Eyeglasses, also called spectacles, consist of a pair of lenses mounted in a frame and worn over the eyes to aid vision.
Although the 13th-century Englishman Roger Bacon is credited with their invention, they probably developed in many countries at an earlier time. About half of the U.S. population wears corrective lenses.
The function of eyeglasses is to correct errors of refraction (focusing), which result from differences in the length of the eye or the shape of the cornea or lens. The most common refractive errors are farsightedness (hyperopia) and nearsightedness (myopia). Hyperopia, the decreased ability to focus on nearby objects, results from having an eye that is smaller than the normal eye. It is
corrected by using a convex lens. Myopia, the decreased ability to focus on distant objects, usually results from a larger eye and is corrected by a concave lens. These lenses are spherically uniform unless the disorder is complicated by an astigmatismÑa corneal or lenticular misshapenness resulting in an oblong-shaped lens surface that focuses differently in perpendicular planes. Toric or spherocylindrical lenses must then be used to obtain the optimum correction.
As a person ages, the lens of the eye slowly loses its ability to reshape itself to focus for near work. This condition, known as presbyopia, usually develops after age 43. It is often corrected with bifocal lenses that have small convex lenses added to the lower part of the
glasses, allowing the top part to focus for distance and the bottom part to correct for near work. The progressive lens eliminates the sharp linear distinction between these two segments and blends it over a gradual multifocal area.
Eyeglass frames are fitted by measuring the distance between the pupils of the eyes so that the lenses will be centered correctly. Temple length is determined by measuring from the top of the ear to the end piece of the frame and then adding two inches. The horizontal width of the lens is determined by the shape of the head, by the shape of the nasal bridge, and considerations for lens weight, refractive error, and style. Modern frames can be made with special spring hinges and flexible alloys to increase durability.
Special shatter-resistant glasses, which may be made of plastic resins or, less often, of case-hardened glass, give protection against flying particles if the lenses break. Polarizing lenses and antireflective lenses reduce glare from smooth surfaces such as roads and bodies of water. Photochromic glasses contain special chemicals that can lighten or darken themselves depending on the brightness around them. Sunglasses, depending on their tint or color, can block a proportion of the light from entering the eyes. Ultraviolet light can damage the lens and the retina, so good sunglasses include special ultraviolet light absorbers that are added to the lens surface.