Letting light through
Imagine that you are a beam of light that is just leaving the Sun. You travel across space and in about eight minutes you shine down into the Earth's atmosphere. You are made of billions of light rays all travelling in the same direction. Will they all make it to the ground?

About four kilometres above the ground there is a cloud. You strike the edge of it as you pass by. A cloud is made from tiny drops of water, dust and ice. The light rays in your beam that strike the cloud hit these small particles. The rays bounce off them, travel a little, bounce off some more and travel a little further. As there are billions of particles in a cloud the light rays are bounced in all directions. Eventually some of them emerge below the cloud and move down to the ground.

By now you have reached the ground. Some light rays have struck the top of a tree. The leaves and branches stop them going any further towards the ground so below them is an absence of light. It is dark and shady. Near the tree is a car with a sunroof. The sunroof is closed with a piece of transparent glass. Your light rays strike the surface of the glass. Some are reflected back up into the sky but most pass through. There is a person sitting in the car having a picnic. She has some sandwiches wrapped up in greaseproof paper. When some of your light rays strike the paper they are reflected. They travel up from the paper towards the person's face and then into her eyes. When the light rays reach inside her eyes she sees the parcel of sandwiches. Some light rays pass through the greaseproof paper and make the inside of the parcel light. This helps a caterpillar in a salad sandwich see where it is moving.

The picnicker looks out through the glass in her car window. The countryside is struck by huge numbers of light rays every second. Most of them strike the objects on the ground and are reflected in all directions. Some of these light rays pass through the car window and enter the picnicker's eye so she can see the view. In the distance is dark cloud. It is dark because it is so full of water droplets. They not only scatter the light rays that enter the cloud but they also absorb them so that little light leaves. Below the cloud it is raining and the light is passing through the raindrops in a special way.

Air and water are both transparent materials but water is much thicker than air. In fact air is so thin that you cannot feel it. When light rays strike the surface of a raindrop at a certain angle something unusual happens. The raindrop splits up into different coloured rays. Each ray travels through the raindrop but when it reaches the other surface it does not pass through. It is reflected back through the raindrop and out into the air. When billions of light rays strike millions of raindrops in this way, a rainbow is made.
A rainbow reveals the secret of a light ray from the Sun. It is not just one ray but is made from seven rays. Each one is a different colour from the rest. The seven colours are red, orange, yellow, green, blue, indigo and violet. When they travel together in a light ray they are mixed up and make the ray of white light. When they pass from one transparent substance to a thicker one they split up and can be seen separately.
The colours in a rainbow give us a clue as to how we see the colours of things around us. Let us think about the first thing that a sunbeam shines through as it comes close to the Earth: the air. The air is full of tiny particles, which are even smaller than those in a cloud. These particles are called atoms and they make up the gases in the air. When a sunbeam shines through the air, the atoms scatter some of the blue light rays and this makes the sky blue.

When a light ray strikes an opaque object on the ground, some of the light is absorbed and some is reflected. However, all the objects on the surface of the Earth are not white like the white light. They are different colours. What has happened to the white light? Some of the colours in it are absorbed and some are reflected. For example, if you are wearing a red jacket, the material in the jacket absorbs six colours and reflects one ? red. It you look at grass, it is absorbing six colours and reflecting green.

Look out of the window at daylight and think about the sunbeams shining down on the Earth. Even if it is cloudy, some of the rays get through and reach the ground. When they do, some colours are reflected to give us the colourful view we see.

Does all the sunlight come to the Earth?
No. The Sun spreads out light in all directions. Only a tiny part of it comes to the Earth. A little goes to the other planets in the Solar System but most travels out to the rest of the universe.

Does the Earth send any light into space?
Yes, it does, but almost all of it is reflected light from the Sun. Some of the light, which strikes the tip of the clouds, is reflected. The land and sea reflect the light into space. This means that the Earth can be seen from space. However, the part of the Earth that is away from the sunlight is in darkness. The only light that can be seen then are the lights from cities and the flash of lightning in thunderstorms.

Is a cloud translucent?
Yes, it is. Some light can pass through it but the light rays are scattered when they leave. If clouds were opaque we would not have any light on a cloudy day. If they were transparent, every day would be a sunny day.

What other materials are translucent?
The greaseproof paper in the story is translucent. Tissue paper is also translucent. The glass in bathroom and toilet windows is translucent. A translucent material is one that lets light through but you cannot see clearly what is on the other side.

What is a transparent material?
It is a material which lets most of the light through. The light is not scattered as it passes through the material so you can see clearly what is on the other side. Glass and some kinds of plastic are transparent materials. Air and water are also transparent materials.

If water is transparent why can you not see the bottom of the ocean?
All transparent materials absorb a little of the light as it passes through them. The thicker the material the more light it absorbs. If you put water in a bowl you can see the bottom because the water is only a few centimetres deep (or thick) so little light is absorbed. The light strikes the bottom of the bowl, is reflected and passes up through the water, out into the air and into your eye. An ocean can be a few kilometres deep (or thick). As the light passes down into the ocean more and more of it is absorbed until you reach a place in deep water where it is black because light cannot reach there. As light cannot even reach the ocean floor there is no chance of any being reflected and so you cannot see any rocks, shipwrecks or deep-sea fish.

How does light get into the eyes?
The front of the eye has a transparent window. Light passes through this part of the eye and enters a black hole called the student. The light passes through the eye and strikes the inside of the back wall. It forms a picture of the view the eye is looking at. In the wall of the eye is a nerve. It takes information about the picture to the brain and we see the view.

Can you always see a rainbow in the rain?
No. You can only see a rainbow when the Sun's rays shine from behind you into the rain. This makes the light rays strike the raindrops at the right angle so the colours in them split up. Sometimes if you look carefully, you can see a double rainbow.

What colours are reflected by a white coloured object?
All the colours are reflected equally to make the object appear white.

What colours are reflected by a black object?
A black object absorbs all the colours. We see an object as black because there is an
absence of light. If the surface of a black object is smooth, some white light may be reflected. It makes white marks on the surface called high lights.

Where do the colours come from in a stained glass window?
The glass has materials added to it, which absorb certain colours of light and let other colours of light pass through. For example, a piece of glass, which appears red, absorbs orange, yellow, green, blue, indigo and violet and lets the red light pass through. A piece of blue glass absorbs all the colours except blue and lets the blue light pass through.

What is a light filter?
It is a piece of material such as coloured glass or coloured transparent plastic which absorbs some colours and lets other colours of light pass through. For example, a red filter filters out all the colours of light except red and lets the red light pass through.