glasses

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 lenses are short sight (myopia) by using concave (spherical) lenses, long sight (hypermetropia) by using convex (spherical) lenses, and astigmatism 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. Varifocal glasses have the same effect without any visible line between the two types of lens.

Glasses are said to have been invented in the 13th century by a Florentine monk. Few people found the need for glasses until printing was invented, when the demand for them increased rapidly. Using photosensitive glass, lenses can be produced that darken in glare and return to normal in ordinary light conditions. Lightweight plastic lenses are also common. The alternative to glasses is contact lenses.