### Theory:

Relation between the power of lens and focal length

$P=\frac{1}{f}$

The dioptre is the SI unit of lens power. It is denoted by the letter D. The power of the lens is expressed in ‘D' if the focal length is expressed in ‘m'. As a result, the power of a lens with a focal length of one metre is 1D.

$1\phantom{\rule{0.147em}{0ex}}D\phantom{\rule{0.147em}{0ex}}=1\phantom{\rule{0.147em}{0ex}}{m}^{-1}$

The power of a convex lens is assumed to be positive, whereas the power of a concave lens is assumed to be negative.

*Power of a lens*