Walt Armstrong, age 11, of Idaho Falls, Idaho, for his question:
HOW DO EYEGLASSES WORK?
Eyeglasses aid those whose eyesight is imperfect. Glasses are made of two lenses mounted in a frame so that one lens is in front of each eye. The lenses help to focus rays of light so that they will fall properly on the retina and form a clear image.
If you can see distant objects but cannot focus well on close objects, your problem is called farsightedness. The eyeball is short and the focus of the light rays falls behind the retina.
In the case of farsightedness, a convex glass is used in your eyeglasses. A convex lens, which has a surface that is curved or rounded outward, is used to slant the light rays together, or converge them.
If you cannot focus rays which come from distant objects, this problem is called nearsightedness. The eye focuses the image so that it falls short of the retina. The image on the retina is blurred, like the image a motion picture projector puts on the screen when it is out of focus.
To correct a case of nearsightedness, a concave lens is used which diverges, or spreads out, the light rays so they do not form an image until they reach the retina.
A problem called astimagtism can be found in some people who may have imperfections in the curvature of the cornea. The surface of the eye may present a certain curvature in one direction and a different one in another. The rays which pass through the eye will not focus properly.
Astigmatism can be corrected by putting a toric lens into glasses. This type of lens compensates for the variations in curvature.
Bifocal glasses were invented by Benjamin Franklin to save the trouble of changing his glasses when looking from near objects to distant ones. Each lens has two parts. And each part has its own focusing power.
Lenses that are made to fit directly over the eyeballs, beneath the eyelids, are called contact lenses. They are practically invisible. Some people, however, find discomfort by wearing this type of lens.
Most vision defects in healthy eyes can be corrected by properly fitted eyeglasses. But the glasses must be made to correct the deficiency of each eye, or they will cause discomfort and blurred vision.
Historians tell us that eyeglasses go back to ancient times. Doctors, however, say that these old glasses were not very helpful.
The ability to grind lenses wasn't developed until the 1700s. It was at this time that man learned how light rays are bent when they travel through glass.