CBSE Class 10 Science 2017 · Paper 31/1/C · Question 3

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