# Power of a Lens

The degree of Convergence or Divergence of light rays achieved by a lens is expressed in terms of its POWER. The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The power P of a lens of focal length f is given by

----------- (10.11)

The SI unit of power of a lens is â€˜dioptreâ€™. It is denoted by the letter D. If f is expressed in metres, then the power is expressed in dioptres. Thus one dioptre is the power of a lens whose focal length is one metre.

1D = 1m-1

The power of a convex lens is Positive.

The power of a concave lens is Negative.

If an optical instrument consists of more than one lens, the net power can be obtained by adding the individual powers

P = P1 + P2 + P3 + â€¦â€¦