The power of a lens is the degree by which light rays are converged or diverged by the lens. It is measured as 1/f (in Dioptre), where f is the focal length of the lens in metre.

If the lens has smaller focal length, then it will cause greater bending of light rays about the focus of the lens, implying that it has greater power. On the other hand, a lens with larger focal length will cause less curved rays, implying lesser power of lens. Hence, power is inversely proportional to the focal length of the lens.