Power of a lens is a measure of the convergence or divergence, which a lens introduces in the light falling on it. Clearly, a lens of shorter focal length bends the incident light more, while converging it in case of a convex lens and diverging it in case of a concave lens. The power P of a lens 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.

`tandelta=h/f`

if `h=1` `=>` `tandelta=1/f`

or `delta=1/f` for small value of `delta`

Thus,

`P=1/f`

The SI unit for power of a lens is dioptre (D).

Power of a lens is positive for a converging lens and negative for a diverging lens.