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ctsLight is all around

You ride your bike home from school on a hot, summer’s day. Getting home, you wander into the backyard to see your little brother splashing around in the pool. As he stands to wave to you, you realise that his legs look much shorter than they really are. You turn to sit at an outdoor table and take your keys out of your pocket. As you pick them up, your crystal key ring catches the light and spreads tiny rainbows of colour across the table in front of you. You pick up a magazine, but as it looks blurry, you put your glasses on and flick through its pages.Talk to a person in your class about any observations that you can make about this scenario. See if the two of you can suggest a possible explanation for everything that happened. Add some diagrams to assist your explanations. Share your ideas with two other pairs of students. Were any of your thoughts the same?

Successful completion of this chapter will allow you to:

• explain how light bends or refracts as it passes through different media

•discuss the phenomenon of total internal reflection of light and its uses, such as in optical fibres

•compare the effects of concave and convex lenses on light and explain how they form images

•describe common vision problems and the types of lenses that correct them

• describe how white light can be split into its constituent colours

•explain the interaction of white light with coloured objects and describe how coloured filters are used

• discuss the wavelike nature of light and how light is polarised.

Figure 4.1 How does light travel through space? Can it bend around corners?

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Say `Cheese'!

Only 300 years ago, the thought of retaining an image in the form of a photograph was pure fantasy.

In 1725, Professor Johann Schultze discovered by accident that silver nitrate was sensitive to light, and could capture images. The world waited until 1826 for Joseph Niepce to take the first photograph. This famous image was of a courtyard at Gras, in France. In a small group, discuss how far the methods of recording images have developed from these humble beginnings. Research significant stages in the development of photographic techniques.Prepare a research presentation, in which you explain how:ü lenses can bend light raysü different types of lenses can work together in an

optical system such as a cameraü a colour film records an imageü coloured filters are used in photography for

various effectsü a digital image is recorded using a charge-couple

device (CCD).Following your research, choose one aspect of photography to investigate in a practical manner. Depending on your resources, this could be studying different types of lenses, the effect of colour filters, varying exposure times or another related area. Check your experimental design with your teacher and then start developing your work!As a group, decide upon the method that you wish to use to present your findings. You could create a brochure on photography for students, a documentary-style video about the history of photography or a website or blog about your subject. Upon completing this mission, evaluate the strengths and weaknesses of how your group worked together on this project, using the table in Chapter 11 Skills link. Could you suggest any changes to the way in which your group completed this task?

Figure 4.2 Capturing an image onto a surface was once pure fantasy. These days, it happens with the click of a button.

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RefractionHave you ever looked down when in a swimming pool and noticed that your legs weren’t as long as you thought they were? Or perhaps you’ve tried to clean a fish tank and had trouble coordinating your hand and eye. These things occur because of refraction. When light travels from one substance into another, its speed changes. Light travels faster through air than through water or glass. This change of speed causes light to bend as it enters or leaves another substance. The only time this change in speed does not produce bending is when light hits a surface at right angles, and passes through unchanged.

In Heinemann Science Links 2 Chapter 3, we discussed the fact that light is part of the electromagnetic spectrum. It is an electromagnetic wave, and travels at a speed of 300 000 km every second.

Changing direction4.1

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Physics 12 : Figure 864cGuy Holt

Figure 4.3 The specific nature of light is complex. We know that it is electromagnetic radiation, so named because it consists of electric and magnetic fields which travel at right angles to each other.

For centuries, scientists have argued about whether light is made up of travelling tiny particles, or whether it travels as a wave. The great Isaac Newton observed that light travels in straight lines and cannot bend around corners like waves. He concluded that light was a stream of particles rather than a wave. Other scientists have since shown that in certain circumstances, such as when it spreads out from a narrow slit, light does bend and has properties of a wave. Scientists now believe that light is an electromagnetic wave, which travels in bundles of energy (somewhat like particles) called photons.

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Figure 4.4 Science fiction movies often involve fight scenes with laser blasters. If you shone a laser beam on Earth, you would only see the beam if it travelled through a stream of particles that it could reflect from, such as talc or chalk dust. What does this mean for the famous laser battles we’ve come to know and love?

Light is a form of energy that travels at 300 000 km every second. Even though it travels so quickly, it still takes light 8 minutes to reach us from the Sun. So, if the Sun stopped shining right now, we probably wouldn’t notice until 8 minutes later, when we’d be left in the dark!

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Although we know that light travels as a wave, we still refer to light rays that travel together as a beam, in order to help us understand some of the effects of light. Light travels in a straight line path. When light reflects from an object, such as a flower, into our eyes, we can see the flower; otherwise it would be invisible to us.

