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.

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.

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.