Radiation of light

Where does light come from? Natural light comes from luminous objects such as the Sun and light bulbs. We say that these objects emit light.

Light travels through space at a speed of 300 000 kilometers per second. We say that energy is transferred by radiation. The energy of the light is transferred through space as electromagnetic waves in straight lines.

Let's look at how light travels. We will make a simple camera to investigate how light travels.

Did you see an upside down image? Why is it upside down?

We see objects because light reflects off them and enters our eyes. If the image is upside down it means that the light from the bottom of the object has arrived at the top of the screen and the light from the top of the object has reached the bottom of the screen, as shown in the following diagram.

When you moved closer to the object, the image appeared bigger, as shown in the following diagram.

What does this mean? It means that light must be travelling in straight lines. This is called the rectilinear propagation of light.

Ray diagrams

A ray diagram is a drawing that shows the path of light. Light rays are drawn using straight lines and arrowheads, because light travels in straight lines. The figure below shows some examples of ray diagrams.

Spectrum of visible light

The visible light spectrum is the light that we are able to see with our naked eyes. Have you ever wondered why everything is colourful and not just black and white? Have you ever seen a rainbow and wondered where the colours have come from? The colours that we see everyday are part of the visible light spectrum. Let's investigate the visible light spectrum.

Splitting white light

This activity is very simple and usually gives clear results. Try to darken the room as much as possible in order to get clear spectra. A ray box and power source are not essential for this activity. You can make your own simple ray box by using a piece of cardboard with a small slit cut into it. Hold the cardboard in front of a light bulb and the light will shine through the slit in a single beam of light. Use a table lamp or set up a circuit with a high wattage light bulb as a source of light.

MATERIALS:

• triangular perspex prism
• ray box and power source

INSTRUCTIONS:

Connect the ray box to the power source. If you do not have a ray box, your teacher will show you how to use a piece of cardboard with a slit cut into it.

Remember that if you do not have a ray box then you can use a light bulb with a cardboard screen to produce a coherent beam of light.

Place the triangular prism on a white background.

Shine a beam of white light through the side of the prism.

Make sure that the learners rotate the prism until they get it at the right angle to refract the light and see the colours.

QUESTIONS:

Draw a picture showing what you observe.

The drawing should show the beam of white light entering the prism, passing through and emerging on the other side as the 7 colours of the visible spectrum. This is a typical image, which learners will see later in the chapter when we discuss refraction of light. They must note the relative bending of red versus blue light.

Write a description of what you observed.

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The white light enters the prism, passes through it and emerges on the other side as a beam of 7 colours (a rainbow).

Write down the order in which the colours appear.

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Red, orange, yellow, green, blue, indigo, violet.

If you repeat the experiment, does the order of the colours change?

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No, the order is always the same.

What do the different colours we see tell us about the composition of white light?

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They tell us that white light is a mixture/blend/combination of the 7 colours of the visible spectrum.

There are actually a large range of colours, but our eyes allow us to distinguish 7 colours.

So, what have we learned so far? Light radiates from luminous objects and always travels in straight lines. The white light that we see is made up of the 7 different colours of the spectrum. When the 7 colours are travelling together we see them as white light.

The 7 colours of the visible spectrum are Red, Orange, Yellow, Green, Blue, Indigo and Violet. Each colour has a different wavelength and frequency. Have a look at the following image which shows the spectrum of visible light.

Colour spinning wheels

This is a very simple, fun activity to show that the 7 colours combine to make white light. You can either get learners to each make their own, or else make a couple before class yourself and hand them out for learners to experiment with.

MATERIALS:

• white cardboard
• coloured pens or pencils (red, orange, yellow, green, blue, indigo, violet)
• string
• scissors
• round object

INSTRUCTIONS:

To do this accurately, find the centre of the circle and mark a dot there. Then draw a straight line from the centre to the edge of the circle. Next, align the straight edge of a protractor with the line you just drew, placing the end of the protractor right on the center of your circle. Look for 52 degrees and make a dot to mark this angle. Draw a line from the centre dot to this dot on the edge. The angle you drew is 1/7 of the circle. Repeat this until you have measured and drawn all segments. A complete rotation is 360 degrees and 360/7 = 51.4 which is why each segment you draw should be about 52 degrees. The correct angle for 6 segments would be 60 degrees.

Draw a circle on the cardboard. You can trace around a round object such as a cup or saucer to do this. Cut out the circle.

Now divide the circle into 7 equal segments. If you do not have indigo and violet colours, but just one purple pen or crayon, then you can divide the circle into 6 equal segments rather.

Shade in each segment a different colour, in the order red, orange, yellow, green, blue, indigo, violet (or just purple if you do not have indigo and violet).

Next, make two holes, one on either side of the centre as shown below.

Thread the string through the holes and tie it in a loop.

You are now ready to spin the wheel. Holding the ends of the loop in each hand, twirl the string over, like you would a skipping rope, so that the string twists. Once the string is tightly twisted, pull your hands apart, then bring them back together. Continue bringing your hands in and out and watch the circle spin.

What do you observe about the colour of the wheel as it spins faster?

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Learners should observe that the colours appear to 'mix'. Depending on the quality of the pens or pencils used, you should see a light grey. The goal is to see white, but this might take some more experimenting.

So far we have been talking about the visible light spectrum. As we mentioned in the beginning, this is the light that we can see. We also spoke about how light travels in electromagnetic waves. We can only see light with a certain range of wavelengths. What does this mean?

The size of a wave is measured in wavelengths. A wavelength is the distance between two corresponding points on two consecutive waves. Normally this is done by measuring from peak to peak or from trough to trough. Have a look at the following diagram which illustrates a wavelength.

The wavelengths of the different colours of visible light are different lengths, as shown in the following diagram.

We can also talk about the frequency of a wave. If a wave has a long wavelength, then it has a low frequency; if it has a short wavelength, then it has a high frequency.

