1. In This Section We Will Be Studying The Following Topics:
LIGHT
In This Section We Will Be Studying The Following Topics:
The sources of light.
Straight lines & shadows.
How light behaves with different materials.
Seeing.
Reflection of light.
Bending light.
Light & colour.

2. THE SOURCES OF LIGHT
Light is a type of energy and it is called the visible spectrum because we can see it.
Anything that radiates energy gives off light and also heat.
The light that is given out travels in straight lines.
Light always travels from the source, so we draw lines and place arrows on the ends of those lines to show where it is coming from.
That is probably why people draw the Sun as if it has lines coming out of it.

3. STRAIGHT LINES & SHADOWS
Light travels in straight lines and we can prove this because if we put an object between the sun and the ground, a shadow is formed because the rays of light get blocked out.
Remember:
We can only see things properly if rays of light strike them.

4. STRAIGHT LINES & SHADOWS
We use lines to show the direction of light and if we set up a simple experiment like this one, we can see that the light travels in straight lines because it cannot pass around the black blocks, only through the gaps.
Only a very small part of the light reaches the last black block.
What would you expect to find behind the walls?
Shadows.

5. HOW LIGHT BEHAVES WITH DIFFERENT MATERIALS
We have seen that light travels in straight lines This is said to be organised light.
Organised light can either pass through a material or it is blocked by the material. This depends on the type of material.
Materials that let light through can be transparent or translucent.
Materials that do not let light through are called opaque. They do not let light through because they reflect the light or absorb the light energy.

6. HOW LIGHT BEHAVES WITH DIFFERENT MATERIALS
When light travels through a material it is called transmission. The light is said to be transmitted.
A transparent material transmits light keeping it organised, so that we can clearly see an object through the material.
For example:
Transparent materials do not affect the light transmitted.
Window panes.
Mobile phone screen.

7. HOW LIGHT BEHAVES WITH DIFFERENT MATERIALS
A translucent material transmits light but does not keep it organised. The light is scattered.
This makes the object look fuzzy or broken up.
Translucent materials scatter the light transmitted.
Can you think of an everyday example?
Bathroom window panes.

8. HOW LIGHT BEHAVES WITH DIFFERENT MATERIALS
Opaque materials do not transmit light. The light is either reflected, absorbed or reflected and absorbed.
Reflected light.
Absorbed light.
Usually opaque materials absorb most of the light energy and heat up. The rest of the light is reflected.
So that is why objects feel hot after they have been left in the sunshine.
Can you think of an object that absorbs all the light?
Black cats in the dark.

9. HOW LIGHT BEHAVES WITH DIFFERENT MATERIALS
Opaque materials that reflect light either reflect organised light or scattered light.
Do you know any objects that reflect organised light?
A mirror or highly polished metal.
Mirror
Organised light.
Organised reflection.
Scattered reflection.
A good example of a material that scatters reflected light is paper.
A page from a notebook or printer paper does not have a smooth surface.
Paper.

10. Sun, light bulbs and fire.
SEEING
As you are reading this text, light from the computer monitor, travelling in straight lines, enters your eyes. This enables you to see the screen.
The computer monitor gives out light, so it is a source of light It is said to be luminous.
Can you name any more luminous objects?
Sun, light bulbs and fire.
We see non-luminous objects because light from a light source is reflected off them into our eyes.
Scattered reflected light.
Hey, I’m in the spotlight now.

11. The light is reflected off these objects so that we can see them.
SEEING
The light is reflected off these objects so that we can see them.
Non luminous.
These objects make their own light. A clue that they are making light is that they also make heat.
Luminous.
Which objects are luminous and which are non luminous?
Non luminous.
Non luminous.
Luminous.
Luminous.
Non luminous.
Luminous.
Non luminous.
Luminous.
Luminous.

