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A dioptre (also diopter) is a unit of curvature equal to one per metre. A circle with radius 1/2 metre has a curvature of 2 dioptres. It is also used to measure focusing power of lenses and (curved) mirrors; the power is the inverse of the focal length. Diverging lenses have negative power.

If the curvature of a lens is C and the index of refraction is n, the focusing power is F=(n-1)C. If both surfaces of the lens are curved, consider their curvatures as positive toward the lens and add them. For a mirror the focusing power is F=2C.