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:
Experiment 45. Fill a china cup with water. Put a pencil in it, letting the pencil rest at a slant from left to right. Lower your head until it is almost level with the surface of the water. How does the pencil look?
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.
Experiment 46. Fill a glass with water. Put the pencil into it in the same way you put it in the cup in the previous experiment, letting the pencil slant from left to right. Lower your head this time until it is on a level with the water in the glass, and look through the glass and water at the pencil. Notice what happens where the pencil goes into the water.
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 of the pencil above the water comes straight to your eye, of course; so you see it just as it is. But the light from the part of the pencil in the water is bent when it comes out of the water into the air on its way to your eye. This makes it come to your eye from a different direction and makes the lower part of the pencil seem to be in a place to one side of the place where it really is. The pencil, therefore, looks 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 bent. And when light is bent before reaching your eyes it usually makes things seem to be where they are not.
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 cold, will be bent. And this is why you see what are called "heat waves" above a stove or rising from a hot beach or sidewalk. Really these are just waves of hot air rising, and they bend the light that comes through them so as to give everything behind them a wavy appearance.
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.
Application 35. Explain why an unevenness in your eye will keep you from seeing clearly; how glasses can help this; why good mirrors are made from plate glass, which is very smooth, instead of from the cheaper and more uneven window glass; why fishes in a glass tank appear to be where they are not.
Inference Exercise
Explain the following:
201. The fire in the open fireplace ventilates a room well by making air go up the chimney.
202. A drop of water glistens in the sun.
203. Dust goes up to the ceiling and clings there.
204. When you look at a person under moving water, his face seems distorted.
205. You sit in the sun to dry your hair.
206. Paste becomes hard and unfit for use when left open to the air.
207. In laundries clothes are partly dried by whirling them in perforated cylinders.
208. Circus balloons are filled by building a big fire under them.
209. Unevenness in a window pane makes telephone wires seen through it look crooked and bent.
210. You can see the image of a star even in a shallow puddle.
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.
How can you take pictures with a camera?
What causes the picture in the camera to be inverted?
Why is a magnifying glass able to set things on fire when you let the sun shine through it?
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.
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, the light from the point will get all mixed up with the light from the base, as shown in Figure 70. Naturally, if the light from the point of the candle flame is mixed up with the light from the base and the beams are all crisscross, you will not get a clear picture of the flame.
Fig. 71. The reading glass is a lens which focuses the light from the candle flame and forms an image.
Experiment 47. Fasten a piece of paper against a wall and place a lighted candle about 4 feet in front of it. Look at the paper. Is there any picture of the candle flame on it? Now hold a magnifying glass (reading glass) near the candle, between the candle and the paper, so that the light will shine through the lens on to the paper. (The magnifying glass is a lens.) Move the lens slowly toward the paper until you get a clear picture of the candle flame. Is it right side up or upside down?
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 another point (Fig. 72). In the diagram you see all the light from the point of the candle flame spreading out in every direction. But the part that goes through the lens is brought together at one point, called the focus. Of course the same thing happens to the light from the base of the candle flame (Fig. 73). Just as before, all the light from the base of the flame is brought to a focus. The light spreads out until it reaches the lens. Then the lens bends it together again until it comes to a point.
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.
Experiment 48. Set up the candle and piece of paper as you did for the last experiment, but move the magnifying glass back and forth between the paper and the candle. Notice that there is one place where the image of the candle is very clear. Does the image become clearer or less clear if you move the lens closer to the candle? if you move it farther from the candle?
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.
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 the paper, naturally you again have a blur of light instead of a point, and the image is not sharp and definite (Fig. 77).
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 out toward the middle instead of bulging, and so on. But the only lens that most people make much use of (except opticians) is the convex lens that bulges out toward the center. The convex lens makes a clear image and it is the only kind of lens that will do this.
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.
Application 36. Explain why there is a lens in a moving-picture machine; why a convex lens will burn your hand if you hold it between your hand and the sun; why the front of a good camera is made so that it can be moved closer to the plate or farther away from it, according to the distance of the object you are photographing; why there is a lens in your eye.