When it comes to visible light, we only see wavelengths of 400 to 700 billionths of a meter. This is called the visible spectrum. But, light waves are just part of the wave spectrum. There is invisible light with shorter wavelengths, such as ultraviolet light, and there are longer wavelengths, such as infrared light.

Have you ever looked through a window and wondered why it is made of glass? Let's find out how light behaves when it strikes the surface of different types of materials in the next section.

Opaque and transparent substances

Three different things happen when light hits a surface, it can be reflected (bounce off), absorbed or transmitted (pass through). Glass reflects some light but most of the light is transmitted straight through. That's why we can see objects on the other side of a closed window.

We say that glass is transparent. Let's find out more about what this means. If a substance is not transparent, it is opaque.

Shadow Play

This activity will show learners that opaque objects cast shadows. You can give them specific shapes to cut out from cardboard or allow them to be creative with their designs. Have them cut out various shapes of different sizes from cardboard. This will allow them to see that larger objects cast larger shadows. The learners can use a white piece of paper as a screen or use the wall of the classroom. If they hold the shape on the desk then the shadow would be cast on the desk but a screen would be more useful. The classroom should not be brightly lit when doing this activity as overhead lights may affect the shadows.

MATERIALS:

• cardboard
• clear plastic
• plastic shopping bag
• scissors
• light source (ray box or light bulb)

INSTRUCTIONS:

Cut out three shapes from your cardboard. All of the shapes should be similar but three different sizes: small, medium and large.

Switch on the light source.

Hold your first shape a short distance in front of the light source.

Look at the shadow that forms. Write down what you observe.

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The shadow forms on the side of the shape which is furthest from the light. It is a dark colour.

Hold your second shape the same distance in front of the light source.

Look at the shadow that forms. Write down what you observe.

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The shadow is formed on the side furthest from the light source. It is dark in colour and larger than the first shadow.

Hold your third shape the same distance in front of the light source.

Look at the shadow that forms. Write down what you observe.

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The shadow is formed on the side furthest from the light source. It is dark in colour and larger than the first and second shadows.

Use your first cardboard shape as a template and cut the shape from the clear plastic and the plastic shopping bag.

Hold the clear plastic shape the same distance from the light source. Write down what you observe.

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No shadow is formed by the clear plastic shape. There may be a slight outline of the shape as a shadow. This sometimes happens if the cut edges of the shape have curled over, the double thickness reduces the transparency. If any of the learners notice this you should explain it to them.

Hold the plastic shopping bag shape the same distance from the light source. Write down what you observe.

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The shadow that forms is on the side opposite the light source but it is significantly lighter than the cardboard shadows. It has a darker outline and a lighter centre.

QUESTIONS

When you held the cardboard up to the light, did it allow light to pass through it? How do you know this?

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No, light did not pass through as it forms a shadow on the opposite wall.

Is the cardboard shape opaque or transparent?

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It is opaque.

What did you notice about the shadows formed by the different size cardboard shapes?

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The larger the shape, the larger the shadow.

Draw a diagram to show how the shadow is formed behind the opaque shape. Use straight lines with arrowheads to represent the rays of light.

This is an example of the type of diagram the learners may draw. They need to show the opaque object between the light source and a screen. They need to show rays of light leaving the light source and moving in straight lines on either side of the shape.

The distance between the shape and the light source was kept the same. What do you think would have happened to the shadow if the distance was increased?

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The answer calls for learners to predict something they have not tested. The shadow should become larger if the object is closer to the light source and smaller if the object is further from the light source.

Test your idea from question 5 by moving your cardboard shapes closer to and further away from the light source. What do you see? Were you correct in your prediction?

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This answer is learner dependant because it depends on their prediction for question 5. Learners should describe seeing that the size of the shadow decreased as the distance increased or that the size of the shadow increased as the distance decreased.

Is the clear plastic shape opaque or transparent?

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The clear plastic is transparent.

Did the clear plastic cast a shadow?

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No

Explain why the cardboard casts a shadow but the clear plastic does not.

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Light travels in straight lines. It cannot bend around an object. Light cannot pass through the cardboard and so a shadow is formed. Light can pass through the clear plastic and so the area behind the plastic is bright.

Is the plastic shopping bag shape opaque or transparent?

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It is neither completely transparent or completely opaque. The shopping bag is translucent or semi-transparent.

Explain why the shopping bag casts a lighter shadow.

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Some of the light can pass through the translucent plastic but not all of it, this means that the shadow is not as dark.

What have we learned? Shadows are formed because light travels in straight lines and cannot pass through opaque objects.

Substances which transmit most of the light and only absorb or reflect a little bit are called transparent. Can you list some everyday objects which are transparent?

Substances which completely reflect or absorb light without transmitting any are called opaque. Can you list some everyday objects which are opaque?

Some substances, such as the plastic shopping bag, allow some light to pass through, but not all of it. This substance is translucent, or semi-transparent.

We can use transparent objects to make filters. If we want red light we use a red glass bulb or a red plastic film placed in front of the light. Only red light is able to transmit through the red glass or plastic. The other colours are absorbed by the filter.

Now that we have seen some examples of transparent and opaque substances, let's take a closer look at what it means to absorb or reflect light.

Absorption of light

Look at this picture of a ladybird. Why is it red and black? And why is the leaf so green? How do we see the different colours? It all has to do with what happens when light hits a surface.

When light hits a surface, some of the light is absorbed and the rest is reflected. It is the reflected light that reaches our eyes and allows us to see the object. Previously, we learned that white light is a mixture of different colours. When white light from the Sun hits the red shell of the ladybird all of the colours are absorbed, except red. Red light is reflected back to our eyes and so we see a red ladybird.

The green leaf absorbs all the colours except green which it reflects back into our eyes.

