lens, device for forming an image of an object by the refraction of light. In its simplest form it is a disk of transparent substance, commonly glass, with its two surfaces curved or with one surface plane and the other curved. Lenses are used singly or in groups in such instruments as cameras, projectors, microscopes, telescopes, binoculars, opera glasses, and eyeglasses. The lens of the eye is known as a crystalline lens.
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