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.

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-glass electric lamp, you cannot see the filament; the light passing through all the rough parts of the glass 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.

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 passes 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 glass 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.