Shadows that grow and shrink
Imagine that you are lost in a forest. The Sun is setting and it is becoming dark. You follow one path after another but seem to be getting nowhere. You are about to sit down when you see a light in the distance. You run through the bushes and out into a clearing. There in front of you is a house. There are lights on inside the house. You feel a little frightened so decide to creep up to the house and look in a window. Everywhere is quiet. Even the owls have stopped hooting. As you step slowly across the clearing, you put your foot on a stick. It snaps and makes a loud crack. You stop and hold your breath. Then the door opens.

A bright beam of light shines out across the clearing. It startles a rabbit, which hops away. You stay perfectly still and wait. A shadow appears in the beam of light. It is the outline of a person but it is huge. You begin to feel very frightened and almost dare not take a peep towards the open door. As your eyes glance along the shadow and back to the door, a small voice calls out, "Who's there?" You look up and see a little old man in the doorway. He calls out your name and says that someone has telephoned him to say that you were missing. You feel the little old man is a friend and go to meet him. He telephones your friends and soon they come to pick you up. As you ride away from the house in the wood you wonder why the little man had such a big shadow.

Once home you try an experiment. You switch on a torch and shine it on a wall. Next you make your hand into the shape of a dog's head and put it into the beam of light. A shadow of a dog's head appears on the wall. For a moment you are very pleased with your shadow and move your fingers to make the dog's mouth open and waggle its tongue. You then remember why you are trying the experiment and try to make the dog's head shadow change size. You move your hand across the beam but the shadow of the panting dog does not change in size. You move your hand up and down but the shadow stays the same. You are about to give up in despair when you move your hand nearer the torch. When this happens, the shadow changes. It becomes larger. You keep moving your hand nearer and nearer to the torch and you see the shadow becoming larger and larger. You quickly predict that if you move your hand back the shadow should become smaller. Slowly, you move your hand away from the torch and towards the wall. As you look at the shadow, you see it becoming smaller and smaller. When you place your hand close to the wall you see that the shadow is hardly larger than your hand.

You repeat your experiment to see if the same thing will happen again. It does. From what you tried you found that when an object is near a source of light it casts a large shadow and when it is further away from a source of light it casts a small shadow. This explains why the little man had a large shadow. He was close to the source of light inside the house. This made a shadow that was like that of a giant.

You may wonder why the light in the house shone a bright beam through the doorway but formed a shadow of the man. It is because of the way light travels and what the man is made from. A beam of light is made up from many tiny beams called light rays. The light rays travel in straight lines. The man is made from opaque material. This is material which blocks light rays. When the man stood in the doorway his body blocked a large number of light rays. Around the edges of his body light rays grazed by and shone onto the ground in the wood. However the ground in front of the man could not be lit up because his body had blocked the light rays. This meant that the area remained dark and a shadow formed.

Now it is time for you to investigate the size of a shadow. Take a torch, shine it on a wall and make your hand look like a dog's head. Can you make the dog's head swell and shrink?

How can you make your hand cast a shadow like a dog's head?
Switch on your torch and aim it at a wall. Hold out your hand as if you were going to shake someone's hand. Raise the thumb a little and put your hand in the torch beam. The thumb should make an ear. The outstretched fingers should make the rest of the dog's head and nose. If you lower your little finger a bit, the shadow would look like a dog with its mouth open. If you lower your little finger a little more, and lower the next finger a small amount, the shadow should look like a dog with a tongue in its mouth.

Can you investigate shadow size without using your hand?
Yes. Just stand a book up on a table. Place a torch about half a metre away from the book and shine a light on the book. Put an object near the book and look at its shadow. Now move the object nearer to the torch and see how the shadow changes. You should see the shadow become larger.

Why does the shadow become larger when the object is near the torch?
Many of the light rays coming from the torch have been brought together into beam by the torch's reflector. However there are also many light rays which have not struck the reflector and are spreading out from the front of the torch bulb in all directions. When the object is close to the bulb it is blocking many of these rays before they have had a chance to spread out, so a large shadow is cast.

Why does the shadow become smaller when the object is further away from the torch?
If the object is further away from the torch is gives the light rays more space to spread out. This means that fewer light rays are blocked by the object and a smaller shadow is made.

What would happen if light rays could curve?
Think of holding a stick in a stream. The water curves round it and perhaps makes a few ripples. If the water behaved like light rays it would travel in straight lines and the water which struck the stick would be stopped and there would be a gap on the other side of the stick. This does not happen because the water curves round the stick. Now think of light rays behaving like water. The light rays which strike the object are blocked but those that just graze its edges do not carry on in a straight line and make a shadow with a sharp edge. They curve round the back of the object and then carry on. If this were to happen there would be no shadows.

You do not see shadows on a cloudy day. Does that mean the light is curving?
No. The light rays, which pass through cloud, are scattered in all directions. They are also reflected off the surfaces of everything in all directions. As the light is not coming from one particular direction shadows are not made.

The person in the story used something they had seen to help them make an experiment. Should people do this?
Yes, they should. In science you make observations or look at things, then try and test what you have seen as did the person in the story.

The person in the story made a prediction. What is a prediction?
It is a kind of guess but it is slightly different from a guess. In a prediction you think about why something may happen and make your guess to match what you believe. In a guess you may just think of anything.

In science are predictions always right?
Not always. A scientist may have an idea how something works and make a prediction using the idea. When the experiment is made something different may happen. This may be due to an error in making the experiment or due to the scientist's idea being wrong. There is nothing bad about an idea being wrong. It is the idea that helps you make the experiment. If you did not have ideas you would not do any experiments. The experiments help you find out whether an idea is good or not.

In the story the person repeated the experiment. Is that normal?
Yes, it is or it should be. You should not do just one experiment. You should repeat it a few times. If you only did the experiment once you would not know if the results were true or just due to luck. Always repeat experiments to make sure of the results.