How is it that you can see yourself in a mirror?
What makes a ring around the moon?
Why can we see clouds and not the air?
Why is a pair of new shoes or anything smooth usually shiny?
If we turn off a switch labeled Reflection of Light on our imaginary switchboard, we think at first that we have accidentally turned off Radiation again, for once more everything instantly becomes dark around us. We cannot see our hands in front of our faces. Although it is the middle of the day, the sky is jet black. But this time we see bright stars shining in it. And among them is the sun, shining as brightly as ever and dazzling our eyes when we look at it. But its light does no good. When we look down from the sky toward the earth, everything is so black that we should think we were blind if we had not just seen the stars and sun.
Groping our way along to an electric lamp, we turn it on. It shines brightly, but it does not make anything around it light; everything stays absolutely invisible. It is as if all things in the world except the lights had put on some sort of magic invisible caps.
We can strike a match and see its flame. We can see a fire on the hearth. We may feel around for the invisible poker, and when we find it, we may put it in the fire. When it becomes hot enough, it will glow red and become visible. We can make a match head glow by rubbing it on a wet finger. We can even see a firefly, if one comes around. But only those things which are glowing of themselves, like flames, and red-hot pokers, and fireflies, will be visible.
The reason why practically everything would be invisible if there were no reflection of light is this: When you look at anything, as a man, for instance, what you really see is the light that hits him and bounces back (reflects) into your eyes. Suppose you go into a dark room and turn on an electric light. Instantly ripples of light flash out from the lamp in every direction. As soon as they strike the object you are looking at, they reflect (bounce back) from it to your eyes. When light strikes your eyes, you see.
Of course, when you look at an electric lamp, or a star, or the sun, or anything that is incandescent (so hot that it shines by its own light), you can see it, whether reflection exists or not. But most things you look at do not shine by their own light. This book that you are reading simply reflects the light in the room to your eyes; it would not give any light in a dark room. The paper reflects a good deal of light that strikes it, so it looks very light; the print reflects practically none of the light that strikes it, so it looks dark, or black, just as a keyhole looks black because it does not reflect any light to your eyes. But without reflection, the book would be entirely invisible. The only kind of print you could read if there were no reflection would be the electric signs made out of incandescent lamps arranged to form letters.
What the ring around the moon is; what sunbeams are. The reason you sometimes see a ring around the moon is that some of the moonlight reflects from tiny droplets of water in the air, making them visible. In the same way, the dust in the air of a room becomes visible when the sun shines through it and is reflected by each speck of dust; we call it a sunbeam. But we are not really looking directly at the sunlight; we are seeing the part of the sunlight that is reflected by the dust specks.
Have you ever noticed that when you stand a little to one side of a mirror where you cannot see your own image in it, you can sometimes see that of another person clearly, while he cannot see his own image but can see yours? It is easy to understand this by comparing the reflection of the light from your face to his eye and from his face to your eye, to the bouncing of a ball from one person to another. Suppose you and a friend are standing a little way apart on sandy ground where you cannot bounce a ball, but that between you there is a plank. If each of you is standing well away from the plank, neither one of you can possibly bounce the ball on it in such a way that he can catch it himself. Yet you can easily bounce it to your friend and he can bounce it to you.
