  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. 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. 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 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 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:
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.
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 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.
Inference Exercise
Section 22. Reflection.
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 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 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. Fig. 62. 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 Fig. 63. 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 glass is very smooth, and you can see yourself in it, especially if there is not much light coming through the glass from the other side to mix up with your reflection. But if the pane of glass is silvered so that Why smooth or wet things are shiny. When a surface is very smooth, we say it is shiny or glossy. Even black shoes, if they are polished, become smooth enough to reflect much of the light that strikes them; of course the parts where the light is being reflected do not look black but white, as any one who has tried to paint or draw a picture of polished shoes knows. Anything wet is likely to be shiny, because the surface of water is usually smooth enough to reflect light rather directly. If a surface is uneven, like a pool with ripples on it, the light reflects unevenly, and you see a distorted image; your face seems to be rippling and moving in the water. Fig. 64. Fig. 64. How should the mirror be placed?
Inference Exercise
Section 23. The bending of light: Refraction.
Light usually travels in straight lines. If the light from an object comes from straight in front of you, you know that the object is straight in front of you. But you can bend light so that it seems to come from a different place, thus making things seem to be where they are not.
The reason the chalk and chair seem to be where they are not is that the prism bends the light that comes from them and makes the light seem to come from somewhere else. As you already know, when you look at a chair you see the light that reflects from it. You judge where Here in a is an object the same height as the eye. The light comes straight to the eye, and one knows that the object is level with the eye. In b the object is in the same position as in a, but the prism bends the light so that it strikes the eye with an upward slant. So the person thinks the object is below the eye at c. Here is another experiment with bending light:
Fig. 66. Fig. 66. The pencil is not bent, but the light that comes from it is.The reason the pencil looks bent is because the light from the part of it under the water is bent when it passes from the water into the air on its way to your eye; so the slant at which it comes to your eye is the same slant at which it ordinarily would come from a bent pencil.
What you see is explained in the same way as are the things that took place in the other experiments in refraction, or bending of light. The light from the part Fig. 67. Fig. 67. The bending of the light by the water in the glass causes the pencil to look broken.Whenever light passes first through something dense like water or glass, and then through something rare or thin like air, it is bent one way; whenever it passes from a rare medium into a dense one, it is bent the other way. Light passing from a fish to your eye is bent one way; light passing from you to the fish's eye is bent the other way, but the main point is that it is Fig. 68. Fig. 68. The light is bent when it enters a window pane and is bent again in the opposite direction when it leaves it.If light goes through a perfectly smooth, flat pane of glass, it is bent one way when it goes into the glass and back the other way when it comes out; so it seems to be perfectly straight and we see things practically as they are through a good window. But if the window glass has flaws in it, so that some parts are a little thicker than others, the uneven parts act like prisms and bend the light to one side. This makes anything we look at through a poor window seem bent out of shape. Of course the things are not bent any more than your pencil in the water was bent, but they look misshapen because the light from them is bent; the reflected light is all we see of things anyway. The air itself is uneven in a way. The parts of the air that are warm, as you already know, are thinner and more expanded than are the cold parts. So light going from cold air into warm or from warm air into Stars twinkle for much the same reason. As the starlight comes down through the cold air and then through the warm air it is bent, and the star seems to be to one side of where it really is; but the air does not stand still,—sometimes it bends the light more and sometimes less. So the star seems to move a little back and forth. And this is what we call "twinkling." Really it is the bending of light.
Inference Exercise
Fig. 69. Fig. 69. When the light from one point goes through the lens, it is bent and comes together at another point called the focus.Section 24. Focus.
In your eye, right back of the pupil, there is a flattened ball, as clear as glass, called the lens. If the lens were left out of your eye, you never could see anything except blurs of light and shadow. If you looked at the sun it would dazzle you practically as much as it does now. However, you would not see a round sun, but only a blaze of light. You could tell night from day as well as any one, and you could tell when you stepped into the shade. If some one stepped between you and the light, you would know that some one was between you and the light or that a cloud had passed over the sun,—you could not be quite sure which. In short, you could tell all degrees of light and dark apart nearly as well as you can now, but you could not see the form of anything. Fig. 70. Fig. 70. The light from each point of the candle flame goes out in all directions.In the front of a camera there is a flattened glass ball called the lens. If you were to remove it, the camera would not take any pictures; it would take a blur of light and shade and nothing more. In front of a moving-picture machine there is a large lens, a piece of glass rounded out toward the middle and thinner toward the edges. If you were to take that lens off while the machine was throwing the motion pictures on the screen, you would have a flicker of light and shade, but no picture. It is the lens that forms the pictures in your eye, on a photographic plate or film, and on a moving-picture screen. And a lens is usually just a piece of glass or something glassy, rounded out in such a way as to make all the spreading light that reaches it from one point come together in another point, as shown in Figure 69. As you know, when light goes out from anything, as from a candle flame or an incandescent lamp, or from the sun, it goes in all directions. If the light from the point of a candle flame goes in all directions, and if the light from the base of the flame also goes in all directions, Fig. 71. Fig. 71. The reading glass is a lens which focuses the light from the candle flame and forms an image.
