1 dioptre of power of a lens is described as the unit of measurement of the optical power of a lens or curved mirror equivalent to the reciprocal of focal length (f). Measurement of the focal length in a meter.
The SI unit of power of lens is dioptre whose focal length is one meter, which is denoted by the letter D.
1 dioptre (D) = P = 1/f
Where f = focal length