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 gla.s.s 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 pa.s.sed right out through the vacuum of the lamp, and much of it went on through the gla.s.s to your hand. You already know what a poor conductor of heat gla.s.s is; yet it lets a great deal of radiant heat pa.s.s through it, just as it does light. As soon as the lamp stops glowing, the heat stops coming; the gla.s.s 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 s.p.a.ce: the heat from both comes to us by radiation.
If a lamp glows for a long time, however, the gla.s.s 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 gla.s.s by convection), and it is partly because the gla.s.s 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 gla.s.s 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 gla.s.s 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.
[Ill.u.s.tration: FIG. 61. How a thermos bottle is made. Notice the double layer of gla.s.s in the broken one.]
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 s.p.a.ce 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 s.p.a.ce.
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 doork.n.o.b _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?
SECTION 22. _Reflection._
How is it that you can see yourself in a mirror?
What makes a ring around the moon?
Why can we see clouds and not the air?
Why is a pair of new shoes or anything smooth usually shiny?
If we turn off a switch labeled REFLECTION OF LIGHT on our imaginary switchboard, we think at first that we have accidentally turned off RADIATION again, for once more everything instantly becomes dark around us. We cannot see our hands in front of our faces. Although it is the middle of the day, the sky is jet black. But this time we see bright stars shining in it. And among them is the sun, shining as brightly as ever and dazzling our eyes when we look at it. But its light does no good. When we look down from the sky toward the earth, everything is so black that we should think we were blind if we had not just seen the stars and sun.
Groping our way along to an electric lamp, we turn it on. It shines brightly, but it does not make anything around it light; everything stays absolutely invisible. It is as if all things in the world except the lights had put on some sort of magic invisible caps.
We can strike a match and see its flame. We can see a fire on the hearth. We may feel around for the invisible poker, and when we find it, we may put it in the fire. When it becomes hot enough, it will glow red and become visible. We can make a match head glow by rubbing it on a wet finger. We can even see a firefly, if one comes around.
But only those things which are glowing of themselves, like flames, and red-hot pokers, and fireflies, will be visible.
The reason why practically everything would be invisible if there were no reflection of light is this: When you look at anything, as a man, for instance, what you really see is the light that hits him and bounces back (reflects) into your eyes. Suppose you go into a dark room and turn on an electric light. Instantly ripples of light flash out from the lamp in every direction. As soon as they strike the object you are looking at, they reflect (bounce back) from it to your eyes. When light strikes your eyes, you see.
Of course, when you look at an electric lamp, or a star, or the sun, or anything that is incandescent (so hot that it shines by its own light), you can see it, whether reflection exists or not. But most things you look at do not shine by their own light. This book that you are reading simply reflects the light in the room to your eyes; it would not give any light in a dark room. The paper reflects a good deal of light that strikes it, so it looks very light; the print reflects practically none of the light that strikes it, so it looks dark, or black, just as a keyhole looks black because it does not reflect any light to your eyes. But without reflection, the book would be entirely invisible. The only kind of print you could read if there were no reflection would be the electric signs made out of incandescent lamps arranged to form letters.
WHAT THE RING AROUND THE MOON IS; WHAT SUNBEAMS ARE. The reason you sometimes see a ring around the moon is that some of the moonlight reflects from tiny droplets of water in the air, making them visible.
In the same way, the dust in the air of a room becomes visible when the sun shines through it and is reflected by each speck of dust; we call it a _sunbeam_. But we are not really looking directly at the sunlight; we are seeing the part of the sunlight that is reflected by the dust specks.
Have you ever noticed that when you stand a little to one side of a mirror where you cannot see your own image in it, you can sometimes see that of another person clearly, while he cannot see his own image but can see yours? It is easy to understand this by comparing the reflection of the light from your face to his eye and from his face to your eye, to the bouncing of a ball from one person to another.
Suppose you and a friend are standing a little way apart on sandy ground where you cannot bounce a ball, but that between you there is a plank. If each of you is standing well away from the plank, neither one of you can possibly bounce the ball on it in such a way that he can catch it himself. Yet you can easily bounce it to your friend and he can bounce it to you.
[Ill.u.s.tration: FIG. 62. The ball bounces from one boy to the other, but it does not return to the one who threw it.]
The mirror is like that plank; it is something that will reflect (bounce) the light directly. The light from your face goes into the mirror, just as you may throw the ball against the plank, and the light is reflected to your friend just as the ball is bounced to him; so he sees your image in the mirror. If he can see you, you can see him, just as when you bounce the ball to him he can bounce it to you.
But you may be unable to see yourself, just as you may be unable to bounce the ball on the plank so that you yourself can catch it.
In other words, when light strikes against something it bounces away, just as a rubber ball bounces from a smooth surface. If you throw a ball straight down, it comes straight up; if light shines straight down on a flat, smooth surface, it reflects straight up. If you throw a ball down at a slant, it bounces up at the same slant in the opposite direction; if light strikes a smooth surface at a slant, it reflects at the same slant in the opposite direction.
[Ill.u.s.tration: FIG. 63. In the same way, the light bounces (reflects) from one boy to the other. It does not return to the point from which it started and neither boy can see himself.]
But to reflect light directly and to give a clear image, the surface the light strikes _must_ be extremely smooth, just as a tennis court must be fairly smooth to make a tennis ball rebound accurately.
Any surface that is smooth enough will act like a mirror, although naturally, if it lets most of the light go through, it will not reflect as well as if it sends all the light back. A pane of gla.s.s is very smooth, and you can see yourself in it, especially if there is not much light coming through the gla.s.s from the other side to mix up with your reflection. But if the pane of gla.s.s is silvered so that no light can get through, you have a real mirror; most of the light that leaves your face is reflected to your eyes again.