If we were to go back to our imaginary switchboard we should find a switch, between the heat and the light
switches, labeled RADIATION. Suppose we turn it off:
Instantly the whole world becomes pitch dark; so does the sky. We cannot see the sun or a star; no electric
lights shine; and although we can “light” a match, it gives no light. The air above the burning match is hot,
and we can burn our fingers in the invisible flame, but we can see nothing whatever.
Yet the world does not get cold. If we leave the switch off for years, while the earth remains in darkness and
we all live like blind people, it never gets cold. Winter and summer are alike, day and night are just the same.
Gradually, after many ages, the ice and snow in the north and in the far south begin to melt as the warmth
from the rest of the world is conducted to the polar regions.
And the heat from the interior of the earth makes
all the parts of the earth’s surface warmer.
Winds almost stop blowing. Ocean currents stop flowing. The land
receives less rainfall, until finally everything turns to a desert; almost the only rain is on the ocean. Animals
die even before the rivers dry up, for the flesh eaters are not able to see their prey, and since, without light, all
green things die, the animals that live on plants soon starve. Men have to learn to live on mushrooms, which
grow in the dark.
The world is plunged into an eternal warm, pitch-black night.
Turning off the radiation would cause all these things to happen, because it is by radiation that we get all our
heat from the sun and all our light from any source. And it is by radiation that the earth loses heat into space in
the night and loses still more heat into space during the winter.
We do not get our heat from the sun by conduction; we cannot, because there is nothing between us and the
sun to conduct it.
The earth’s air
The earth’s air, in amounts thick enough to count, goes up only a hundred miles or so. It is
really just a thin sort of blanket surrounding the earth. The sun is 93,000,000 miles away. Between us and the
sun there is empty space. There are no molecules to speak of in that whole vast distance. So if heat traveled
only by conduction,–that is, if radiation stopped,–we should be so completely shut off from the sun that we
should not know there was such a thing.
But even if we filled the space between us and the sun with copper or silver, which are about the best
conductors of heat in the world, it would take the heat from the sun years and years to be conducted down to
us. Yet we know that the sun’s heat really gets to us in a few minutes. This is because heat can travel in a very
much quicker way than by conduction.
It radiates through space, just as light does. And it can come the whole
93,000,000 miles from the sun in about 8 minutes. This is so fast that if it were going around the world instead
of coming from the sun, it would go around 7-1/2 times before you could say “Jack Robinson,”–really,
because it takes you at least one second to say “Jack Robinson.”
We are not absolutely sure how heat gets here so fast. But what most scientists think nowadays is that there is
a sort of invisible rigid stuff, not made of molecules or of anything but just itself, called ether. (This ether, if
there really is such a thing, is not related at all to the ether that doctors use in putting people to sleep. It just
happens to have the same name.) The ether is supposed to fill all space, even the tiny spaces between
Fast moving particles
The fast moving particles of the sun joggle the ether up there, and make ripples that spread out
swiftly all through space. When those ripples strike our earth, they make the molecules of earth joggle, and
that is heat. The ripples that spread out from the sun are called ether waves.
But the important and practical fact to know is that there is a kind of heat, called radiant heat, that can pass
through empty space with lightning-like quickness. And when this radiant heat strikes things, it is partly
absorbed and changed to the usual kind of heat.
This radiant heat is closely related to light. As a matter of fact, light is only the special kind of ether waves
that affect our eyes. Radiant heat is invisible. The ether waves that are visible we call light. In terms of ether
waves, the only difference between light and radiant heat is that the ripples in light are shorter. So it is no
wonder that when we get a piece of iron hot enough, it begins to give off light; and we say it is red hot. What
happens to the ether is this: As the molecules of iron go faster and faster (that is, as the iron gets hotter and
hotter), they make the ripples in the ether move more frequently until they get short enough to be light instead
of radiant heat. Objects give off radiant heat without showing it at all; the warmth that you feel just below a
hot flatiron is mainly radiant heat.
When anything becomes hot enough to glow, we say it is incandescent. That is why electric lamps are called
incandescent lamps. The fine wires–called the filament–in the lamp get so hot when the electricity flows
through them that they glow or become incandescent, throwing off light and radiant heat.
It is the absorbing of the radiant heat by your hand that makes you feel the heat the instant you turn an electric