Cambridge Encyclopedia » Cambridge Encyclopedia Vol. 45

lens

optical light

A transparent optical element comprising two refracting surfaces, at least one of which is curved; parallel light rays passing through the lens may converge (and focus at a point) or diverge, depending on the lens shape. Lenses are characterized by their focal length (the distance at which the image of a distant object is most sharply defined) and their aperture or f-number (the light transmission). They are widely used in optical instruments, binoculars, projectors, and cameras, and are usually made of glass, sometimes of plastic.

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