What about the black spots of the ladybird? Is black a colour? The black spots on the ladybird absorb all the colours and no light is reflected. That is why they appear black.

Do you remember learning about heat as energy transfer in Gr 7? We looked at the absorption of heat. We saw that black, matt objects absorbed all of the light energy, while white objects reflected all of it. Black, matt (not shiny) objects absorb all of the colours of light and reflect none and so appear black to our eyes.

What about a white object? Why do you think white objects look white? Have a look at the following diagram for a clue.

Let's now look more at what we mean by reflection of light.

Reflection of light

When light hits a surface it is often reflected off the surface. This photograph shows how light is reflected off a still lake, creating a mirror image of the tree. The still, flat surface of the lake has acted as a mirror.

Have some fun with these photos of reflections in water. One photograph is the right way up and the other one is upside down! Which one is which?

Most surfaces reflect light. When light strikes a reflective surface, it can change direction. Let's look at how this happens.

When light reflects off a surface the ray which hits the surface, it is called the incident ray. The ray of light which is reflected from the surface is called the reflected ray. When we draw diagrams of reflection we also draw in an imaginary line to help us measure different angles. This line is called the normal. The normal line is always drawn perpendicular to the surface.

Between the normal line and the incident and reflected rays, there are two angles. These are:

• angle of incidence - the angle between the incident ray and normal line

• angle of reflection - the angle between the reflected ray and normal line

The following diagram explains these concepts.

Let's investigate the relationship between the angle of incidence and the angle of reflection.

Is there a relationship between the angles of incidence and reflections?

Learners will see that the angle of reflection is equal to the angle of incidence. Learners must save some of the sheets for the next activity where you will use a piece of crumpled aluminium foil instead of the mirror.

Another way to do this investigation is to use a sheet of corrugated cardboard instead of paper. Learners can then stick pins into the cardboard along the light ray and then draw in the lines later.

AIM: To investigate the reflection of light from a surface.

INVESTIGATIVE QUESTION:

Look at the diagram above and try to formulate an investigative question for this investigation.

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Learners will have their own versions. An example of an appropriate question would be: 'How is the angle of reflection related to the angle of incidence of the incident ray?' It is important that the question relates the two angles in some way.

HYPOTHESIS: The angle of incidence is equal to the angle of reflection

MATERIA LS AND APPARATUS:

• mirror
• white paper
• pencil
• protractor
• ruler
• ray box

A laser pointer also works very well instead of a ray box.

METHOD:

1. Put a white piece of paper on the desk.
2. Use your ruler to draw a straight line near the top of the white paper.

3. Use your protractor to make a right angle in the middle of your pencil line. This is the normal line.

1. Place your mirror upright along the first line.

2. Shine a light from the ray box along the paper so that it "hits" the mirror where your normal line and your mirror meet.

1. Use a pencil to mark the incident light ray.

1. Use a pencil to mark the reflected light ray.

1. Remove the mirror and switch off the ray box.

2. Use a ruler and pencil to draw a line from the points you have marked on each ray to the normal line.

1. Mark the angle of incidence (i) and angle of reflection (r).

1. Turn the ray box on again to confirm that your pencil lines follow the rays.

1. Use a protractor and measure the angle of incidence and the angle of reflection and record your results in the table.

2. Repeat this method 3 more times, each time using a different angle of incidence.

Keep one of the sheets with your drawn ray diagram for the next activity.

RESULTS:

Fill your results into the following table.

Repeat	Angle of Incidence	Angle of Reflection
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3
4

The answers in the table will depend on the angles of incidence which the learners use for their investigation. It is important that they see that the angles of incidence and reflection are equal to each other in each repetition.

ANALYSIS:

Has your investigation provided everything you need to answer your investigative question?

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This answer would be learner-dependent as it would depend on the investigative question they chose.

How could you improve this investigation to get more accurate results?

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This answer is learner dependent. An example of an improvement could be to use a protractor printed on the page already in order to measure the angles accurately.

CONCLUSION:

What can you conclude based on your results?

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In reflection, the angle of reflection is always equal to the angle of incidence.

Whenever light is reflected from a surface, the angle of incidence to equal to the angle of reflection. On a smooth surface all the light rays are reflected in the same way and so the image is clear and focused.

A mirror is an example of a smooth surface. The image you see is focused and clear. As you can see in the photograph, the scientists and engineers are clear and focused in the mirror image.

What happens when we do not have a smooth surface? Have a look at the photo.

Can you now see why reflections off rippled water are not clear, but rather blurred? This is because the light rays have not reflected parallel to each other as they do from a smooth surface, but have scattered in different directions.

The following table shows the difference between a smooth surface and a rough surface. Straight parallel rays are approaching the surface. You need to draw in the reflected rays to show specular (clear) reflection from a smooth surface and diffuse (unclear) reflection from a rough surface.

Specular diffusion from a smooth surface	Diffuse reflection from a rough surface.

Here are the answers for what learners should draw.

Specular diffusion from a smooth surface	Diffuse reflection from a rough surface.

Visible light is the range of frequencies of light that are visible to the human eye, and is responsible for the sense of sight. Are you curious to find out how we actually see light? Let's discover more in the next section.

How do we see light?

How is it that we are able to see light? Light that is absorbed by objects does not enter the eye. Only reflected light or direct light from luminous objects can enter the eye and be interpreted. Have a look at the following image which shows the outer structure of the eye.

We can see the iris, the pupil and the sclera. The sclera is a the tough white, outer part of the eye, which acts as protection. The iris is the coloured part of the eye which differs from person to person. It is circular and surrounds the pupil. Light enters the eye through the pupil.

Let's take a look at the internal structure of the human eye. The following diagram shows a cross section through the eye. The eye is actually a large ball, and only a small part is visible on the outside. Covering the iris is a tough, transparent layer called the cornea. Behind the iris is the lens. Both the cornea and the lens help you to focus the light entering your eyes, as we will learn about in the next section.