12. Luminous and non-luminous.
SEEING
To see an object, light from that object must enter our eye.
What are the two types of object that we can see?
Luminous and non-luminous.
It helps to understand how our eyes work. We can think of them as a simple camera. The simplest is a pinhole camera.
Image.
Notice, the image is upside down. It is said to be inverted.
Pinhole.
Object.
Screen.
The image is upside down (inverted) because the rays of light from the top and bottom of the object cross over as they go through the pinhole.

13. SEEING Now, let’s have a look at how our eyes work.
Cornea
Light from the object hits the cornea and changes direction slightly to go through the pupil.
Object
Light enters the eye through the pupil, like the hole in a pinhole camera.
Image.
The light then goes through the lens. It changes direction as it enters and leaves the lens. The lens can change shape to make it easier to see objects, whether they are near or far away.
Finally the light image falls on the retina. The retina is a layer of cells that send electrical signals to the brain when light falls on them. Like the pinhole camera the image is upside down, but the brain turns it the right way around.

14. SEEING
The pupil, which is the black circle in the middle of the eye, can change size.
The iris can make the pupil bigger to let more light in if you are in a dark room. Or it can make it smaller to let less light in if you are in bright sunshine.
Look at the pupils of a friend, then ask them to cover their eyes for about 10 seconds. When they remove their hands quickly have a look at their pupils and you should see a difference in size.
Thinking of the eye lens, why do some people need to wear glasses?
Because their lenses can’t change shape very well.
Our pupils have become adjusted to the dark cinema.
Why, when we leave a cinema while it is daylight, do things seem very bright?

15. REFLECTION OF LIGHT
Organised light.
Organised reflection.
Organised light hitting a mirror or a flat shiny surface is reflected in an organised way.
Mirror.
There is a special name for a ray of organised light. It is called an incident ray.
Incident ray.
Reflected ray.
Mirror.

16. REFLECTION OF LIGHT
The direction that a reflected ray leaves the surface depends on the direction that the incident ray hits the surface.
When a ray of light hits at 90o to the surface it is reflected straight back in the direction that it came from. This line is called the normal.
The normal can be thought of as an imaginary line at 90o to the reflecting surface.
Reflected Ray.
Reflecting Surface.
Incident Ray.
90o
Reflecting Surface.
90o
Normal.

17. REFLECTION OF LIGHT
Other rays of light that hit the surface to the left of the normal are reflected to the right of the normal and rays from the right are reflected to the left.
Reflecting Surface.
Normal 1 2
Reflecting Surface.
Normal 3 4
Which are the incident rays?
1 and 4.
Which are the reflected rays?
2 and 3.
The angle between the incident ray and the normal is called the angle of incidence and the angle between the normal and the reflected ray is called the angle of reflection.
Reflecting Surface.
Normal
Angle of Incidence (labelled io).
Angle of Reflection (labelled ro).

18. REFLECTION OF LIGHT
The law of reflection:
The angle of reflection always equals the angle of incidence. r
Reflecting Surface
Normal. i
For example: if i = 45o then r = 45o

19. REFLECTION OF LIGHT
When you look into a mirror you see your reflection. If you look at the reflection of a book you will see that the words appear reversed. This is know as mirror writing.
Object.
Image.
Notice that in the image, the letters F and N have been flipped over horizontally. This is called lateral inversion.
Lateral Inversion.
The letter A does not appear inverted. This is because it is a symmetrical letter.
On the front of police cars and ambulances, so that drivers will see the words POLICE or AMBULANCE correctly in their mirrors.
Where is mirror writing often seen?

20. This is a diagram of a pencil resting in a glass beaker.
BENDING LIGHT
We have seen that light travels in a straight line and that it can be reflected, but can it be bent?
If we add water to the beaker the pencil looks different.
This is a diagram of a pencil resting in a glass beaker.
What has happened to the pencil?
It looks as if the pencil has been broken. This is caused by the water which has slightly bent the light reflected from the pencil. This bending is called refraction.
What else seems to have happened to the pencil in the water?
It appears to look bigger.

