Why distant things look dim and small
Imagine that you are out in the countryside at night. There is a cloudy sky, which is cutting out the light from the Moon and stars so that it is very dark. You stumble along a path between trees and bushes. Every few moments you seem to be hit in the face by the tips of twigs because they are too difficult to see in the gloom. Suddenly in the distance you see a light. It seems to be raised above the ground but you cannot be sure. You move cautiously towards it. You move out of the trees and bushes onto open land. Now there are large tufts of grass to contend with but you keep walking forwards.

As you draw a little closer, you see that the light is not above the ground but on top of a small hill. In a few more steps you see that the light is not shining steadily. It flickers. One moment there is a great deal of light then gradually it dies down. A few moments later it grows in brightness again. As you move a little nearer you can see that this change in light is due to flames. The light is coming from a fire.

Why should the flames die down and then build up again? You are intrigued now and move forward more quickly, then stop. You can see some shapes around the fire. They are people. You wonder if you should approach, when a twig you are standing on snaps. One of the people turns and looks towards you. She waves to you and you walk on.

From a distance the people appeared to be wearing dark clothes but now as you come near the fire you can see that their clothes are many different colours. They greet you with smiles and a small boy holds out a cup of soup. As you stand round the fire and talk, you look around and see that there are tents on the other side of the hill. The people are camping in them and this is their campfire.

You wonder about why the fire seemed to go bright and then die back but find your answer. A little way from the fire is a large pile of wood. Every time the fire died down some people put more wood on and the fire burst into flames again.

This incident can tell you a lot about light. The source of light was the fire. It sent out light in all directions. You could tell this because you could see light from the fire on one side of the hill as you approached but also when you reached the hilltop you could see the tents on the other side of the hill by firelight that was reflected from them. Although you could see the light from a distance you could not see much detail. At first you thought the light was in the sky, then when you were nearer and receiving more light you realised it was on a hilltop. In a similar way you could see the intensity of the light increasing and decreasing but you could not tell why until you had come close enough to see the light was made by flames. Finally, the firelight that was reflected from the people was much weaker than the light coming from the fire. Even when you were quite close you only received enough light to make out their outlines and their clothes looked dark. Only when you came up to the people did your eyes receive enough light to tell that the clothes had colours in them.

As the fire died down and some people went to their tents you looked up into the sky. The clouds had cleared away now and you could see the stars. Each one looked like a point of light. You began to imagine what it would be like to climb into a spacecraft and travel out from the Earth towards them. How would a star change as you went towards it and what might you find when you reached it?

Don't all objects give out light?
No. There are only a few objects which give out light. They are called light sources. The fire is an example. Electric lights, television screens, computer screens are other examples of light sources. All other objects reflect light. You see them by the light they reflect from a light source.

How does a flame give out light?
When anything burns a change takes place. The burning materials change into other materials. One of the substances that is made in this change is a substance called carbon. It is black and found in the lead of pencils. When something burns, the carbon forms very fine particles which rise above the burning material and become so hot that they glow white and yellow. It is the glowing carbon particles in the flame that gives out the light.

How would the light change if you walked away from the fire?
Your eyes would receive less and less light as you walked backwards away from the fire. This would mean that the light became dimmer.

If the fire is giving out the same amount of light as you walk away why does it look dimmer in the distance?
Light rays travel in the following way. A group of light rays come out from the flame. They are close together. If you stand nearby, your eyes receive a large number of them and you can see a bright light. As the light rays move away from the fire, they spread out from each other. If you stand further away from the fire, your eyes receive a smaller number of light rays so you do not see the light as brightly as before. The light appears dimmer. The further away you go the fewer light rays your eyes receive, owing to them spreading out even more, and the dimmer the fire appears.

Why could the people not be seen when the fire was first noticed?
The people were not light sources. They could only be seen by the light that was reflected from them. When light from the fire strikes the people's clothes, some of it is absorbed by the material. This means that the reflected light is much weaker than the light from the light source. The rays of the weaker light spread out just like this from the stronger light. When the fire was first seen you were a long way from it. By the time these few weak light rays reached you they were so spread out that your eyes could not detect them. So you could not see the people.

Why could the colour in the clothes only be seen when you were close to the fire?
Your eyes contain two kinds of light sensitive cells. One kind, called rod cells, is very sensitive to light and can detect light shining from a very weak or distant source. The other cells are called cone cells because they are shaped like cones. They are sensitive to the colours in light but only work when a large number of light rays strike them. When you were far from the fire you were seeing everything just by using your rod cells. When you came close to the fire, the cones cells could work and let you see colours.

Why did the fire appear larger as you approached it?
The picture that you see with your eyes is called your field of view. Everything in your field of view appears to be a certain size. The size that an object appears to be is due to the light coming from the top and the bottom of it. Sit back and look at this computer monitor. It appears to go smaller. Sit forwards and it appears to get larger. When you sit back, you create more space for the light rays from the top and bottom of the monitor to come together and this allows them to enter your eye at a very small angle to each other. When you sit forwards, you create less space for the light rays to come together and they enter your eye at a much larger angle to each other. Your eye judges the size of something by the angle at which the light rays enter it. Find out how this works by picking up two objects which are different sizes (two pebbles will do) and hold the smaller one close to your eye and the larger one further away. Move them about until they both seem to be the same size.

What might happen if you approached a distant star?
It would become larger and brighter. When you got very close you might see planets by the starlight they reflect, just as you saw people in the campfire story by the firelight they reflected.