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. [Illustration: FIG. 62. The ball bounces from one boy to the other, but it does not return to the one who threw it.] The mirror is like that plank; it is something that will reflect (bounce) the light directly. The light from your face goes into the mirror, just as you may throw the ball against the plank, and the light is reflected to your friend just as the ball is bounced to him; so he sees your image in the mirror. If he can see you, you can see him, just as when you bounce the ball to him he can bounce it to you. But you may be unable to see yourself, just as you may be unable to bounce the ball on the plank so that you yourself can catch it. In other words, when light strikes against something it bounces away, just as a rubber ball bounces from a smooth surface. If you throw a ball straight down, it comes straight up; if light shines straight down on a flat, smooth surface, it reflects straight up. If you throw a ball down at a slant, it bounces up at the same slant in the opposite direction; if light strikes a smooth surface at a slant, it reflects at the same slant in the opposite direction. [Illustration: FIG. 63. In the same way, the light bounces (reflects) from one boy to the other. It does not return to the point from which it started and neither boy can see himself.] But to reflect light directly and to give a clear image, the surface the light strikes _must_ be extremely smooth, just as a tennis court must be fairly smooth to make a tennis ball rebound accurately. Any surface that is smooth enough will act like a mirror, although naturally, if it lets most of the light go through, it will not reflect as well as if it sends all the light back. A pane of glass is very smooth, and you can see yourself in it, especially if there is not much light coming through the glass from the other side to mix up with your reflection. But if the pane of glass is silvered so that no light can get through, you have a real mirror; most of the light that leaves your face is reflected to your eyes again.