21. BENDING LIGHT
To see what happens when light is refracted we can shine a beam of light through a Perspex TM block.
If a ray of light hits the block at 90o to the surface, it travels straight through the block and is not refracted. As with reflection this is the normal.
Angle of Incidence io
Angle of Refraction ro
The route taken by a ray of light hitting a Perspex block at less than 90o.
The Angle of Refraction is the angle the ray of light makes with the normal as it leaves a surface.

22. BENDING LIGHT
The ray of light is refracted as it enters and leaves the block. We can say that light is refracted each time it crosses a surface between two materials.
Let us look at the ray of light leaving the block.
Angle of Incidence io
Angle of Refraction ro
Notice that the rays of light entering and leaving the block are parallel.
This means that the angle of incidence of the light entering the block is the same as the angle of refraction of the light leaving the block.

23. BENDING LIGHT
Also the angle of incidence and refraction in the block are always the same.
What can we say about how the ray bends as it enters and leaves the block?
The ray is bent towards the normal as it enters and away from the normal as it leaves.

24. Light from the object is refracted at the water surface.
BENDING LIGHT
I’ve been thinking.
Is the fact that things under water seem closer to the surface than they are something to do with refraction?
Yes you are right Moley. It is an effect called apparent depth. Let us have a look at it.
Light from the object is refracted at the water surface.
After refraction the light appears to come from here. So the object does not appear as deep as it actually is.
Object.

25. The light is refracted by rain drops.
BENDING LIGHT
There is a special instance of refraction which shows that sunlight is made by combining coloured light.
This is when white light is shone through a glass prism.
But, different coloured lights are bent by different amounts. This splits the light up into a range of colours called a Spectrum.
The light is refracted at both surfaces.
The splitting of light is called dispersion. The colours of the spectrum are usually listed as:
Red, Orange, Yellow, Green, Blue, Indigo and Violet
Rainbows are sometimes seen after rain. What makes the sunlight disperse?
The light is refracted by rain drops.

26. LIGHT AND COLOUR
If white light is made up of all the colours, how do we see green leaves or a red car and how do they get coloured spotlights in a theatre or disco?
Coloured lights are produced by shining white light through filters.
White Light.
Blue Filter.
Blue Light.
The blue filter will let blue light through, it transmits blue light. The other colours are absorbed by the filter.
What would happen if we now put a red filter in front of the blue light?
White Light.
Blue Filter.
Blue Light.
Red Filter.
No light is transmitted because blue light contains no red light and the blue light is absorbed by the red filter.
No Light.

27. LIGHT AND COLOUR
So why is the car red?
White Light.
Red Light Reflected.
When white light hits the red car all the colours are absorbed by the car except red, which is reflected. Therefore we see the car as red.
It is the same with this plant. The flower reflects the blue light and the leaves reflect the green light.
An object is white because all the colours are reflected and black because all the colours are absorbed.
What would happen if we looked at the plant in red light?
It would look black because in red light there is no blue or green light to be reflected.

28. LIGHT AND COLOUR YELLOW CYAN MAGENTA
There are three primary colours which can be added together to make other colours including white.
Red
Blue
Green
Can you name the three primary colours?
The primary colours can be mixed in many combinations to make all the colours of the spectrum.
YELLOW
CYAN
MAGENTA

29. CHECK YOUR LEARNING How does light travel? Why are shadows formed?
Why do we see an object?
What are the two types of reflected light?
Which type of reflected light comes from a mirror?
Which part of the eye controls the amount of light that enters?
What can we say about the angle of incidence and the angle of reflection?
What is refraction?
If white light is shone on a blue bike why do we see the bike as blue?

30. CHECK YOUR LEARNING - ANSWERS
In straight lines.
Because objects block the light.
Because light is reflected from objects.
Organised and Scattered.
Organised.
The iris.
The angle of incidence and the angle of reflection are equal.
Where light is bent when it passes from one medium to another.
The blue light is reflected and all the other colours that make up white light are absorbed.