The lens has brought the light from the candle flame to a focus; all the light that goes through the lens from one point of the flame has been brought together at Fig. 72. Fig. 72. The light from the tip of the candle flame is focused at one point.Fig. 73. Fig. 73. And the light from the base of the flame is focused at another point.Fig. 74. Fig. 74. The light from the tip and base (and from every other point) of the flame is, of course, focused at the same time. In this way an image of the flame is formed.But of course the light from the base of the flame is focused at the same time as the light from the point; so what really happens is that which is illustrated in Figure 74. In this diagram, we have drawn unbroken lines to show the light from the point of the candle flame and dotted lines to show the light from the base of the flame. This is so that you can follow the light from each part and see where it goes. Compare this diagram with the one where the light is shown all crisscrossed (Fig. 70), and you will see why the lens makes an image, while you have no image without it. By looking at the last diagram (Fig. 74) you can also see how the image happens to be upside down.
The explanation is this: After the light comes together into a point, it spreads out again beyond the point, as shown in Figure 75. So if you hold the lens in such a way that the light comes to a focus before it reaches the paper, the paper will catch the spreading light and you will get a blur instead of a sharp image. It is as shown in Figure 76. Fig. 75. Fig. 75. The light spreads out again beyond the focus.Fig. 76. Fig. 76. So if the light comes to a focus before it reaches the paper, the image will be blurred.On the other hand, if you hold your lens in such a way that the light has not yet come to a focus when it reaches Fig. 77. Fig. 77. Or if the light reaches the paper before it comes to a focus, the image will be blurred.And that is why good cameras have the front part, in which the lens is set, adjustable; you can move the lens back and forth until a sharp image is formed on the plate. Motion-picture machines and stereopticons likewise have lenses that can be moved forward and back until they form a sharp focus on the screen. Even the lens in your eye has muscles that make it flatter and rounder, so that it can make a clear image on the sensitive retina in the back of your eye. The lens in the eyes of elderly people often becomes too hard to be regulated in this way, and so they have to wear one kind of glasses to see things near them clearly and another kind to see things far away. The kind of lens we have been talking about is the convex lens. "Convex" means bulging out in the middle. There are other kinds of lenses, some flat on one side and bulging out on the other, some hollowed Fig. 78. Fig. 78. Lenses of different kinds.Why you can set fire to paper with a magnifying glass. A convex lens brings light to a focus, and it also brings radiant heat to a focus. And that is why you can set fire to things by holding a convex lens in the sunlight so that the light and heat are focused on something that will burn. All the sun's radiant heat that strikes the lens is brought practically to one point, and all the light which is absorbed at this point is changed to heat. When so much heat is concentrated at one point, that point becomes hot enough to catch fire.
Inference Exercise
Section 25. Magnification.
Everybody knows, of course, that a convex lens in the right position makes things look larger. People use convex lenses to make print look larger when they read, and for that reason such lenses are often called reading glasses. For practical purposes it is not necessary to understand how a convex lens magnifies; the important thing is the fact that it does magnify. But you may be curious to know just how a magnifying glass works. First, you should realize that the image formed by a convex lens is not always larger than the object. Repeat Fig. 79. Fig. 79. A section of the eye.The important point in this experiment is for you to see that if the lens is nearer to the image on the paper than it is to the candle, the image is smaller than the candle. That is why a photograph is usually smaller than the thing photographed; it would be impossible to take a picture of a house or a mountain if the lens in the camera gave a magnified image.
