colours?

3. Yellow, cyan and magenta are referred to as secondary colours because they can be made by adding pairs of primary colours together. Yet in printing, paints and dyes, the primary colours are referred to as yellow, cyan and magenta. Why are these three colours listed as primary colours?

4. What happens to white light as it passes through a red filter?

5. What happens to white light as it passes through a magenta filter?

6. Which living cells in the retina enable you to detect colour?

7. Which three primary colours can be detected by your eye?

8. Why is it so difficult to identify a car by its colour at night?

Think1. Which two primary colours

pass through a yellow filter?2. What colour would a bright

red shirt appear to be under:(a) red lights?(b) blue lights?(c) yellow lights?Explain how you obtained each of your answers.

3. Explain in your own words how a blue shirt is seen to be blue in sunlight.

4. Compare the way that colour images are produced by colour television and colour printing?

CreateCreate a colour wheel with a disc of cardboard. Colour one third of the cardboard red, another third green and the final third blue. Make a hole in the centre of the cardboard disc so that a pencil can be inserted through it. The pencil needs to fit tightly enough so that the wheel spins when you spin the pencil. What colour do you see when the disc is spun quickly?

Activities

7.97.9


Light and its behaviour have always fascinated scientists. The ancient Greeks understood that light could be refracted and reflected. Aristotle thought that both light and sound moved about in waves, very much like the waves in the sea. He believed that there had to be air present for light or sound to travel. He thought that if scientists could make a perfect vacuum, it could be proved that light and sound waves were not able to travel through a vacuum. He also thought that it would be impossible to create a vacuum.

By the 17th century, many observations of light had been made, but no one theory explained why light travelled in straight lines, cast shadows or why it split into different colours when it travelled through a prism.

Around 1670 two famous scientists put forward two very different theories that set the scientific community debating, discussing and experimenting for the next 200 years.

One of these scientists was Sir Isaac Newton. Using the work of Evangelista Torricelli, who succeeded in creating a vacuum, Newton found that light did shine through a vacuum. He now knew that Aristotle had been wrong on this point. Newton claimed that light was made up of particles he called ‘corpuscles’. This theory became known as the particle theory of light. Newton used his particle theory to explain why light reflected. His logic was that when these corpuscles hit a surface, they simply bounced off like a tennis ball. He also explained refraction by saying that the corpuscles travelled faster or slower through glass, air or water.

The other scientist with a theory on light was Christiaan Huygens. He suggested that light did indeed travel in waves. His theory was called the wave theory of light. Like Aristotle, Huygens believed light travelled like waves rippling on a pond. He was able to explain the spectrum of light through a prism by saying that each colour had a different wavelength. The amount of bending depended on the wavelength of the light. Those parts of light with the shortest wavelength were bent, or refracted, the most.

The two theories presented the scientific community with a dilemma. Huygens’ wave theory explained the visible spectrum, but not reflection or why light cast shadows. Another part of the dilemma was that waves usually need some material to move across, but the light that reaches Earth comes though space, where there is nothing for the waves to move in. For a very long time, Newton’s particle theory was the most popular explanation of this scientific mystery.

In 1801 Thomas Young, a doctor who had been studying the eye, made a breakthrough. He had become fascinated by the fact that the eye could create all colours just by recognising three colours — red, blue and green. This work led him to perform what is now known as Young’s double-slit experiment. This proved that light was in fact made up of waves by showing interference patterns. His discovery upset many scientists, as they were not willing to accept that Newton had been wrong.

Now the remaining problem was explaining how light travelled through empty space. To solve this, scientists came up with the idea of a special material called luminiferous (light-producing) material, in which the waves travelled.

