CHAPTER FIVE
RADIANT HEAT AND LIGHT
Section 21. How heat gets here from the sun; why things glow when they become very hot.
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.
Fig. 60. It is by radiation that we get all our heat and light from the sun.
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.