define power of a lens.

Power of a lens is defined as the reciprocal of its focal length.i.e. P=1/f

Its S.I. unit is dioptre(D). Dioptre is the unit of power of a lens whose focal length is in meters.

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A dioptre (uk), or diopter (us), is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres). It is thus a unit of reciprocal length.
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