The light travels through the eye and hits the retina at the back of the eyeball. The retina is a layer of tissue lining the back of the eyeball, as indicated in the diagram, it is the yellow layer. The retina consists of cells which are sensitive to light. Light enters the eye and forms an image on the back of the eyeball. Light hitting the retina is similar to light hitting the screen in the pinhole camera. The receptor cells convert the light energy into electrical nerve impulses. These impulses travel out of the eye through the optic nerve and to the brain where they are interpreted as sight.

So how do we see colour? Do you remember when we spoke about why the ladybird appears red and black? Look at the following diagram again.

The white light hits the ladybird's surface. The white light has all the colours of light, but when it hits the red surface, only the red light is reflected. The other colours are absorbed by the red surface. This means that when we look at the red parts of the ladybird, we only get red light reflected into our eyes. Therefore, when this reflected light hits our retina and the electrical impulse is sent to our brains, we see the red colour.

Seeing colours

MATERIALS:

• coloured pens or pencils

INSTRUCTIONS:

1. Answer the following questions about how we see objects.
2. Draw a ray diagram to accompany your written answer.
3. An example has been done for you.

Look at the picture of a sunflower.

We can draw a ray diagram to show why we see the green leaves as green, as shown below. The green surface of the leaves absorb all the colours of white light except green light which is reflected into our eyes.

Now explain why the petals appear yellow and the centre appears black. Use the concepts of absorption and reflection in your explanation. Draw diagrams to support your answer.

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The white light that strikes the sunflower has all the colours. The yellow petals absorb all the colours of the spectrum except yellow which is reflected into our eyes. The black centre absorbs all of the colours of the spectrum and does not reflect any light into our eyes, hence our brain interprets a lack of light/colour as black.

Heath has bought himself a blue car. Explain why we see the car as blue by using the absorption and reflection of light. Draw a diagram to support your answer.

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White light from the Sun hits the car. All of the colours of light, except blue, are absorbed by the surface of the car. Only blue light is reflected from the surface of the car and enters our eyes. Our brain can only see the blue light and so we perceive that the car is blue.

We have looked at opaque and transparent substances, absorption of light, reflection of light and how we see light. We are now going to go back to transparent substances and see how light can interact with these materials.

Refraction of light

Do you remember the last time you drank a cold drink with a straw? Did you notice that the straw did not look straight anymore once it was in the water or cool drink?

Let's investigate this by examining what happens to light when it passes through a glass block.

What happens to light when it passes through a glass block

You do not need a ray-box for this investigation. A laser, such as those found on keyrings, or a light bulb can be used. If you use a light bulb, you need to make a cardboard screen. Cut a thin slit into the cardboard and hold it in front of the light bulb, this will create a ray of light suitable for the investigation.

We are going to investigate what happens to a ray of light when it passes from air and into a glass block and then from the glass block back into air. We are going to use a glass block with parallel sides.

Before we start the investigation, we need to think about how we are going to determine if light changes direction or not. Do you remember in the investigation on reflection where we measured the angle of incidence and the angle of reflection? What did we find in this investigation?

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The angle of incidence equals the angle of reflection.

When light passes through a transparent substance, we can also measure the angles. Look at the following diagram. The angle of incidence (i) is measured between the incident light ray and the normal line. As the light passes through the transparent substance, the angle of refraction (r) is the angle between the refracted light ray and the normal.

Learners can come back to this diagram and mark in which is the more dense medium (glass) and which is the less dense medium (air). In this diagram, medium 1 is air and medium 2 is glass.

In the diagram above, you can see that the angle of refraction is smaller than the angle of incidence. Therefore, the refracted light ray changed direction when it entered the transparent medium. We can also say something about which direction it bent towards. Did the light ray bend towards or away from the normal line?

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The refracted ray bent towardsthe normal line.

The next diagram shows another outcome.

Learners can come back to this diagram and mark in which is the more dense medium (glass) and which is the less dense medium (air). In this diagram, medium 1 is now the glass and medium 2 is air.

In the diagram above, does the refracted ray change direction when it enters the transparent medium? Give a reason for your answer.

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Yes, it changes direction as the angle of incidence is not equal to the angle of refraction. The angle of incidence is smaller than the angle of refraction.

In which direction did the refracted ray change?

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The refracted ray bent away from the normal line.

We are now ready to start our investigation.

AIM: To determine whether light changes direction when it passes through a parallel-sided glass block.

HYPOTHESIS: Write a hypothesis for this investigation.

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Learners must hypothesize about whether they think the light ray will change direction or not when it passes through the glass block.

MATERIALS AND APPARATUS:

• glass block
• ray box, laser pointer or other light source
• protractor

METHOD:

1. Put the glass block in the centre of a piece of white paper and trace around it.
2. Shine a ray of light into the glass block. The ray should be at an angle to the surface of the block.

1. Trace the light ray with pencil and mark the point at which it enters the glass block.
2. The light ray emerges on the other side of the glass block. Mark the point at which it emerges with a pencil and trace the emergent ray.

1. Remove the glass block. Your diagram should look similar to the one above.
2. Draw a line joining the incident ray and emergent ray. You have traced the refracted ray through the glass block.
3. Draw the normal lines where the incident ray meets the block and where the emergent ray leaves the block.

1. Measure the angles labelled 1, 2, 3 and 4 as shown on the diagram with a protractor.
2. Fill in the measurements in the table.
3. Repeat the steps above three times using different angles of incidence (angle 1).

The emergent ray from a parallel sided block is parallel to the incident ray.

RESULTS AND OBSERVATIONS:

Fill your results into the following table.

Experimental repeat	Angle 1	Angle 2	Angle 3	Angle 4
1
2
3
4

Which pairs of angles are equal in the measurements you have taken?

