hing rare or thin like air, it is bent
one way; whenever it passes from a rare medium into a dense one, it is
bent the other way. Light passing from a fish to your eye is bent one
way; light passing from you to the fish's eye is bent the other way,
but the main point is that it is bent. And when light is bent before
reaching your eyes it usually makes things seem to be where they are
not.

If light goes through a perfectly smooth, flat pane of glass, it is
bent one way when it goes into the glass and back the other way when
it comes out; so it seems to be perfectly straight and we see things
practically as they are through a good window. But if the window glass
has flaws in it, so that some parts are a little thicker than others,
the uneven parts act like prisms and bend the light to one side.
This makes anything we look at through a poor window seem bent out of
shape. Of course the things are not bent any more than your pencil
in the water was bent, but they look misshapen because the light from
them is bent; the reflected light is all we see of things anyway.

[Illustration: FIG. 68. The light is bent when it enters a window pane
and is bent again in the opposite direction when it leaves it.]

The air itself is uneven in a way. The parts of the air that are warm,
as you already know, are thinner and more expanded than are the cold
parts. So light going from cold air into warm or from warm air into
cold, will be bent. And this is why you see what are called "heat
waves" above a stove or rising from a hot beach or sidewalk. Really
these are just waves of hot air rising, and they bend the light
that comes through them so as to give everything behind them a wavy
appearance.

Stars twinkle for much the same reason. As the starlight comes down
through the cold air and then through the warm air it is bent, and the
star seems to be to one side of where it really is; but the air does
not stand still,--sometimes it bends the light more and sometimes
less. So the star seems to move a little back and forth. And this is
what we call "twinkling." Really it is the bending of light.

_APPLICATION 35._ Explain why an unevenness in your eye will
keep you from seeing clearly; how glasses can help this; why
good mirrors are made from plate glass, which is very smooth,
instead of from the cheaper and more uneven window glass; why
fishes in a glass tank appear to be where they are not.

INFERENCE EXERCISE