Common Science Part 21

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 gla.s.s than under other kinds of gla.s.s?

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 gla.s.ses_. 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 gla.s.s 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?

[Ill.u.s.tration: 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.

[4]Your 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.]

[Ill.u.s.tration: FIG. 80. How an image is formed on the retina of the eye.]

[Ill.u.s.tration: FIG. 81. A simpler diagram showing how an image is formed in the eye.]

[Ill.u.s.tration: FIG. 82. A diagram showing how a reading gla.s.s causes things to look larger by making the image on the retina larger.]

[Ill.u.s.tration: FIG. 83. Diagram showing how a reading gla.s.s 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 gla.s.s, or convex lens, if put in the right place, will do this. In the next diagram, Figure 82, we shall include the magnifying gla.s.s, leaving out all lines except the two outside ones shown in Figure 81.

You will notice that the magnifying gla.s.s 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.

[Ill.u.s.tration: FIG. 84. Diagram of a microscope.]

The two important points to notice are these: First, the magnifying gla.s.s 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 gla.s.s 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.

[Ill.u.s.tration: FIG. 85. This is the way a concave mirror forms a magnified image.]

[Ill.u.s.tration: 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 gla.s.s 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 gla.s.s, 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.

[Ill.u.s.tration: 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).

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 gla.s.s 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 gla.s.s 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 s.h.i.+ny when it gets old.

230. When you look at a window from the sidewalk, you frequently see images of the houses across the street.