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.
[Illustration: 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-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.
[Illustration: 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 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.