hrough 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.

[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 mak