Define 1 dioptre of power of a lens.
Power of lens is defined as the reciprocal of its focal length. If Pis the power of a lens of focal length Fin metres, then
P= 1/f(metres)
The S.I. unit of power of a lens is Dioptre. It is denoted by D.
1 dioptre is defined as the power of a lens of focal length 1 metre.
∴1 D = 1 m - 1
