dioptre

Also known as a reciprocal meter, a unit used to express the focal power of optical lenses. One dioptre is the power of a lens that brings parallel light rays to a focus at a point one meter from the lens, after passing through it. A stronger lens brings light rays to a focus at a point closer to it than a weaker lens and has a higher dioptric power. The power of a converging lens in dioptres is a positive number equal to the reciprocal of its focal length in meters. Diverging lenses have negative powers.

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