Michelle Husted, age 11, of Waukesha, Wis., for her question:
HOW DO EYEGLASSES WORK?
One popular type of glasses is sunglasses, which use colored lenses to protect the eyes from the sun's glare. Some use polarized material which cuts glare. Some unglasses have lenses that darken in the sunlight but become almost colorless indoors. Welders also use special glasses to cut off harmful light rays that come from a ~ elding arc.
Glasses are composed of two lenses mounted in a .`rame that puts one lens in front of each eye. The lenses then focus light rays so that they fall properly on the Letina and form a clear image.
Today the lenses in glasses are made of either heat¬treated optical glass or plastic. The lenses are shaped and ground to correct the defect of each eye. An optometrist or an ophthalmologist, after a professional eye examination, can determine the type of glasses needed.
If your eyes cannot correctly focus rays which come from distant objects your problem is called nearsightedness. Your eye then focuses the image so that it falls short of the retina. The image on the retina is blurred. A concave lens will be used to diverge, or spread out, the light rays so they do not form an image until they reach the retina.
If you can see distant objects but cannot focus on close objects your problem is farsightedness. With this problem, the focus of the light rays falls behind the retina. A convex glass is used to correct the problem since it slants the light rays together, or converges them.
If you have imperfections in the curvature of the cornea astigmatism is your problem. The rays which pass through the eye will not focus properly. This problem is corrected by a toric lens which compensates for variations in curvature.
Glasses that are bifocals provide both near and far vision. Each lens has two parts of different focal power. The lower section is smaller and is used for closer work, while the upper section is used for distant vision. Trifocal lenses are also made for some people who require a third section for medium distances.
Well fitted glasses can correct most vision defects in healthy eyes. It is necessary that a specialist fit the glasses and that they be changed whenever the eyes change their focusing power.
Contact lenses are made to fit directly over the eyeballs and beneath the eyelids. They are almost invisible and can be ground to meet any type of vision problem. Many people find the lenses can only be worn for limited periods of time without discomfort.
Grinding of lenses came during the 1700s when man developed an understanding of light refraction and how light rays could be bent when they traveled through glass.