Question

Define power of lens and write its formula and s. i. unit

Answer 1

The diopter is the unit of measure for the refractive power of a lens. Thepower of a lens is defined as the reciprocal of its focal length in meters, or D = 1/f, where D is the power in diopters and f is the focal length in meters. Lens surface power can be found with the index of refraction and radius of curvature

Answer 2

power of lens is reciprocal of its focal length in meters

that is

P=1/f in meters

it's SI unit is dioptre