—dioptral, adj./duy op"teuhr/, n.1. Optics. a unit of measure of the refractive power of a lens, having the dimension of the reciprocal of length and a unit equal to the reciprocal of one meter. Abbr.: D2. an instrument, invented by Hipparchus, to measure the apparent diameter of the sun or moon or to estimate the size or elevation of distant objects.Also, esp. Brit., dioptre.[1585-95; < L dioptra < Gk: instrument for measuring height or levels, equiv. to di- DI-3 + op- (for ópsesthai to see) + -tra n. suffix of means]
* * *▪ opticsin optics, unit of magnifying power of a lens or lens system. Because the power of a lens is proportional to unity (one) divided by the focal length (see lens), the power of a lens in diopters is numerically equal to 1 m divided by the focal length in metres. The algebraic sign of the magnifying power indicates whether the lens causes an incident pencil of parallel light rays to converge or to diverge. Thus, a diverging lens having a focal length of 1 m has a power of -1 diopter, and a converging lens of focal length 0.5 m has a power of two diopters. The power of a combination of two or more thin lenses in contact is equal to the sum of the powers of the individual lenses. For example, a lens of -10 diopters combined with a lens of 30 diopters gives a converging lens of 20 diopters (of 5 cm focal length).
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