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Hunters who use spears to catch fish know about refraction. They quickly learn that they cannot always trust their eyes. Light bends as it leaves the water so the fish always appears closer to the surface than it really is. Aiming below where the fish appears to be is more successful. To understand what is happening, we need to first draw an imaginary line at right angles to the boundary between the two materials that light travels through (in this case water and air). This line is called the normal.

Figure 4.5 I’ve shrunk in the wash!

Figure 4.6 You may also notice this fooling of your eyes with a paintbrush or a pencil resting in water.

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Figure 4.7 a Whenever light enters a less dense medium, it will bend away from the normal. We can define the ‘angle of incidence’ (i) as the angle between the incoming ray and the normal, and the ‘angle of refraction’ (r) as the angle between the refracted ray and the normal. In this example, r > i.b Light entering a more dense medium will bend towards the normal. In this example, r < i.

Whenever light travels into a less dense medium, such as from water into air, it speeds up. Instead of passing into the air in a straight line path, it always bends away from the normal. This type of bending is shown in Figure 4.7a. When the opposite occurs and light travels into a more dense medium, it slows down. In this case, it bends towards the normal, as shown in Figure 4.7b.

To see how this works in practice, consider the fish that a boy is observing in Figure 4.8. Light travels upwards from the fish, and must reach the little boy’s eyes for him to see it. This light speeds up and bends away from the normal as it leaves the water and emerges into the air. The boy is seeing rays of light from the fish that have already bent

How well can you see if you open your eyes under water? Probably not very well! Refraction is to blame. When light travels in water, it moves at a slower speed than in air. This means that when it hits your cornea, it does not bend as much as it would if it had come from air. This makes it difficult for our eyes to focus properly. A solution to this problem is to wear goggles or a mask, so that the light enters your eyes from air rather than water. So, next time you’re combing the ocean floor or the depths of the local pool, remember your eyewear!

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Figure 4.8 To the boy, the fish appears to be closer to the surface than it really is, in the same way that the boy’s legs in Figure 4.5 appear to be closer to the surface than they really are.

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S C I E N C E work

Refraction magicTest out these strange effects of refraction for yourself!

What you need

• coin

• cup (opaque)

• pencil (or pen)

• beaker of water

What to do

1. Put the coin in the cup (that is not see-through) and move back until the coin is just out of sight. Stand in this position while a friend pours water into the cup. What happens? Try to draw a diagram to explain what has happened, using Figure 4.9 as a starting point.

2. Now, put the pencil in the beaker of water. Look at the pencil from above, and describe what you see. Look at the pencil from the side of the beaker. Sketch what you observe. Try to draw on the path of the light rays as they leave the water and enter the air.

3. Talk to your partner about another way you may be able to observe the effects of refraction. Using equipment found in your laboratory, set up another activity and see if your classmates can explain what has happened.

4. Prepare a summary report about your refraction activity. You could take digital photos or video clips and prepare a multimedia presentation, or create a poster or cartoon.

Activity 4 . 1

Figure 4.9 Can you make a coin appear before your eyes? Just add water!

In 1621, the Dutch scientist Willebrord Snell discovered that the ratio of the angle of incidence to the angle of refraction was unique for every transparent substance. He called this characteristic bending power of a substance its refractive index. Investigators can link pieces of broken glass to a window pane smashed at a crime scene if the refractive indices of both samples of glass match.

scifileon their journey. However, his brain knows that light usually travels in straight lines. For this reason, the boy thinks that the fish is located in a straight line from his eyes to the water. This is why it appears to be closer to the surface. Our brains are fooled by the bending of light!

Total internal reflectionWhen light passes from a dense material to a material that is less dense, such as from glass into air, most of the light continues on its path into the air, bent away from the normal. However, a small fraction of this light is actually reflected at the boundary of the glass and air, and bounces back up into the glass. As we increase the angle to the normal at which the ray hits, the refracted ray bends progressively further away from the normal. This continues until the refracted ray leaves along the boundary of the two materials (i.e. at an angle of 90º to the normal). At this point, we call the angle of incidence between the ray and the normal the critical angle. For any angles of incidence larger than the critical angle, there is no refracted ray at all; instead all of the incoming light is reflected. This phenomenon is called total internal reflection.

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Figure 4.10 a For angles smaller than the critical angle for this type of glass, some light is reflected at the boundary but most continues through to the air.b For angles larger than the critical angle, no light enters the air. It is all reflected back into the glass.

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Figure 4.12 Why do diamonds sparkle? Because of total internal reflection! The critical angle for a diamond–air boundary is very low (about 23°) because diamond is an optically dense substance. Jewellers cut the facets of the diamond on angles so that most of the light entering it from the front will strike the back facets at an angle larger than the critical angle. This means it will bounce around the facets and then back out again. Can you now explain why they sparkle?

Figure 4.11 Light is refracted and totally internally reflected twice within the diamond, explaining its brilliance.

