What is meant by power of a lens ? When is it positive or negative? Define one dioptre.

Explanation

Power of a lens is ability of the lens to converge the beam of light falling on it.
Numerically, power of a lens is reciprocal of focal length of the lens, i.e., `P=(1)/(f)`.
For a convex lens, f is , p is +
For a concave lens, f is - , P is negative. One dioptre is the power of a lens of focal length one meter.