Fig. 80. Fig. 80. How an image is formed on the retina of the eye.Fig. 81. Fig. 81. A simpler diagram showing how an image is formed in the eye.Fig. 82. Fig. 82. A diagram showing how a reading glass causes things to look larger by making the image on the retina larger.Fig. 83. Fig. 83. Diagram showing how a reading glass enlarges the image on the retina. More lines are drawn in than in Figure 82.You can understand magnification best by looking at Figures 80, 81, 82, and 83. In Figure 80 there are a candle flame, the lens of an eye, and the retina on which the image is being formed. Figure 81 is the same as Figure 80, with all the lines left out except the outside ones that go to the lens. It is shown in this way merely for the sake of simplicity. All the lines really belong in this diagram as in the first. In both diagrams the size of the image on the retina is the distance between the point where the top line touches it and the point where the bottom line touches it. In order to make anything look larger, we must make the image on the retina larger. A magnifying glass, or convex lens, if put in the right place, will do this. In the next diagram, Figure 82, we shall include the magnifying glass, leaving out all lines except the two outside ones shown in Figure 81. You will notice that the magnifying glass starts to Fig. 84. Fig. 84. Diagram of a microscope.The two important points to notice are these: First, the magnifying glass is too close to the eye for the light to be brought to a focus before it reaches the eye; the light is bent toward a focus, but it reaches the eye before the focus is formed. The focus is formed for the first time on the retina itself. Second, the magnifying glass bends the light on its way to your eye so that the light crosses sooner in your eye and spreads out farther before it comes to a focus. This forms the larger image, as you see in the simple diagram, Figure 82. Fig. 85. Fig. 85. This is the way a concave mirror forms a magnified image.Fig. 86. Fig. 86. The concave mirror forms an image of the burning candle.How the microscope works. But the microscope is different. It works like this: The first lens is put very near the object which you are examining. This lens brings the light from the object to a focus and forms an image, much larger than the object itself, high up in the tube. If you held a piece of paper there you would see the image. But since there is nothing there to stop the light, it goes on up the tube, spreading as it goes. Then there is another lens which catches this light and bends it inward on its way to your eye, just as any magnifying glass does. Next the lens in the eye forms an image on the retina. The diagram (Fig. 84) will make this clearer. (A real microscope is not so simple, of course, and usually has two lenses wherever the diagram shows one.) What actually happens is that the Fig. 87. Fig. 87. The great telescope of the Yerkes Observatory at Lake Geneva, Wisconsin.A mirror that magnifies. A convex lens is not the only thing that can magnify. A concave mirror, which is one that is hollowed out toward the middle, does the same thing. When light is reflected by such a mirror, it acts exactly as if it had gone through a convex lens (Fig. 85).
How telescopes are made. Astronomers use convex lenses in some of their telescopes; in others, called reflecting telescopes, they use concave mirrors. Both do the same work, making the moon, the planets, and the sun look much larger than they otherwise would.
Inference Exercise
Section 26. Scattering of light: Diffusion.
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 Fig. 88. 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 Fig. 89. 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
Inference Exercise
Section 27. Color.
Fig. 90. Fig. 90. Making a rainbow on the wall.Color is merely a kind of light. We say that a sweater is red; really the sweater is not red, but the light that it reflects to our eyes is red. We speak of a piece of red glass, but the glass is not red; it is the light that it lets pass through it that is red. White is not really a color; all colors put together make white. Experiments 50 and 51 will prove this. Fig. 91. Fig. 91. The prism separates the white light into the rainbow colors.
