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.
[Illustration: FIG. 70. The light from each point of the candle flame
goes out in all directions.]
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.
[Illustration: 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.
[Illustration: FIG. 72. The light from the tip of the candle flame is
focused at one point.]
[Illustration: FIG. 73. And the light from the base of the flame is
focused at another point.]
[Illustration: 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.
[Illustration: FIG. 75. The light spreads out again beyond the focus.]
[Illustration: 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 the paper, naturally you
again have a blur of light instead of a point, and the image is not
sharp and definite (Fig. 77).
[Illustration: 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 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.
[Illustration: 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.
_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.