Power of lens:
A convex lens with short focal length converges the light rays with greater degree nearer to principal focus and a concave lens with short focal length diverges the light rays with greater degree nearer to principal focus.
The degree of divergence or convergence of ray of light by a lens is expressed in terms of the power of lens. Degree of convergence and divergence depends upon the focal length of a lens. The power of a lens is denoted by ‘P’. The power of a lens is reciprocal of the focal length.
Power = `1/("Focal length f")`
or, P = `1/"f"`
The SI unit of Power of lens is dioptre and it is denoted by ‘D’.
Power of a lens is expressed in dioptre when the focal length is expressed in metre. Thus, a lens having 1 metre of focal length has power equal to 1 dipotre.
Therefore, 1 D = 1 m−1
A convex lens has power in positive and a concave lens has power in negative.