State and define the SI unit of power of a lens.
The reciprocal of focal length of the lens is called the power of the lens (P).
P (in dioptre) =
The S.I unit of power of lens is diopter denoted by D.
From the formula of power of lens.
1 D =.
Thus 1 Dioptre is the power of the convex lens having focal length of 1m.