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.

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, 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 molecules. 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 lamp on. Try this experiment:

EXPERIMENT 42. Turn on an incandescent lamp that is cold. Feel it with your hand a second, then turn it off at once. Is the glass hot? (The lamp you use should be an ordinary 25, 40, or 60 watt vacuum lamp.)

The radiant heat from the incandescent filament in the lamp passed right out through the vacuum of the lamp, and much of it went on through the glass to your hand. You already know what a poor conductor of heat glass is; yet it lets a great deal of radiant heat pass through it, just as it does light. As soon as the lamp stops glowing, the heat stops coming; the glass is not made hot and you no longer feel any heat. In one way the electric filament shining through a vacuum is exactly like the sun shining through empty space: the heat from both comes to us by radiation.

If a lamp glows for a long time, however, the glass really does become hot. That is partly because there is not a perfect vacuum within it (there is a little gas inside that carries the heat to the glass by convection), and it is partly because the glass does not let quite all of the radiant heat and light go through it, but absorbs some and changes it to the regular conducted heat.

One practical use that is made of a knowledge of the difference between radiant and conducted heat is in the manufacture of thermos bottles.

EXPERIMENT 43. Take a thermos bottle apart. Examine it carefully. If it is the standard thermos bottle, with the name "thermos" on it, you will find that it is made of two layers of glass with a vacuum between them. The vacuum keeps any conducted heat from getting out of the bottle or into it. But, as you know, radiant heat can flash right through a vacuum. So to keep it from doing this the glass is silvered, making a mirror out of it. Just as a mirror sends light back to where it comes from, it sends practically all radiant heat back to where it comes from. Heat, therefore, cannot get into the thermos bottle or out of it either by radiation or conduction. And that is why thermos bottles will keep things very hot or ice-cold for such a long time.

Fill the thermos bottle with boiling water, stopper it, and put it aside till the next day. See whether the water is still hot.

If we could make the vacuum perfect, and surround all parts of the bottle, even the mouth, with the perfect vacuum, and if the mirror were perfect, things put into a thermos bottle would stay boiling hot or icy cold forever and ever.

WHY IT IS COOL AT NIGHT AND COLD IN WINTER. It is the radiation of heat from the earth into space that makes the earth cooler at night and cold in winter. Much of the heat that the earth absorbs from the sun in the daytime radiates away at night. And since it keeps on radiating away until the sun brings us more heat the next day, it is colder just before dawn than at midnight, more heat having radiated into space.

For the same reason it is colder in January and February than in December. It is in December that the days are shortest and the sun shines on us at the greatest slant, so that we get the least heat from it; but we still have left some of the heat that was absorbed in the summer. And we keep losing this heat by radiation faster than we get heat from the sun, until almost spring.

APPLICATION 33. Distinguish between radiant and conducted heat in each of the following examples:

(a) The sun warms a room through the window. (b) A room is cooler with the shades down than up, when the sun shines on the window. (c) But even with the shades down a room on the sunny side of the house is warmer than a room on the shady side. (d) When a mirror is facing the sun, the back gets hot. (e) If you put your hand in front of a mirror held in the sun, the mirror reflects heat to your hand. (f) If you put a plate on a steam radiator, the top of the plate gradually becomes hot. (g) If anything very hot or cold touches a gold or amalgam filling of a sensitive tooth, you feel it decidedly. (h) The handle of your soup spoon becomes hot when the bowl of it is in the hot soup. (i) The moon is now very cold, although it probably was once very hot.

INFERENCE EXERCISE

Explain the following:

181. Trees bend in the wind, then straighten up again. Why do they straighten up?

182. A cloth saturated with kerosene and placed in the bottom of a clock will oil the clockworks above it.

183. In cold weather the doorknob inside the front door is cold.

184. It is cool in the shade.

185. Clothes get hot when you iron them.

186. Potatoes fried in deep fat cook more quickly than those boiled in water.

187. If you hold your hand near a vacuum electric lamp globe that is glowing, some of the heat will go out to your hand at once.

188. Rubbing silver with fine powder polishes it.

189. A mosquito can suck your blood.

190. A hot-water tank becomes hot at the top first, then gradually heats downward. When you light the gas under an ordinary hot-water heater, the hot water circulates to the top of the boiler, while the cold water from the boiler pushes into the bottom part of the heater, as shown in Figure 59. What causes this circulation?