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S C I E N C E work

Bending light

Aim

To investigate how a light beam refracts through a transparent block, and to observe light being totally internally reflected.

Materials

• plastic or glass rectangular and semicircular blocks

• ray box • power supply

• paper • ruler

• protractor • pencil

Method

1. Put the rectangular glass block in the centre of the first sheet of paper and trace around it with a grey lead pencil. Rule a normal line from the centre of the block as shown in Figure 4.13.

2. Connect the ray box to a power supply and place the single slit in the box to produce a single ray of light.

EXPERIMENT 4 .2

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Figure 4.13 Draw the normal at 90º to the glass boundary.

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3. Direct a ray of light to enter the block at its centre, where the normal was drawn, as shown in Figure 4.13.

4. View the path the light takes as it enters, passes through the block, and emerges at the far side. Make small pencil marks to trace the ray up to and away from the block.

5. Remove the block and join the dots to show the path the light has taken.

6. Draw another normal at the point where the light left the block. Use a protractor to measure the angles of incidence and refraction between the ray and the normal. Record your findings as follows:

Light entering glass from air Angle of incidence =

Angle of refraction =

Light entering air from glass Angle of incidence =

Angle of refraction =

7 . Position the semicircular block on another sheet of paper. Direct a ray at the curved edge so it travels along a radius of the semicircle. This way it will strike the edge along the normal and enter the block without bending. Position the ray box so that the ray strikes the inside straight edge at about 45° as shown in Figure 4.14.

8. You should be able to see a refracted ray and a reflected ray. Draw what you can see.

9. Move the ray box in an arc away from the centre to increase the angle between the incident ray and the normal to the straight edge. Can you find the critical angle for this block?

10. What happens for any angles larger than this?

Discussion

1. (a) Does light bend towards or away from the normal as it enters the glass block?

(b) Which way does it bend as it leaves?

2. Which way do you think light will bend as it enters water from air? Explain.

3. Explain how you located the critical angle using the semicircular block and what is important about this particular angle.

Evaluation

1. Did you find that the light bent in the directions that you would have predicted in your experiment?

2. Were there any times when you observed a result that you didn’t expect?

3. Comment upon any aspects of this task that were difficult and any possible sources of error in your experiment.

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Figure 4.14 Direct a ray of light into the glass towards what would be the centre of this circle.

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Figure 4.15 A single optical fibre is made up of a central glass (or plastic) core, surrounded by a glass coating called cladding. Light is totally internally reflected as it hits the boundary of the core and cladding in the optical fibre and is reflected back inside the core and travels through a series of reflections along the fibre.

Optical fibres are also used to make multipurpose bendable tubes called endoscopes. Doctors use bundles of optical fibres in the endoscope to examine inside patients. There is no need to perform major surgery—the endoscope can be inserted through a small cut into a patient’s body, or through a natural opening, such as the mouth, anus or urethra. Light passes down some fibres and is reflected back up along others to a video camera and a viewing screen. Surgeons then watch the screen, which shows a magnified image, to guide instruments as they operate. Procedures include manipulating tiny suction tubes to remove loose bone fragments, laser surgery, taking tissue samples for further testing, or locating and removing tumours from the lungs or digestive tract.

Figure 4.16a Surgeons can use the flexible endoscope to navigate and perform tasks with minimal post operative complications to patients. b These stomach polyps that can be removed using such a process.

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When you were younger, would you have liked to own a bendy pipe that would let you see around corners? What about a flexible tube that could show you what your intestines look like from the inside? All this is possible, with a handy device called an endoscope.

An endoscope is made from a bundle of optical fibres. An optical fibre can be called a light pipe, and enables us to send a message using light. An optical fibre is about as narrow as a human hair and consists of a central core that is made from glass or plastic, surrounded by a second layer of glass or plastic—the cladding—as shown in Figure 4.15. Light pulses from a laser are sent along the central glass core. Because messages can be sent at great speeds and with very little signal loss, optical fibres have extensive uses in long-distance communications. (These are explored in Chapter 5.)

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endoscopesquestions

1. Create a labelled diagram of an optical fibre.

2. An optical fibre can be referred to as a pipe that carries messages. If so, in what form are these messages sent?

3. Explain why the light entering an optical fibre does not simply refract out of the edges of the glass tube.

4. What is an endoscope? State two ways that this could be used in surgery.

5. For what purpose were experts at the Australian Photonics Cooperative Research Centre specifically designing a miniature endoscope?

6. How useful would an endoscope the width of a human hair be? Discuss your ideas with a partner, and then choose the one that you both like the best. Draw a design and state the purpose of your new super small endoscope.

There is a variety of endoscopes, each for a particular function:

ü A gastroscope is used to examine the stomach and digestive tract.

ü A bronchioscope is used to examine the lungs.

ü A colonoscope is used to investigate the bowel.

ü An arthroscope examines skeletal joints for problems.

