How do glasses help your eyes?
On a hot day, how is it that you see "heat waves" rising from
the street?
What makes the stars twinkle?
Light usually travels in straight lines. If the light from an object
comes from straight in front of you, you know that the object is
straight in front of you. But you can bend light so that it seems to
come from a different place, thus making things seem to be where they
are not.
EXPERIMENT 44. Hold a triangular glass prism vertically
(straight up and down) in front of one eye, closing the other
eye. Look through the prism, turning it or your head around
until you see a chair through it. Watch only the chair through
the prism. When you are sure you know just where it is, try to
sit down in it.
Now look for a pencil or a piece of chalk through the prism,
in the same way. When you think you know where it is, try to
pick it up.
The reason the chalk and chair seem to be where they are not is that
the prism bends the light that comes from them and makes the light
seem to come from somewhere else.
As you already know, when you look at a chair you see the light that
reflects from it. You judge where the chair is by the direction from
which the light is coming when it reaches your eye. But if the light
is bent on its way, so that it comes to your eye as it ordinarily
comes from an object off to one side, naturally you think the thing
you are looking at is off to one side. Maybe the diagram (Fig. 65)
will make this clearer.
[Illustration: FIG. 65. In passing through the prism the light is bent
so that an object at _b_ appears to be at _c_.]
Here in _a_ is an object the same height as the eye. The light comes
straight to the eye, and one knows that the object is level with the
eye. In _b_ the object is in the same position as in _a_, but the
prism bends the light so that it strikes the eye with an upward slant.
So the person thinks the object is below the eye at _c_.
Here is another experiment with bending light:
EXPERIMENT 45. Fill a china cup with water. Put a pencil in
it, letting the pencil rest at a slant from left to right.
Lower your head until it is almost level with the surface of
the water. How does the pencil look?
[Illustration: FIG. 66. The pencil is not bent, but the light that
comes from it is.]
The reason the pencil looks bent is because the light from the part of
it under the water is bent when it passes from the water into the air
on its way to your eye; so the slant at which it comes to your eye is
the same slant at which it ordinarily would come from a bent pencil.
EXPERIMENT 46. Fill a glass with water. Put the pencil into
it in the same way you put it in the cup in the previous
experiment, letting the pencil slant from left to right. Lower
your head this time until it is on a level with the water in
the glass, and look through the glass and water at the pencil.
Notice what happens where the pencil goes into the water.
What you see is explained in the same way as are the things that took
place in the other experiments in refraction, or bending of light. The
light from the part of the pencil above the water comes straight to
your eye, of course; so you see it just as it is. But the light from
the part of the pencil in the water is bent when it comes out of the
water into the air on its way to your eye. This makes it come to your
eye from a different direction and makes the lower part of the pencil
seem to be in a place to one side of the place where it _really_ is.
The pencil, therefore, looks broken.
[Illustration: FIG. 67. The bending of the light by the water in the
glass causes the pencil to look broken.]
Whenever light passes first through something dense like water or
glass, and then through something 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
Explain the following:
201. The fire in the open fireplace ventilates a room well by
making air go up the chimney.
202. A drop of water glistens in the sun.
203. Dust goes up to the ceiling and clings there.
204. When you look at a person under moving water, his face
seems distorted.
205. You sit in the sun to dry your hair.
206. Paste becomes hard and unfit for use when left open to
the air.
207. In laundries clothes are partly dried by whirling them in
perforated cylinders.
208. Circus balloons are filled by building a big fire under
them.
209. Unevenness in a window pane makes telephone wires seen
through it look crooked and bent.
210. You can see the image of a star even in a shallow puddle.
[Illustration: FIG. 69. When the light from one point goes through
the lens, it is bent and comes together at another point called the
focus.]