The next major breakthrough occurred in the 1860s when James Clerk Maxwell connected light, electricity and magnetism. He said that light was a wave in which energy was carried from a source by electric and magnetic fields. This type of wave could travel through a vacuum. He used mathematical calculations to support his theory. He determined that light was just a small part of a wide range

Light historyAristotle(b. 384 Macedonia –d. 322 BC Greece)

Sir Isaac Newton(b. 1642 England –d. 1727 England)

Evangelista Torricelli(b. 1608 Italy –d. 1647 Italy)

Christiaan Huygens(b. 1629 Holland –d. 1695 Holland)

Thomas Young(b. 1773 England –d. 1829 England)

James Clerk Maxwell(b. 1831 Scotland –d. 1879 England)


of electromagnetic waves, which we know today as the electromagnetic spectrum. This spectrum ranges from gamma rays to radio waves.

Towards the end of the 19th century, most scientists were convinced that light was a wave. However, doubts started to creep in when, in 1887, Edward Morley and Albert Michelson proved that there was no such thing as luminiferous ether.

In 1900, while studying heat, Max Planck determined that like heat, light was a form of electromagnetic radiation that travelled in packets that he called quanta. He worked this out using a mathematical formula, but even he found the results difficult to believe.

It was Albert Einstein who really understood what Planck’s results meant. Before Planck, Philipp Lenard had discovered that when light struck certain metals, electrons were emitted. This was called the photoelectric effect. However, it was Einstein who came up with the explanation of these extraordinary results. He said that light could be thought of as little packets of energy called photons, and not as waves. It looked like Newton’s theory was in fact correct. However, Einstein’s photons were not thought of as little balls and they behaved like waves. Einstein said that light was in fact both a particle and a wave. This theory is still accepted today. It is this dual nature of light that makes it one of the most interesting mysteries of the universe. Einstein also worked out that nothing travels faster than light. In his famous theory of relativity where E = mc2, c is the speed of light. There is, of course, more of this story yet to be discovered . . .

Arab scholars such as Alhazen (c. 965–1038) knew enough

about the refraction of light to develop lenses for spectacles.

Then in 1666 Sir Isaac Newton discovered that a prism placed in

a beam of light split the light into different colours and that a

second prism could recombine the colours. These colours were

known as the light spectrum. There were only six colours: red,

orange, yellow, green, blue and violet. But because seven was

considered a lucky number at the time, violet was split into

indigo and violet.

The measurement standard for the metre is defined as the

distance light travels through a vacuum in of a

second. Using a stabilised iodine laser as the source of light,

scientists all over the world can reproduce their own accurate

standard of length because light in a vacuum travels at the same

speed everywhere.

1

299 792 458--------------------------------

Remember 1. Who first thought light energy

was a wave?2. Explain the difference between

the wave theory of light and the particle theory of light.

3. Who finally solved the problem of whether light was a wave or a particle? Explain the solution.

4. Why were scientists upset by Thomas Young’s discovery?

Investigate 1. Find out more about the life of

one scientist mentioned in this section.

2. What is the difference between a law and a theory? List the laws related to light that you understand.

Imagine Imagine you were a scientist working on light in the 19th century. Make a list of the problems you might have in developing your theories and communicating with other scientists.

Activities

Edward Morley(b. 1838 United States –d. 1923 United States)

Albert Michelson(b. 1852 Prussia –d. 1931 United States)

Max Planck(b. 1858 Germany –d. 1947 West Germany)

Albert Einstein(b. 1879 Germany –d. 1955 United States)

Philipp Lenard(b. 1862 Hungary –d. 1947 West Germany)


it all togetherPutting Putting it all together

Word list

magenta

cone

focus

away

internal

printing

optical

absorbed

power

scatter

diverging

speed

image

refraction

cornea

biconvex

dispersion

direction

accommodation

long-sightedness

white

fibres

Copy and complete the statements below to compile a summary of this unit. The missing words can be found in the word list below.

1. You can see beams of light only when particles in substances like air some of the light towards your eyes.

2. When light meets a boundary between two different substances, it can be reflected,

or transmitted.

3. Everything that you see is an .

4. When light travels from one substance into another, it changes and, unless it crossed the boundary at right angles, changes

as well. This process is called .

