Chapter-9|Ray Optics and Optical Instrument|NCERT 12th Physics:
Power of a Lens:
The ability of a lens to converge or diverge the rays of light incident on it is called the power of the lens.
The SI unit of power of lens is dioptre(D).
The power of a lens is measured as the reciprocal of its focal length(in metre).
P= 1/f
