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?
[Illustration: 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.]
[Illustration: FIG. 80. How an image is formed on the retina of the
eye.]
[Illustration: FIG. 81. A simpler diagram showing how an image is
formed in the eye.]
[Illustration: FIG. 82. A diagram showing how a reading glass causes
things to look larger by making the image on the retina larger.]
[Illustration: 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.
[Illustration: FIG. 84. Diagram of a microscope.]
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.
[Illustration: FIG. 85. This is the way a concave mirror forms a
magnified image.]
[Illustration: 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 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.
[Illustration: 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 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.