n.
Lenses set in frames to wear in front of the eyes to aid vision or correct vision defects (see ophthalmology , optometry ).
Their use for farsightedness and nearsightedness has been known since the late Middle Ages. In 1784 Benjamin Franklin invented bifocals, with divided lenses for distant and near vision. Eyeglasses can also correct astigmatism . Most lenses are made of glass or plastic (lighter and more shatterproof than glass but easily scratched). Sunglass lenses are tinted to reduce glare and often treated to reduce ultraviolet light exposure. See also contact lens .