5. When light travels from water into the air, it bends from the normal.

6. A lens is curved outwards on both sides. It converges light towards a point called a .

7. A lens spreads light out.

8. The action of the lens in obtaining a sharp image on the retina is called .

9. Most of the bending of light done by the human eye occurs at the .

10. If the combined focusing of the lens and cornea is too weak for the length of the eye, images of nearby objects become blurry. This condition is commonly known as

.

11. Endoscopes, which include bundles of , can be used

to look inside the human body.

12. The total reflection of light when it travels from glass into air is called total reflection.

13. The separation of white light into its colours, called , occurs because different colours of light are bent by different amounts as they refract.

14. Red, green and blue light can be combined together to produce light.

15. The colours yellow, and cyan are used in , paints and dyes because they can be combined in different proportions to produce a wide range of colours, including white.

16. The receptor cells on the retina detect the brightness and colour of light. It is the

cells that respond to colour.

Summing upSumming up


1. Thin beams of light are projected from a ray box towards four objects. Each object is hidden from view by a tile. The beams emerging from each of the objects are shown in the diagrams below. Name or describe the object behind each tile.

What is behind each of the tiles?

2. Explain the difference in the meaning of each of the following pairs of words.(a) ray and beam(b) reflection and scattering(c) refraction and dispersion(d) transparent and translucent(e) converging and diverging(f) concave and convex(g) focus and virtual focus(h) gastroscope and arthroscope(i) cones and rods

3. Redraw the diagram of the eye below. Complete all the labelling and state the main function of each labelled part of the eye.

4. Explain how the eye detects colour and why some people are colour blind.

5. The diagram below shows how rays from a distant object arrive at the retina of a person with blurry distance vision.

How can this problem be corrected with a lens?

(a) What is the name of the condition illustrated above?

(b) What does the correcting lens need to do to the incoming light in order to correct the problem?

(c) Draw a diagram to show how anappropriate lens placed in front of the eye shown above changes the path of light sothat a clear image of a distant object fallson the retina.

6. Using diagrams where appropriate, explain:(a) why your legs look short when you stand in

clear water(b) why the sky is blue(c) why the coloured photographs in most

books contain only dots of four different colours.

ray box

tileA

ray box

tileB

ray box

ray box

tileD

tileC

to brain

opticnerve

ciliary muscle

pupil


blurryimage

Looking back

Extension Extension


Fun in the sunSlip on a shirt, slop on somesunscreen and slap on a hat!Protect yourself from that invis-ible ultraviolet radiation thatcauses sunburn and skin cancer.However, a shirt, sunscreen anda hat are not enough! Ultravioletradiation can threaten your sightas well as your skin. A good pairof sunglasses can protect youreyes from the hidden dangers ofultraviolet radiation.

The depletion of the ozonelayer high in the atmosphereincreases the risk of damage byultraviolet radiation. Your cornea,lens and retina can all bedamaged by too much exposureto ultraviolet radiation.

The eyelids and surroundingskin can become burnt,increasing the risk of developingskin cancer and cancer of theeyelids.

The cornea can becomeinflamed and burnt. A sunburntcornea (known as photokera-titis) is very painful andespecially common when sun-light is reflected from snow.Repeated or extended exposurecan kill cells in the cornea,reducing the amount of light thatcan get through.

The ultraviolet light absorbedby the lens can cause cataracts,which can eventually result inthe need for surgical removal ofthe lens.

The cells on the retina can beseverely damaged by the smallamount of ultraviolet radiationthat is not absorbed by thecornea and lens. This type ofdamage is the most commoncause of blindness in olderadults.

Dangerous timesThe dangers of ultraviolet radi-ation are greatest between 10 amand 2 pm (11 am and 3 pm whendaylight-saving is in operation), athigher altitudes and when sun-light is reflected from sand orsnow.

