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.

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.

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.