Common Science Part 19

WHY SMOOTH OR WET THINGS ARE SHINY. When a surface is very smooth, we say it is shiny or glossy. Even black shoes, if they are polished, become smooth enough to reflect much of the light that strikes them; of course the parts where the light is being reflected do not look black but white, as any one who has tried to paint or draw a picture of polished shoes knows. Anything wet is likely to be shiny, because the surface of water is usually smooth enough to reflect light rather directly.

If a surface is uneven, like a pool with ripples on it, the light reflects unevenly, and you see a distorted image; your face seems to be rippling and moving in the water.

[Ill.u.s.tration: FIG. 64. How should the mirror be placed?]

_APPLICATION 34._ Some boys were playing war and were in a ditch that they called a trench. They wanted to make a simple periscope so that they could look out of the ditch at the "enemy" without being in danger. They had an old stovepipe and a mirror. Practically all of them agreed that if the mirror were fixed in the top of the stovepipe and if they looked up through the bottom, they would be able to see over the side of the ditch. But they had an argument as to how the mirror should be placed. Each drew a diagram to show how he thought the mirror should be arranged, using dotted lines to show how the light would come from the enemy to their eyes. Three of the diagrams are shown in Figure 64.

The boy who drew the first said: "If you want to see the enemy, the mirror's got to face him. Then it will reflect the light down to your eyes."

The boy who drew the second said: "No, the light would just go back to him again. The mirror must slant so that the light that strikes it at a slant will be reflected to your eye at the same slant."

"How could it get to your eye at all," the third boy said, "if the mirror didn't face you? You've got to have the mirror reflect right down toward your face. Then all the light that strikes it will come down to you."

Which arrangement would work?

INFERENCE EXERCISE

Explain the following:

191. Your hands do not get wet when you put them into mercury.

192. When beating hot candy, we sometimes put it in a pan of water.

193. Electric stoves frequently have bright reflectors.

194. We put ice in the _top_ of a refrigerator.

195. You can jack up the back part of an automobile when you could not possibly lift it up.

196. The sun shines up into your face and sunburns you when you are on the water.

197. People in the tropics dress largely in white.

198. Menthol rubbed into your skin makes it feel very cold afterward.

199. We feel the heat of the sun almost as soon as the sun rises.

200. You can shoot a stone far and hard with a sling shot.

SECTION 23. _The bending of light: Refraction._

How do gla.s.ses 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 gla.s.s 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.

[Ill.u.s.tration: FIG. 65. In pa.s.sing 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?

[Ill.u.s.tration: 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 pa.s.ses 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 gla.s.s 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 gla.s.s, and look through the gla.s.s 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.

[Ill.u.s.tration: FIG. 67. The bending of the light by the water in the gla.s.s causes the pencil to look broken.]

Whenever light pa.s.ses first through something dense like water or gla.s.s, and then through something rare or thin like air, it is bent one way; whenever it pa.s.ses from a rare medium into a dense one, it is bent the other way. Light pa.s.sing from a fish to your eye is bent one way; light pa.s.sing 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 gla.s.s, it is bent one way when it goes into the gla.s.s 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 gla.s.s 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.

[Ill.u.s.tration: 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 gla.s.ses can help this; why good mirrors are made from plate gla.s.s, which is very smooth, instead of from the cheaper and more uneven window gla.s.s; why fishes in a gla.s.s tank appear to be where they are not.

INFERENCE EXERCISE

Explain the following: