Whenever a ray of light passes through a lens (except when it passes through the optical center) it bends. The bending of light rays towards the principal axis is called convergence and bending of light rays away from the principal axis is called divergence. The degree of convergence or divergence of a lens is expressed in terms of its power. A lens of short focal length deviates the rays more while a lens of large focal length deviates the rays less. Thus power of a lens is defined as the reciprocal of its focal length in meters.
The unit of power is dioptre
If there is convex lens of power ID then its focal length is equal 1 meter.
The power of a convex lens is positive and concave lens is negative.
Note : If two thin lenses are placed in contact, the combination has a power equal to the algebraic sum of the powers of two lenses.
