CBSE Class 10 Science 2017 · Paper 531/3 · Question 4

Power of a lens is the degree of convergence or divergence of light rays achieved by the lens. It is the reciprocal of the focal length of the lens: $P=\frac{1}{f}$, with $f$ in metres. The power of a convex lens is positive and that of a concave lens is negative. SI unit: the dioptre (D). One dioptre is the power of a lens whose focal length is 1 metre, so $1\text{D}=1{\text{m}}^{-1}$.