CBSE Class 10 Science 2018 · Paper 31/1 · Question 11(b)

Power of a lens: the degree of convergence or divergence of light rays produced by the lens, that is, its ability to converge or diverge the light passing through it. It is the reciprocal of the focal length in metres, $P=\frac{1}{f}$. SI unit: dioptre (D); $1\text{D}=1{\text{m}}^{-1}$. Lens of focal length $40$ cm: $P=\frac{1}{+0.40\text{m}}=+2.5$ D; it is a converging (convex) lens. Lens of focal length $-20$ cm: $P=\frac{1}{-0.20\text{m}}=-5$ D; it is a diverging (concave) lens.