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Learners should note that angle 1 is equal to angle 4 and angle 2 is equal to angle 3 in all the sets of measurements.

NOTE: Discuss this with your learners as to why angles 2 and 3 are equal. The explanation for this is to do with parallel lines and alternate angles. This links well with what learners would have covered in Mathematics in the beginning of the year. The normal lines are parallel and so the alternate angles between them are equal. You can draw this on the board to explain it in more detail and show that the normal lines are parallel as the corresponding angles are equal (they are 90o).

Which of the angles you measured are the angles of incidence and which are the angles of refraction? Write this down below and mark them on the diagram above.

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Angles 1 and 4 are the angles of incidence and angles 2 and 3 are the angles of refraction.

What do you notice about the angle of incidence and angle of refraction for each of your sets of measurements?

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The angle of incidence is always different to the angle of refraction.

Did the light entering the glass block bend towards or away from the normal line?

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The light bends towards the normal line. NOTE: This is because the light is moving from a less dense to a more dense medium, which will be discussed later on.

Make the angle of incidence zero (make the light ray enter the block perpendicular to the surface). What is the angle of refraction?

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Zero.

CONCLUSION:

What can you conclude from your results?

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The angle of incidence is not equal to the angle of refraction. This means that the light ray changes direction when it passes from the air into the glass block, and again when it passes from the glass block back out into the air.

The angle of incidence is not equal to the angle of refraction because the light has changed direction as it enters the glass. Therefore, when light travels from one medium to another, it bends, or changes direction. This is called refraction. When light enters a different medium at right angles then it does not change direction.

So why does the light refract? Light behaves as a wave does and waves travel at different speeds in different media. For example, light travels faster in air than it does in water. When light enters a different medium, it changes speed, and if it entered at an angle other than 90o, then it also changes direction. The more dense the medium, the slower the light moves.

Do you remember learning about density last term in Matter and Materials? Write down your own definition for density in the space below.

If light moves from a less dense medium, like air, into a denser medium, like glass, then the light slows down. The light will bend towards the normal line.

If light moves from a more dense medium to a less dense medium then the light speeds up and moves away from the normal.

When light refracts and changes direction as it passes through different mediums, it can distort what we see. Think back to the pencil or straw in a glass of water at the start of the section. We can now explain why a drinking straw or pencil in a glass of water looks bent. The light bends when it moves from one medium to another. Light moves from the air to glass to water, and therefore changes direction.

If you have stood in a pool of water before and looked down, have you noticed how short your legs appear to be? Let's have a look at this a bit more in the next activity.

Magic coin trick

This activity will show the learners that bending of light will affect what we are able to see. The coin is not visible until the water is added. The water causes the light rays from the coin to refract (bend) towards the learner's eye. This allows the learner to see the coin.

Watch a video that shows and explains the coin activity.

MATERIALS:

• coin
• prestik
• opaque bowl or cup
• water

INSTRUCTIONS:

1. Work in pairs for this activity.
2. Put a small amount of prestik onto the bottom of the bowl.
3. Stick the coin to the bottom of the bowl.
4. Take small steps back from the desk/table until you cannot see the coin over the lip of the bowl.
5. Ask your partner to slowly pour water into the bowl and observe.

The learners should stick the coin to the bowl in order to keep the coin still when water is poured into the bowl. Often learners do not pour the water in gently and if the coin moves then it will affect the results.

QUESTIONS:

What happened when your partner poured the water into the bowl?

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Learners responses may vary slightly but they should all have seen the coin "appear" when the water was deep enough. When more water is added the entire coin can be seen.

Where does the coin appear to be?

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The coin appears to be higher than it actually is.

Explain why the coin can be seen when the water is added, but not before. The diagrams below will help you explain what is happening in words.

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When there is no water in the bowl there is no direct line of sight from the learner's eye to the coin. When water is added the light from the coin leaves the water and is refracted. The learner's brain detects the refracted light and as the brain knows light travels in straight lines, the coin appears to be higher in the water.

The diagrams used here show the container as transparent so that you can see the coin inside, whereas you will actually be using an opaque container.

Refraction can be used to explain why images appear to be distorted when we view them through transparent mediums. For example, if you are looking at your legs or hands through some water, they will appear closer than they actually are as the light is refracted. Look at the photograph of the glass with water in it in front of diagonal lines. Can you see how the lines are distorted when the light travels through the water and glass compared to when it does not?

Can you remember how we split white light into the separate colours of the visible spectrum in the beginning of this chapter? What did we use to do this in the activity?

We can do this because the different colours of light bend by different amounts. The amount of refraction depends on the type of light entering the medium. Different colours of light will slow down to different speeds.

When the white light entered the prism it refracted. The different colours of light travel at different speeds in the prism so they refracted at different angles and split up. Red light refracts the least and the blue light refracts the most as you can see in the following diagram.

Prisms are not the only objects that can split white light into separate colours. In fact, a rainbow is a good example of white light splitting up.

Light from the Sun enters the raindrops and refracts. The light is then reflected off the back of the raindrop. When the light passes out of the raindrop it is refracted again and the colours split up even more as shown in the diagram.

What colour is at the top of a rainbow and which colour is at the bottom?

Does this match the order which we see in the diagram showing how light is refracted and reflected in a raindrop?

How does this happen? When we see a rainbow, we see a combination of millions of raindrops. Although each raindrop refracts and reflects all 7 colours, we only see only colour of light reflected from each particular raindrop. This depends on the angle of the raindrop from our position. Therefore, the raindrops higher up in the sky reflect red light to us and the rain drops lower down reflect violet light to us. This is shown in the following diagram.

We are now going to look at an application of the refraction of light.

Lenses

• diverge
• converge
• focus

Do you remember when we spoke about how we see light and the structure of the eye, we mentioned that there is a lens just behind the iris? Another place where you may have seen lenses before are in reading glasses which some people wear to correct their vision. Or, have you seen how a magnifying glass makes things appear bigger. What are lenses and how do they work?

