The reciprocal of focal length of the lens is called the power of the lens (P).
P (in dioptre) =\(\frac{1}{f(in\, meters)}\)
The S.I unit of power of lens is diopter denoted by D.
From the formula of power of lens.
1 D = \(\frac{1}{1m}\)= 1m-1
Thus 1 Dioptre is the power of the convex lens having focal length of 1m.