“Power of lens is a measure of the convergence or divergence, which a lens introduces in the light falling on it.”
It is defined as the tangent of the angle by which it converges or diverges a beam of light falling at unit distant from the optical centre.
Thus, Power of a lens is the reciprocal of focal length.
When focal length is measured in metre then power is taken in dioptre (D).
For converging lens,, therefore P=+ve
For diverging lens, , therefore P =– ve.