A lens is a transparent object which focuses or refracts light. When light is spread out, we say it has diverged. Some lenses will diverge light while others will converge light, bringing the light rays together. When light rays are all brought to the same point, we say they have been focused. Let's have a look at this more closely.

Diverging and converging light with lenses

You will need a ray box or light source which lets at least two rays of light through so that learners can observe how they are either focused or dispersed. In the absence of a ray box, you can use any light source and use a piece of cardboard with two slits cut into it to let the light through.

If you are not able to do this activity as you do not have lenses, photographs have been provided so that learners can still answer the questions and see what happens.

MATERIALS:

• ray box or light source
• concave lens
• convex lens
• piece of paper
• pencil

Before we start, it is important that you know the difference between a convex and a concave lens.

Convex lens	Concave lens
A convex lens has one side which curves or bulges outwards. A convex lens converges light.	A concave lens has one side which curves or is hollowed inwards. A concave lens diverges light.

A lens can have two sides which are concave and it is then called a biconcave lens or two sides which are convex and it is then called a biconvex lens.

INSTRUCTIONS:

Place a ray box or light source on one side of a piece of paper and turn it on. Observe the light rays. You might see something as shown in the photograph here.

Turn the ray box off.

Place the convex lens (with the rounded surface) on the piece of paper where the light rays will pass through it. Trace around it.

Turn on the ray box or light source and observe what happens to the rays when they pass through the lens.

Trace the path of the light rays on your piece of paper.

Describe what has happened to the light rays.

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The light rays have been focused as they come to a point.

Mark the point where the light rays cross. This is called the focal point of a convex lens.

Turn off the ray box or light source and place a new piece of paper in front of it.

Now place the concave lens in the path of the light rays and trace around the lens.

Turn on the light source and observe what happens to the rays.

Trace the path of the rays on the piece of paper.

Describe what has happened to the light rays.

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The light rays have diverged as they spread out after passing through the lens.

Turn off the light rays and extend the rays you have drawn until they meet at a point in front of the lens. This is the focal point of a concave lens.

If you still have your pin hole cameras, place a convex and concave lens in front of the camera and observe the image that forms.

Is the image larger or smaller when you observe through a concave lens?

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The image will be larger.

Is the image larger or smaller when you observe through a convex lens?

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The image will be smaller.

We have now seen how lenses can disperse or focus light. Have a look at the following diagrams which show how a biconvex lens converges light and a biconcave lens diverges light.

Converging lens	Diverging lens
A converging lens refracts the light entering it and bends the light rays to a focal point on the other side of the lens.	A diverging lens refracts the light entering it and bends the light rays away from each other. The light rays can be traced back to a focal point in front of the lens.

What do we use lenses for? Think of a magnifying glass. If you hold a magnifying glass over a picture or words then it enlarges the image. Is a magnifying glass an example of a diverging or converging lens?

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A magnifying glass is an example of a converging lens.

Let's think about how this works. Imagine you are looking at the ladybird from the beginning of the chapter through a magnifying glass. The ladybird looks bigger than what it actually is. When the object you are viewing is closer to the lens than the focal point, you see a virtual image of the ladybird that is larger than the object.

Have a look at the first diagram below. Can you see that the ladybird is between the focal point and the lens? The rays reflected from the ladybird are refracted by the magnifying glass and enter the person's eye.

In the next diagram you can see how your eyes see a virtual image of the ladybird which is bigger than the object. The more curved the convex lens is in a magnifying glass, the greater its ability to magnify objects.

When you hold a magnifying glass up and view a distant object, the object appears smaller and upside down. Unlike when viewing the ladybird close up, the distant object isbeyond the focal point of the lens, which results in this effect.

How do lenses work?

Do you remember what the human eye looks like? We have lenses in our eyes to allow us to see. The light enters the eye and passes through the lens. The lens focuses the light onto the back of our retina so that a clear image is formed. What type of lens do we have in our eyes? Give a reason for your answer.

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A biconvex (converging) lens as it needs to focus the light rays onto the back of the retina.

A contact lens is designed to rest on the cornea of the eye and correct vision. Leonardo da Vinci was the first to come up with the idea in the 16th century to help prevent eye infection.

In order for a clear image to form, the lens in our eye needs to focus the light rays coming into our eyes so that the focal point falls on the retina. This depends on the shape of the lens in our eyes. Sometimes, people have lenses in their eyes that cannot focus properly. Have a look at the following diagram which shows a normal eye and then an eye which focuses before the retina (near-sighted) and behind the retina (far-sighted).

Optical glasses, or spectacles, are used to correct near or far-sightedness.

If you are near-sighted you need a diverging lens. Would this be a biconcave or biconvex lens?

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You would need a biconcave lens.

If you are far-sighted you need a converging lens. Would this be a biconcave or biconvex lens?

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You would need a biconvex lens.

The following image shows how lenses can be used to correct far and near-sightedness.

Amicroscope makes a tiny, nearby object look much bigger. A telescope makes a large, distant object look much closer and brighter. In both, light from the object passes through two or more lenses to form an image. The lens shapes and distances between them determine how the image is produced.

Next term in Planet Earth and Beyond we will look at how lenses are used in optical telescopes to view objects in space.

Careers in optics

• optics

Peer through the bionic contact lenses (video)

Have you noticed that many of the words relating to vision start with or have 'op' as part of the word. These words are derived from the Greek wordopsis which means 'vision'. For example, optics, optical, optometrist, myopia and optometer.

Research careers in optics

An interview conducted with an optometrist. http://www.clubx.co.za/careers/so-you-want-to-be-an-optometrist/

There are many different careers in the field of geometric optics.

