Pair of lenses fitted in a frame and worn in front of the eyes to correct or assist defective vision. Common defects of the eye corrected by such lenses are nearsightedness (myopia), corrected by using concave (spherical) lenses, farsightedness (hypermetropia), corrected by using convex (spherical) lenses, and astigmatism, corrected by using cylindrical lenses. Spherical and cylindrical lenses may be combined in one lens. Bifocal glasses correct vision both at a distance and for reading by combining two lenses of different curvatures in one piece of glass. Today, lightweight plastic lenses are common instead of glass.
Eyeglasses are said to have been invented in the 13th century by a Florentine monk. Few people found the need for eyeglasses until printing was invented, when the demand for them increased rapidly. Using photosensitive glass, lenses can be produced that darken in glare and lighten in ordinary light conditions. See also contact lenses.
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