Mike Caswell, age 7, of Montgomery, Alabama, for his question:
What makes some people nearsighted or farsighted?
Your eyes are bright little windows for looking out at the world. They are much cleverer than the very best cameras. Most of the time they work perfectly well all by themselves. But once in a while, they need the help of an eye doctor. And he may decide to help them with glasses.
A little baby does not see well at all. Things around him look blurred and foggy and he cannot see which things are near and which things are far away. But soon the world begins to look clearer and brighter. If all goes well, when he gets to be six or seven years old, his eyes can sort out the scenery with no trouble at all. But some people's eyes never learn to do a perfect job by themselves.
Some people's eyes can read a newspaper only a foot or so away, but to these same eyes, the distant trees and the faraway mountains look blurred and foggy. These people are nearsighted. Farsighted people can see faraway things very clearly but the print in a book is too blurry for them to read. Lots of people are nearsighted and lots more are farsighted. But this does not mean that their eyes have a serious disease.
An eye has many parts that work together. The round black disc in the middle of the eye is the pupil. Actually it is a round hole, a window to let in the light. And the light carries a picture of the scenery. This picture goes straight through the eyeball. On the inside wall of the eyeball there is a screen, somewhat like a miniature movie screen. This sensitive screen is called the retina. The picture that comes through the pupil window falls right on the retina. Then lots of little nerve cells send messages to the brain and the brain figures out what you see.
The round eyeballs are buried safely in the bones of the head. And, it seems, no two people have eyeballs exactly alike. Some are longer, some are shorter than others. The light comes through the pupil in straight, slanted lines. When the slanting lines come together they have a perfect picture. They are in focus. But they must focus together just when they reach the retina screen not too soon and not too late. Then little nerve cells in the retina can see a clear and perfect picture.
If the eyeball is too long, the picture focuses just in front of the retina. This person is nearsighted. He can read fine print but distant trees look blurred. When the eyeball is too short, the slanting lines of light are too long to focus exactly on the retina screen. They come together and focus just behind it. This person is farsighted. A page of print looks foggy but the distant scenery looks bright and clear.
An eye doctor knows just how to help these eye problems. He gives the proper glasses to correct the focusing. Glasses bend the lines of light. The glasses for a nearsighted person make the slanting lines focus a bit farther back. Then the picture falls exactly on the screen. The glasses for a farsighted person bend the lines to focus a little closer in. Instead of focusing behind the screen, they focus exactly on it. The picture falls bright and clear, just where the eyes can see it.