INSTRUCTIONS:

1. Work in groups of 3.
2. Interview someone in the field of geometric optics and find out how they chose their career and what and where they studied.
3. Write a paragraph explaining the career and the study options available in order to qualify for that career.
4. Here are some examples of careers in geometric optics.
   1. Optometry
   2. Ophthalmology
   3. Optoelectronics
   4. Illumination engineering

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Here is some information about each of these careers:

Optometry

Optometrists measure the efficiency of the patient's eyes. They examine eyes for vision problems, disease and other abnormal conditions. They test for proper depth and colour perception and the ability to focus and coordinate the eyes. They specialize in visual defects They are able to prescribe spectacles or contact lenses to rectify or alleviate visual defects such as far-sightedness, short-sightedness, astigmatism (image distortion) and presbyopia (far-sightedness as the result of age).

School Subjects

National Senior Certificate meeting degree requirements for a degree course

National Senior Certificate meeting diploma requirements for a diploma course

Each institution will have its own minimum entry requirements.

Compulsory Subjects: Mathematics, Physical Sciences

Recommended Subjects: Life Sciences

Training

Degree: BOptometry - UJ, UFS, UL. The duration of the course is 4 years of full-time study. After the completion of the degree course, students may be expected to complete a one-year internship before registration as professional optometrists.

Diploma: N.Dip: Optical Dispensing and B.Tech - CPUT. The duration of the course is three years. A fourth year of study culminates in the BTech Optometry. During their third and fourth year, students have contact with patients. Students are required to complete a one-year internship.

Optometrists are required to register with the Interim National Medical and Dental Council (INMDC) of SA before they may practise.

Ophthalmology

Ophthalmologists diagnose and treat diseases of the eye, including glaucoma and cataracts, vision problems, such as near-sightedness, and eye injuries. Most ophthalmologists practice a combination of medicine and surgery, ranging from lens prescription and standard medical treatment to the most delicate and precise surgical manipulations.

School Subjects

National Senior Certificate meeting degree requirements for a degree course

Each institution will have its own minimum entry requirements.

Compulsory Subjects: Mathematics, Physical Sciences

Recommended Subjects: Life Sciences

Note: Competition to enter medical studies is stiff and there are usually many applicants with excellent grades who naturally would be given preference.

Training

MBChB degree at UP, UCT, UFS, Wits, US, UL, UKZN:

• Theoretical training: 6 years
• Student internship: 1 year
• Practical work at a hospital: 1 year (also known as the house doctor year).
• Post-graduate study for specialisation as an ophthalmologist: 3 - 5 years.

Registration: on successful completion of the examination to qualify as a specialist, the candidate must register with the International Medical and diagnostic Centre as an ophthalmologist.

A useful website: http://sun025.sun.ac.za/portal/page/portal/Health_Sciences/English/Departments/Surgical_Sciences/Ophthalmology/General

Optoelectronics

Optoelectronics is the study and application ofelectronic devices that source, detect and controllight, usually considered a sub-field of[link]photonics.

This career would require a degree in electrical engineering which could be obtained at any South African university. Entry requirements will depend on the institution involved.

Illumination engineering

illumination engineering is the study and use of lighting in various situations, buildings and community spaces, such as sports and recreational lighting, lighting industrial facilities, roadway lighting, museum lighting. Illumination engineering can be studied at university by pursuing a degree in electrical engineering. The Illumination Engineering Society of South Africa also offers courses, details are on their website.

The Zooniverse website provides a great overview of the various citizen science projects that learners can get involved in. There is a huge variety of projects, including helping to identify possible planets around stars, analysing real life cancer data, looking at tropical cyclone data and listening to the calls from whales or bats.

Citizen science is scientific research that is conducted in whole or in part by nonprofessional scientists, specifically the general public. Encouraging learners to get involved in some of these projects will open their eyes to the possibilities out there, and also add meaning and value to what they learn within the Natural Sciences classroom. https://www.zooniverse.org/

'Citizen science' is when the general public takes part in and conducts scientific research.

Want to take part in some real science research? Check out these citizen science projects to get involved easily. https://www.zooniverse.org/

Remember to discover more online by visiting www.curious.org.za and by typing the links in the Visit margin boxes into your internet browser to watch any videos, play with simulations or read an interesting article.

• Light travels in straight lines.
• White light consists of all the colours of the visible spectrum.
• The colour spectrum can be seen when white light is dispersed by a prism or a raindrop (rainbow).
• Light cannot pass through opaque objects.
• Light can pass through transparent objects.
• Light is absorbed by some materials.
• A material appears to be a certain colour because it reflects that part of the colour spectrum. Other wavelengths of light are absorbed.
• In reflection, the angle of incidence is equal to the angle of reflection.
• On a smooth surface, parallel rays of light are all reflected at the same angle.
• On rough surfaces, the light is scattered and the image produced is not clear.
• The human eye has specialised cells in the retina which convert light into electrical nerve impulses. The nerve impulses are transmitted to the brain via the optic nerve, where they are interpreted.
• Light travels at different speeds in different media.
• When light enters a different medium at an angle, the light is refracted.
• If the light slows down, the light bends towards the normal line.
• If the light speeds up, the light bends away from the normal line.
• Converging lenses refract and focus light.
• Diverging lenses and triangular prisms refract and disperse light.
• Lenses have many applications, for example, in glasses to correct vision, microscopes, telescopes and magnifying glasses.

Concept map

The concept map on the next page shows how all the concepts relating to visible light link together. Complete the map to reinforce what you have learned in this chapter.

