Common Science Part 22

SECTION 26. _Scattering of light: Diffusion._

Why is it that on a dark day the sun cannot be seen through light clouds?

Why do not the stars come out in the daytime?

If you were on the moon, you could see the stars in the daytime. The sun would be shining even more brightly than it does here, but the sky around the sun would be pitch black, except for the stars shining out of its blackness. The reason is that there is no air on the moon to scatter the light.

WHY WE CANNOT SEE THE STARS IN THE DAYTIME. Most of the sun's light that comes to the earth reaches us rather directly; that is why we can see the image of the sun. But part of the sunlight is scattered by particles of air, and that is why the whole sky is bright in the daytime. You know, of course, that the blue sky is only the air that surrounds the earth. Enough of the light is scattered around to make the sky as bright as the stars look from here; so we cannot see the stars through the sky in the daytime.

HOW A CLOUD CAN HIDE THE SUN WITHOUT CUTTING OFF ALL ITS LIGHT. When a cloud drifts between us and the sun, we no longer see the sun; yet the earth does not become dark. The sun's light is evidently still reaching us. The cloud is made of millions of very tiny droplets of water. When the sunlight strikes the curved sides of these droplets, it is reflected at all angles according to the way it strikes, as shown in Figure 89.

[Ill.u.s.tration: FIG. 88. The sunlight is scattered (diffused) by the clouds. The photograph shows in the foreground the Parliament Buildings, London, England.]

Some of the light is reflected back into the sky; that is why everything becomes darker when the sun goes behind a cloud; but much of the light comes through to us, at all sorts of slants. When it comes all higgledy-piggledy and crisscross like this, no lens can put it together again; it is as hopelessly broken up as Humpty-Dumpty was.

But much of the light gets here just the same; so we see it without seeing the form of the sun. Light that cannot be brought to a focus is called _scattered_ or _diffused light_.

When you look through a ground-gla.s.s electric lamp, you cannot see the filament; the light pa.s.sing through all the rough parts of the gla.s.s gets so scattered that you cannot bring it to a focus. Therefore, no image of the filament in the incandescent lamp can be formed on the retina of your eye.

[Ill.u.s.tration: FIG. 89. How the droplets in a cloud scatter the rays of light.]

A piece of white paper reflects practically all the light that strikes it. Yet you cannot see yourself in a piece of ordinary white paper.

The trouble is that the paper is too rough; there are too many little uneven places that reflect the light at all sorts of angles; the light is scattered and the lens in your eye cannot bring it to a focus.

_APPLICATION 38._ Explain why a scrim curtain will keep people from seeing into a room, but will not shut the light out; why curtains soften the light of a room; why indirect lighting (i.e. light thrown up against the ceiling and then reflected down into the room by the rough ceiling) is better for your eyes than is the old-time direct lighting.

INFERENCE EXERCISE

Explain the following:

231. The alcohol formed by the yeast in making bread light is practically all gone by the time the bread is baked.

232. The oceans do not flow off the earth at the south pole.

233. Lamp globes often have frosted bottoms.

234. A damp dust cloth will take up the dust, without making it fly.

235. The stars twinkle when their light pa.s.ses through the moving air currents that surround the earth.

236. Shears for cutting tin and metal have long handles and short blades.

237. A coin at the bottom of a gla.s.s of water seems raised when you look at it a little from one side.

238. You have to brace your feet to row well.

239. Light from the northern part of the sky, where the sun is not, does not make sharp shadows.

240. Pokers and lifters for stove lids often have open spiral handles.

SECTION 27. _Color._

What makes the ocean look green in some places and blue in others?

What makes the sky blue?

What causes material to be colored?

What makes a rainbow?

What is color?

[Ill.u.s.tration: FIG. 90. Making a rainbow on the wall.]

Color is merely a kind of light. We say that a sweater is red; really the sweater is not red, but the light that it reflects to our eyes is red. We speak of a piece of red gla.s.s, but the gla.s.s is not red; it is the light that it lets pa.s.s through it that is red.

White is not really a color; _all_ colors put together make white.

Experiments 50 and 51 will prove this.

[Ill.u.s.tration: FIG. 91. The prism separates the white light into the rainbow colors.]

EXPERIMENT 50. Hold a prism in the sunlight by the window and make a "rainbow" on the wall. The diagram here shown ill.u.s.trates how the prism breaks up the single beam of white light into different-colored beams of light.

[Ill.u.s.tration: FIG. 92. When the wheel is rapidly whirled the colors blend to make white.]

EXPERIMENT 51. Rotate the color disk on the rotator and watch it. Make it go faster and faster until all the colors are perfectly merged. What color do you get by combining all the colors of the rainbow? If the colors on the disk were perfectly clear rainbow colors, in exactly the same proportion as in the rainbow, the whirling would give a white of dazzling purity.

Since you can break up pure white light into all the colors, and since you can combine all the colors and get pure white light, it is clear that white light is made up of all the colors.

As we have already said, light is probably vibrations or waves of ether. Light made of the longest waves that we can see is red. If the waves are a little shorter, the light is orange; if they are shorter yet, it is yellow; still shorter, green; shorter still, blue; while the shortest waves that we can see are those of violet light. Black is not a color at all; it is the absence of light. We say the night is black when we cannot see anything. A deep hole looks black because practically no light is reflected up from its depths. When you "see"

anything black, you really see the things around it and the parts of it that are not perfectly black. A pair of shoes, for instance, has particles of gray dust on them; or if they are very shiny they reflect part of the light that strikes them as a white high-light. But the really black part of your shoes would be invisible against an equally black background.

A black thing absorbs the light that strikes it and turns it to heat.

Here is an experiment that will prove this to you:

EXPERIMENT 52. (a) On a sunny day, take three bottles, all of the same size and shape, and pour water out of a pitcher or pan into each bottle. Do not run the water directly from the faucet into the bottle, because sometimes that which comes out of the faucet first is warmer or colder than that which follows; in the pitcher or pan it will all be mixed together, and so you can be sure that the water in all three bottles is of the same temperature to begin with. Wrap a piece of white cotton cloth twice around one bottle; a piece of red or green cotton cloth of the same weight twice around the second bottle, and a piece of black cotton cloth of the same weight twice around the third bottle, fastening each with a rubber band. Set all three bottles side by side in the sunlight, with 2 or 3 inches of s.p.a.ce between them. Leave them for about an hour. Now put a thermometer into each to see which is warmest and which is least warm.

From which bottle has most of the light been reflected back into the air by the cloth around it? Which cloth absorbed most of the light and changed it into heat? Does the colored cloth absorb more or less light than the white one? than the black one?

[Ill.u.s.tration: FIG. 93. Which color is warmest in the sunlight?]

(b) On a sunny day when there is snow on the ground, spread three pieces of cotton cloth, all of the same size and thickness, one white, one red or green, and one black, on top of the snow, where the sun shines on them. Watch them for a time. Under which does the snow melt first?

The white cloth is white because it reflects _all_ colors back at once. It therefore absorbs practically no light. But the reason the black cloth looks black is that it reflects almost none of the colors--it absorbs them all and changes them to heat. The colored cloth reflects just the red or the green light and absorbs the rest.