The power of a lens is defined as the ability of the lens to diverge or converge the light rays depending on the focal length. The short focal length of the concave lens causes higher divergence than the one with a longer focal length. The power of the lens is also defined as the reciprocal of its focal length. It is denoted by “P”. The SI unit of power of the lens is ‘dioptre’. 1 dioptre is described as the power of a lens whose focal length is 1 metre.