ü A laparoscope examines the abdominal region.

ü A hysteroscope is used to study the uterus.

A standard endoscope is made up of thousands of optical fibres and is about 5 mm wide. In 2004, photonics experts at the Australian Photonics Cooperative Research Centre at the University of Sydney developed a miniature endoscope, with a diameter of only half a millimetre! This was achieved through a new technique, in which an array of microscopic holes was drilled into the device to enable it to channel more light. Researchers hope that this

may one day be used in cochlear implant surgery to restore hearing. To fit a cochlear implant, electrodes

must be channelled down the cochlea, through one and a half turns of its spiral-shaped structure. In about 10% of

cases, unseen tiny obstructions in the inner ear block the fitting of the electrodes. A super small endoscope could be

used to detect and navigate around these miniature road blocks in the inner ear. The design of this tiny endoscope is being refined

and could one day even give us a look inside our own blood vessels!

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1. Copy and complete the following sentences by selecting the correct alternative in each case.

(a) When light passes from glass into air, it will be refracted towards/away from the normal.

(b) Light travels faster/slower in air than in glass.

(c) It is only possible for total internal reflection to occur for light that is passing from a more dense/less dense material into a more dense/less dense material.

questions 4.1 4. Copy the diagrams in Figure 4.18 and fill in

where you think the rays would continue.

5. Work with a partner to answer the following questions.

If light bends away from the normal as it passes from material 1 into material 2, then:

(a) draw a diagram to illustrate what is happening.

(b) which of the materials is more optically dense?

(c) in which material does the light travel faster? (d) explain why you think this may be the case. (e) suggest some possibilities for what

materials 1 and 2 could be.

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2. (a) Figure 4.17 is a diagram of a ray of light leaving air and entering glass. Copy this diagram and label the normal, air/glass boundary, incident ray, refracted ray, angle of incidence and angle of refraction.

(b) Has the refracted ray bent towards or away from the normal?

3. Total internal reflection occurs only in specific situations. List all of the factors necessary for this to occur.

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Figure 4.19

6. A lucky seagull circling over the ocean spies a tasty morsel. Copy Figure 4.19 and complete the path of the light rays to show how far below the surface the seagull’s snack appears to lie.

7 . Create a rhyme or jingle explaining the meaning of the term refraction.

8. Work with a partner to write a list of ten situations in which you could observe the effects of refraction. If possible, make a video documentary or digital slideshow illustrating some of these situations.

9. Turn back to the photograph of the boy with the short-looking legs in the pool shown in Figure 4.5. Try to explain the reason for this effect to an older or a younger person who has not heard about refraction. Draw a diagram to help you. Ask the person for feedback about the parts of your explanation that were the most effective.

10. Conduct some research to list a few facts about the ways that diamonds and other precious jewels are cut. Do you think this would make much of a difference to their brilliance? Present your answer in a format of your choice.

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Focusing on lenses4.2

Figure 4.20 We’ve seen that light bends or refracts as it enters and leaves different materials. We can use this to our benefit by designing pieces of glass or plastic that are shaped to make light bend in useful ways. Each device shown here uses lenses to bend light.

A lens is a transparent object that has carefully shaped surfaces. Magnifying glasses, movie projectors, telescopes, binoculars and microscopes all rely on systems of lenses. They provide us with images that would otherwise be impossible for us to see.

There are two main types of lenses. Convex lenses are thicker in the middle than at the edges. Concave lenses are thinnest at the centre and thickest at the edges.

S C I E N C E work

Looking at lensesCarefully pick up a concave and then a convex lens. Examine their different shapes. These lenses each bend light in a different way, for a range of particular uses. Working with a partner, look at some of your friends and a number of objects around you through each type of lens. Vary the distance that you hold the lens from your eye, and move it back and forth. Do any of the images change? Are they right way up or upside down? Are they larger or smaller than the object you are looking at? Draw up a chart with headings ‘Concave lenses’ and ‘Convex lenses’ and compile a list of as many features as you can about the types of images you observed through each. Record how close the lens was to your eye in each case, and approximately how far it was from the object being viewed. Can you suggest how we could use each type of lens?

Activity 4 .3

Convex lensesConvex lenses are also called converging lenses because they focus parallel light rays to a point. You are more familiar with these lenses than you might know because you’re carrying two with you at the moment—in your eyes!

If parallel rays of light are directed at a convex lens, they meet at a point. This point is called the principal focus of the lens. The distance from the lens to this point is called the focal length. Usually, the thicker the lens at its centre, the more powerful it is and the smaller the focal length.

If we place a screen (or a piece of paper) where the rays of light from an object intersect, we can see an image that is produced by the lens. This is called a real image because it is produced by the rays of light focusing on the screen. If you try this using a convex lens to obtain an image of an object (such as a window) on a piece of paper, you’ll