Define one dioptre of power of a lens.

Explanation

Power of lens is defined as the reciprocal of its focal length. If P is the power of a lens of focal length Fin meters, then
`P=(1)/("f(in meters)")`
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.
`:. 1D = 1m^(- 1)`