Match the correct definitions to the terms in the following table. Write the letter of the definition next to the correct number below. [12 marks]

Term	Definition
1. Radiation	A. Light cannot pass through.
2. Visible light	B. The angle of incidence equals the angle of reflection when a ray is reflected off a smooth surface.
3. Opaque	C. One of the ways in which energy is transferred, specifically through a vacuum
4. Transparent	D. When light enters a transparent medium it can change direction.
5. Absorption	E. Curved inwards.
6. Reflection	F. The spectrum of light which we are able to see.
7. Retina	G. Bulging outwards.
8. Refraction	H. A transparent object able to refract and focus light.
9. Diverging	I. Light can pass through.
10. Lens	J. When light rays are spread out from a point.
11. Concave	K. A layer of tissue at the back of the eye which is sensitive to light.
12. Convex	L. When the surface of a substance absorbs certain colours of light.

Answers: 1: 2: 3: 4: 5: 6: 7: 8: 9: 10: 11: 12:

Answers: 1: C 2: F 3: A 4: I 5: L 6: B 7: K 8: D 9: J 10: H 11: E 12: G

A beam of white light is shone through a glass prism. It splits up into seven colours which are shone on a screen. A learner took a photograph which is shown below and drew a ray diagram to show the prism. The colours are marked 1 to 7 in the diagram.

1. What does this tell us about white light? [1 mark]

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2. Why does the light do this when it passes through the prism? [3 marks]

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3. What colour is at label 1 and what colour is at label 7? Explain your answer. [3 marks]

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4. What label corresponds to the colour of grass? [1 mark]

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5. Can you see there are two other lighter, white rays emerging from the prism? What do you think this is the result of? [2 marks]

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1. White light is made up of a spectrum of 7 colours.

2. When the light passes from the air into the glass at an angle, it refracts and bends. The colours of the spectrum bend by different amounts causing the light to disperse. When the light leaves the other side of the prism, it refracts again and the colours bend even more and split up showing the seven colours.

3. Colour 1 is red and colour 7 is violet as red light bends the least and violet light bends the most.

4. Label 4, green.

5. These rays are the result of some reflection off the inner surfaces of the prism as not all the light passes directly through.

   Note:This is an extension question.

Why does an opaque object cast a shadow? [2 marks]

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An opaque object casts a shadow as it does not let any light pass through it. The light is either reflected or absorbed. There will be a shadow on the opposite side to the light source as the light cannot reach there due to the object.

Look at the following photograph of water in a pond and answer the questions.

1. How are we able to see the image of the wooden poles sticking up on the edge of the pond? [2 marks]

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2. Why is the image not clear, but blurred? [2 marks]

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1. The light is reflected off the poles and then it reflects off the surface of the water and into our eyes.

2. This is because the light rays are not reflected off a smooth surface, but rather an uneven surface, due to the ripples in the water. The light rays are scattered.

Two learners are discussing the colours of light. They decide that white and black are not really colours of light. If they are not colours, then how can we see them? [5 marks]

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White is a combination of all of the colours in the visible spectrum. White objects reflect all the colours equally and so we see the mixture of colours as white. Black is an absence of colour. Black objects absorb all of the colours and reflect none. This means that we don't see any coloured light from that object.

Explain how we are able to see the different colours on the South African flag. [6 marks]

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Black: All the colours are absorbed and none are reflected.

Yellow: All the colours except yellow are absorbed and the yellow is reflected.

Green: All the colours except green are absorbed and the green is reflected.

Blue: All the colours except blue are absorbed and the blue is reflected

Red: All the colours except red are absorbed and the red is reflected.

White: All the colours are reflected, none are absorbed and so the combined colours appear as white.

Draw a ray diagram in the space provided to show how we see the green part of the flag. [5 marks]

Which diagram shown below correctly shows the path of a ray of light through a triangular piece of glass? [2 marks]

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Complete the following sentence and write it out in full on the lines provided: When light travels from a less dense into a more dense transparent medium, it refracts and bends _____5 the normal line. When light travels from more dense to a less dense medium, it refracts and bends _____5 from the normal line. [2 marks]

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When light travels from a less dense into a more dense transparent medium, the light refracts and bends towards the normal line. When light travels from more dense to a less dense medium, it refracts and bends away from the normal line.

Draw a diagram to show what is meant by 'when the refracted ray bends towards the normal'. Mark the angle of incidence and angle of refraction. Indicate which medium is denser [4 marks]

Study the following diagram and answer the questions that follow.

1. This diagram is a drawing that a learner made during an investigation into the refraction of light. What does the red line represent in this diagram? [1 mark]

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2. What do the blue lines represent? Label this on the diagram. [1 mark]

3. The light passes from the air and into a block of another medium. Is this medium more or less dense than air? Give a reason for your answer. [2 marks]

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4. What type of medium could the block be made from? [1 mark]

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5. Label the incident ray and the emergent ray on the diagram. [2 marks]

6. Label the angles of incidence (i) and angles of refraction ® on the diagram. [2 marks]

1. The ray of light.

3. The block is more dense than air as when the light enters the block, the ray bends towards the normal line indicating that it travels more slowly. The ray then bends away from the normal line when it leaves the block and enters a less dense medium (the air) and travels faster.

4. It must be a transparent medium, such as glass.

6. 0.5 marks for each label. The learner's completed diagram with labels should look as follows:

   

Which diagram shown below shows the path of a light beam passing through a rectangular glass prism correctly? [2 marks]

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Why does it look like the tree trunk in the photograph is skew? [2 marks]

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This is due to refraction. The light that passes through the piece of glass is bent and so the image becomes distorted and looks as though the trunk is skew.

What shape does a lens have to have in order to focus the light? [1 mark]

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It must be convex.

Draw a ray diagram to show how a converging lens focuses light to a point. [4 marks]

Which eyesight defect can be fixed by using a converging lens? Explain what this defect is and why it can be corrected. [4 mark]

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Far-sightedness can be corrected using a convex lens. This is when the light focuses on a point behind the retina so the image is blurred. A convex lens is used to bend the light rays before they enter the eye so that when they do pass through the lens in the eye they are focused clearly on the retina.

Total [74 marks]