Polarised lensesSunglasses with labels that indi-cate that they have polarisedlenses offer no more protectionfrom ultraviolet radiation thanother sunglasses. Polarisedlenses have a layer that reducesthe glare caused by the reflectionof light from smooth surfaces likesnow, sand and water.

Mirrored sunglassesSunglasses that reflect the mostvisible light have lenses with thinmetallic coatings on the surface.The fact that they reflect visiblelight doesn’t guarantee they willreflect ultraviolet radiation.

Choosing the right pairBecause of the dangers of ultra-violet radiation, you shouldchoose sunglasses carefully. Youshould check the label to ensurethat they absorb at least 99 percent of ultraviolet radiation. Sun-glasses that block a lot of visiblelight without absorbing ultravioletradiation can be more dangerousthan no sunglasses at all. The lackof visible light causes the pupil toopen more. This allows even moreultraviolet radiation to enter yourlens and fall on the retina thanwould normally be the case.

Remember1. List the different forms of

damage that can be done to the eyes by ultraviolet radiation.

2. At what time of day is ultraviolet radiation most dangerous?

3. Explain why a poor pair of sunglasses can be more dangerous than no sunglasses at all.

ThinkList, in order of importance, the features that you consider when choosing a pair of sunglasses. Compare your list with those of others in your class.

CreateDraw a poster that warns people about the need for effectively protecting the eyes from the sun.

InvestigateConduct a survey to find out whether paying more for sunglasses means better protection from ultraviolet radiation.

Activities

In the dark


Reflection Reflection

1. Tell a storyChoose one of the following topics and write a creative story describing the process. Your story can be funny, but the science needs to be correct. Write your story from the point of view of a cell, a part of the body, a ray of light or an object.(a) an endoscope examination (of any type)(b) an eye examination(c) a telephone message sent through optical fibres(d) an eye sending messages about colours to the

brain(e) a crystal hanging in a sunbeam.

2. Colour my worldThink about what you now know about colour.

Choose one of the following questions, and use diagrams and words to show what you know.(a) How is colour produced?(b) At a recent dance you noticed your clothing

changed colour at certain times. Explain how this could happen.

(c) How do printers use colour science to produce a full-colour product?

3. Optical illusionsIllusions happen when your eye interprets an image differently from fact, or when it sees something that is not present. Some examples are:• the position of a fish

under water• the appearance of water

on a road in hot weather• sailors seeing islands on

the horizon• pictures that contain two

images.Think of an optical illusion

that you have experienced and use the knowledge you now have about light to explain why it happened.

4. Seeing is believingImagine you are a ray of light entering an eye. Describe your journey through the eye to the brain. What could happen on this journey to make the image the brain sees look fuzzy?

WORK

Can you see two angry faces or a candle?

5. Eye problemsIn a group of three or four, each person should choose a different eye disease or

eyesight problem and research it. Take turns in explaining the symptoms of your diseases and how they are cured or treated. Use diagrams where possible. When you have finished, tell each other why you chose that particular disease. Include stories of any personal examples that you know about.

6. Catching the imageYou use a camera to record images that you want to see again in the future. A camera works like your eye.(a) Examine a camera and see how it works. Can you

name the main parts of the camera? (Hint: The camera’s instruction booklet could be useful.)

(b) Compare the camera with your eye. Draw a diagram of both to explain the similarities and differences.

7. Blind spotEveryone has a blind spot and it’s good to be aware that you have one. When learning to drive, one important lesson is to learn how to compensate for your blind spot.

WORK

F IND YOUR BLIND SPOT

• Close your left eye and focus on the koala with your right eye.

• Move the book back and forth until the kangaroo disappears. When it does you have found your blind spot.

1. Think about the structure of your eye and suggest why we each have a blind spot. How do your eyes compensate for your blind spot?

2. When driving a car, what is meant by your blind spot? How would you compensate for this blind spot to ensure you are a safe driver?

Documents

Cs3-Ch07 Keeping an Eye on Light