Inference Exercise
Explain the following:
211. Cut glass ware sparkles.
212. An unpainted floor becomes much dirtier and is harder to clean than a painted one.
213. If you sprinkle wet tea leaves on a rug before sweeping it, not so much dust will be raised.
214. Food leaves a spoon when the spoon is struck sharply upon the edge of a stewpan.
215. An image is formed on the photographic plate of a camera.
216. Ripples in a pool distort the image seen in it.
217. Cream rises to the top of a bottle of milk.
218. Your eyes have to adjust themselves differently to see things near by and to see things at a distance.
219. A vacuum cleaner does not wear out a carpet nearly as quickly as a broom or a carpet sweeper does.
220. You can see a sunbeam in a dusty room.
Section 25. Magnification.
Why is it that things look bigger under a magnifying glass than under other kinds of glass?
How does a telescope show you the moon, stars, and planets?
How does a microscope make things look larger?
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 Experiment 41, but this time move the lens from near the candle toward the paper, past the point where it makes its first clear image. Keep moving the lens slowly toward the paper until a second image is formed. Which image is larger than the flame? Which is smaller?
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.
4Your eye is a small camera. It has a lens in the front; it is lined with black; and at the back there is a sensitive part on which the picture is formed. This sensitive part of the eye is called the retina. It is in the back part of your eyeball and is made of many very sensitive nerve endings. When the light strikes these nerve endings, it sends an impulse through the nerves to the back part of the brain; then you know that the image is formed. And, of course, since your eyeball is small and many of the things you see are large, the image on the retina must be much smaller than the object itself, and this is because the lens is so much nearer to the retina than it is to the object.
Footnote 4: The following explanation may be omitted by any children who are not interested in it. Let such children skip to the foot of page 156.
Fig. 82. A diagram showing how a reading glass causes things to look larger by making the image on the retina larger.
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 bend the lines together, and that the lens in the eye bends them farther together; so they cross sooner, and the image is larger. Figure 83 shows more of the lines drawn in.
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.
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 first lens makes an image many times as big as the object; then you look at this image through a magnifying glass, so that the object is made to look very much larger than it really is. That is why you can see blood corpuscles and germs and cells through a microscope, when you cannot see them at all with your naked eye.
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).
Experiment 49. Place the lighted candle and the paper about 4 feet apart, as you did in Experiment 47. Hold a concave mirror back of the candle (so that the candle is between the mirror and the paper); then move the mirror back, the mirror casting the reflection of the candle light on the paper, until a clear image of the candle is formed.
Look at your image in the concave mirror. Does it look larger or smaller than you?
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.
Application 37. Explain how a reading glass makes print look larger; how you can see germs through a microscope; what kind of mirror will magnify; what kind of lens will magnify.
Inference Exercise
Explain the following:
221. The water that forms rain comes from the ocean, yet the rain is not salty.
222. Iron glows when it is very hot.
223. You can start a fire with sunlight by holding a reading glass at the right distance above the fuel.
224. Big telescopes make it possible for us to see in detail the surface structure of the moon.
225. A room is lighter if it has white walls than if it has dark walls.
226. Iron is heated by a blacksmith before he shapes it.
227. A dentist's mirror is concave; he sees your teeth enlarged in it.
228. Good penholders usually have cork or rubber tips.
229. A man's suit becomes shiny when it gets old.
230. When you look at a window from the sidewalk, you frequently see images of the houses across the street.
Section 26. Scattering of light: Diffusion.
Why is it that on a dark day the sun cannot be seen through light clouds?
Why do not the stars come out in the daytime?
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 these droplets, it is reflected at all angles according to the way it strikes, as shown in Figure 89.
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 all the rough parts of the glass gets so scattered that you cannot bring it to a focus. Therefore, no image of the filament in the incandescent lamp can be formed on the retina of your eye.
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 rough; there are too many little uneven places that reflect the light at all sorts of angles; the light is scattered and the lens in your eye cannot bring it to a focus.
Application 38. Explain why a scrim curtain will keep people from seeing into a room, but will not shut the light out; why curtains soften the light of a room; why indirect lighting (i.e. light thrown up against the ceiling and then reflected down into the room by the rough ceiling) is better for your eyes than is the old-time direct lighting.
