The power of a lens is a measure of the degree of convergence or divergence of the light rays falling on it. It is defined as the reciprocal of the focal length of the lens and is represented by the letter P. The SI unit of power of a lens is dioptre (D) . The power of a convex lens is positive, while that of a concave lens is negative. The formula for calculating the power of a lens is P = 1/f, where f is the focal length of the lens in meters. Therefore, a lens with a shorter focal length will have a higher power, and a lens with a longer focal length will have a lower power. For example, a convex lens of short focal length bends the light rays through large angles by focusing them closer to the optical center, while a concave lens of very short focal length causes a higher divergence than the one with a longer focal length.