Dioptre

A dioptre, or diopter, is a unit of measurement of the refractive power of a lens or curved mirror which is equal to the reciprocal of the focal length measured in metres (i.e. 1 / metres). For example, a 3-dioptre lens brings parallel rays of light to focus at 1/3 meter. As a measurement of the degree to which light is converged or diverged, lenses with various dioptric values are used during an eye examination to improve focus and determine the refractive error of the eye. Convex lenses have positive dioptric value and generally correct hyperopia (farsightedness) whereas concave lenses have negative dioptric value generally correct myopia (nearsightedness). Typical over the counter reading glasses will be rated at +1.00 to +3.00 dioptres.

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A dioptre is also a unit of measurement of curvature equal to the reciprocal of the radius measured in metres. For example, a circle with a radius of 1/2 metre has a curvature of 2 dioptres. 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.

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Despite being defined by "metres", the dioptre is not considered an SI unit.

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