Inference Exercise
Explain the following:
231. The alcohol formed by the yeast in making bread light is practically all gone by the time the bread is baked.
232. The oceans do not flow off the earth at the south pole.
233. Lamp globes often have frosted bottoms.
234. A damp dust cloth will take up the dust, without making it fly.
235. The stars twinkle when their light passes through the moving air currents that surround the earth.
236. Shears for cutting tin and metal have long handles and short blades.
237. A coin at the bottom of a glass of water seems raised when you look at it a little from one side.
238. You have to brace your feet to row well.
239. Light from the northern part of the sky, where the sun is not, does not make sharp shadows.
240. Pokers and lifters for stove lids often have open spiral handles.
Section 27. Color.
What makes the ocean look green in some places and blue in others?
What makes the sky blue?
What causes material to be colored?
What makes a rainbow?
What is color?
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.
Experiment 50. Hold a prism in the sunlight by the window and make a "rainbow" on the wall. The diagram here shown illustrates how the prism breaks up the single beam of white light into different-colored beams of light.
Experiment 51. Rotate the color disk on the rotator and watch it. Make it go faster and faster until all the colors are perfectly merged. What color do you get by combining all the colors of the rainbow? If the colors on the disk were perfectly clear rainbow colors, in exactly the same proportion as in the rainbow, the whirling would give a white of dazzling purity.
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 that we can see is red. If the waves are a little shorter, the light is orange; if they are shorter yet, it is yellow; still shorter, green; shorter still, blue; while the shortest waves that we can see are those of violet light. Black is not a color at all; it is the absence of light. We say the night is black when we cannot see anything. A deep hole looks black because practically no light is reflected up from its depths. When you "see" anything black, you really see the things around it and the parts of it that are not perfectly black. A pair of shoes, for instance, has particles of gray dust on them; or if they are very shiny they reflect part of the light that strikes them as a white high-light. But the really black part of your shoes would be invisible against an equally black background.
A black thing absorbs the light that strikes it and turns it to heat. Here is an experiment that will prove this to you:
Experiment 52. (a) On a sunny day, take three bottles, all of the same size and shape, and pour water out of a pitcher or pan into each bottle. Do not run the water directly from the faucet into the bottle, because sometimes that which comes out of the faucet first is warmer or colder than that which follows; in the pitcher or pan it will all be mixed together, and so you can be sure that the water in all three bottles is of the same temperature to begin with. Wrap a piece of white cotton cloth twice around one bottle; a piece of red or green cotton cloth of the same weight twice around the second bottle, and a piece of black cotton cloth of the same weight twice around the third bottle, fastening each with a rubber band. Set all three bottles side by side in the sunlight, with 2 or 3 inches of space between them. Leave them for about an hour. Now put a thermometer into each to see which is warmest and which is least warm.
From which bottle has most of the light been reflected back into the air by the cloth around it? Which cloth absorbed most of the light and changed it into heat? Does the colored cloth absorb more or less light than the white one? than the black one?
(b) On a sunny day when there is snow on the ground, spread three pieces of cotton cloth, all of the same size and thickness, one white, one red or green, and one black, on top of the snow, where the sun shines on them. Watch them for a time. Under which does the snow melt first?
The white cloth is white because it reflects all colors back at once. It therefore absorbs practically no light. But the reason the black cloth looks black is that it reflects almost none of the colors—it absorbs them all and changes them to heat. The colored cloth reflects just the red or the green light and absorbs the rest.
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" to throw back the red lights and keep all the other kinds (absorb them and change them to heat), we can understand what the dye in a red sweater does. The dye is not really trained, of course, but for a reason which we do not entirely understand, some kinds of dye always throw back (reflect) any red that is in the light that shines on them, but they keep all other kinds of light, changing them to heat. Other dyes or coloring matter always throw back any green that is in the light that shines on them, keeping the other colors. Blue coloring matter throws back only the blue part of the light, and so on through all the colors.
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 lights are long, inclined tubes which glow with a greenish-violet light. No matter how good the color of a person is in ordinary light, in that light it is ghastly.
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 is no red in the light to reflect, obviously a thing cannot look red in that light.
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 lake or an ocean, you can notice it decidedly when you look straight down into the water itself.
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, green, blue, indigo, and violet. As the white light of the sun passes through the raindrops, the violet part of the light is bent more than any of the rest, the indigo part is bent not quite so much, and so on to the red, which is bent least of all. So all the colors fan out from the single beam of white light and form a band of color, which we call the rainbow.