Definition
“The reciprocal of its focal length is called Power of the lens.”
It is represented by the letter P.
The power P of a lens of focal length f (in m) is given by.
Power of lens (P) = 1/f ( in m).
The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D.
The power of a convex lens is positive and a concave lens is negative.
Thus, the lens with a focal length of +50 cm is convex and that with -50 cm is convex. Their powers are +2 D and -2 D respectively.