Since you can break up pure white light into all the colors, and since you can combine all the colors and get pure white light, it is clear that white light is made up of all the colors. As we have already said, light is probably vibrations or waves of ether. Light made of the longest waves A black thing absorbs the light that strikes it and turns it to heat. Here is an experiment that will prove this to you:
Maybe you will understand color better if it is explained in another way. Suppose I throw balls of all colors to you, having trained you to keep all the balls except the red ones. I throw you a blue ball; you keep it. I throw a red ball; you throw it back. I throw a green ball; you keep it. I throw a yellow ball; you keep it. I throw two balls at once, yellow and red; you keep the yellow and throw back the red. I throw a blue and yellow ball at the same time; you keep both balls. Now suppose I change this a little. Instead of throwing balls, I shall throw lights to you. You are trained always to throw red light back to me and always to keep (absorb) all other kinds of light. I throw a blue light; you keep it, and I get no light back. I throw a red light; you throw it back to me. I throw a green light; you keep it, and I get no light back. I throw a yellow light; you keep it, and I get no light back. I throw two lights at the same time, yellow and red; you keep the yellow and throw back only the red. But yellow and red together make orange; so when I throw an orange light, you throw back the red part of it and keep the yellow. Now if we suppose that instead of throwing lights to you I throw them to molecules of dye which are "trained" So if you throw a white light, which contains all the colors, on a "red" sweater, the dye in the sweater picks out the red part of the white light and throws that back to your eyes (reflects it to you) but it keeps the rest of the colors of the white light, changing them to heat; and since only the red part of the light is reflected to your eyes, that is the only part of it that you can see; so the sweater looks red. The "green" substance (chlorophyll) in grass acts in the same way; only it throws the green part of the sunlight back to your eyes, keeping the rest; so the part of the light that reaches you from the grass is the green light, and the grass looks green. Anything white, like a piece of paper, reflects all the light that strikes it; so if all the colors (white light) strike it, all are reflected to your eyes and the object looks white. You have looked at people under the mercury-vapor lights in photo-postal studios, have you not? The Fig. 94. Fig. 94. A mercury-vapor lamp.Go into the kitchen tonight, light a burner of the gas stove, turn out the light and sprinkle salt on the blue gas flame. The flame will leap up, yellow. Look at your hands, at some one's lips, at a piece of red cloth, in this light. Does anything look red? The reason why nothing looks pink or red in these two kinds of light is this: The light given by glowing salt vapor or mercury vapor has no red in it; if you tried to make a "rainbow" from it with a prism, you would find no red or orange color in it. A thing looks red when it absorbs all the parts of the light that are not red and reflects the red light to your eyes. If there When you look through a piece of colored glass, the case is somewhat different. A piece of blue glass, for instance, acts as a sort of strainer. The coloring matter in it lets the blue light through it, but it holds back (absorbs) the other kinds of light. So if you look through a piece of blue glass you see everything blue; that is, only the blue part of the light from different objects can reach your eyes through this kind of glass. Anything that is transparent and colored acts in a similar way. Why the sky is blue. And that is why the sky looks blue. Air holds back all colors of light except blue; that is, it holds them back a little. A room full of air holds the colors back hardly at all. A few miles of air hold them back more; mountains in the distance look bluish because only the blue light from them can reach you through the air. The hundred or more miles of air above you hold back a considerable amount of the other colors of light, letting through much more of blue than of any other color. So the sky looks blue; that is, when the air scatters the sunlight above you, it is chiefly the blue parts of the sunlight that it allows to reach your eyes. Why bodies of water look green in some places and blue in others. Water acts in a similar way, but it lets the green light through instead of the blue. A little water holds back (absorbs) the other colors so slightly that you cannot notice the effect in a glass of water. But in a swimming tank full of water, or in a When you look at a smooth body of water at a slant on a clear day, the blue sky is reflected to you and the water looks blue instead of green. And it may even look blue when you look straight down in it if it is too deep for you to see the bottom and the sky is reflected from the surface. Why the sky is often red at sunset. Dust lets more of red and yellow light through than of any other color, and for this reason there is much red and yellow in the sunset. Just before the sun sets, it shines through the low, dusty air. The dust filters out most of the light except the red and yellow. The red light and yellow light are reflected by the clouds (for the clouds are themselves "white"; that is, they reflect all the colors that strike them), and you have the beautiful sunset clouds. Sometimes there is a purple in the sunset, and even green. But since the air itself is blue (that is, it lets mostly blue light go through), it is easy to see how this blue can combine with the red or yellow that the dust lets through, to form purple or green. But we could not have sunset colors or all the colors we see on earth, if it were not that the sunlight is mostly white—that it contains all colors. And that, too, is why we can have a rainbow. How rainbows are formed. You already know fairly well how a rainbow is formed, since you made an imitation of one with a prism. A rainbow appears in the sky when the sun shines through the rain; the plain white light of the sun is divided up into red, orange, yellow, How we can tell what the sun and stars are made of. When a gas or vapor becomes hot enough to give off light (when it is incandescent), it does not give off white light but light of different colors. An experiment will let you see this for yourself.
Copper vapor always gives this greenish-blue light when it is heated. The photographer's mercury-vapor light gave a greenish-violet glow. When you burn salt or soda in a gas flame, you remember that you get a clear yellow light. By breaking up these lights, somewhat as you broke up the sunlight with the prism, chemists and astronomers can tell what kind of gas is glowing. The instrument they use to break up the light into its different colors is called a spectroscope, and the band of colors formed is called the spectrum. With the spectroscope they examine the light that comes from the sun and stars and by the colors of the spectra they can tell what these far-distant bodies are made of.
Inference Exercise
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