CBSE Class 10 Science 2017 · Paper 531 · Question 22

(a) The power of a lens is the degree of convergence or divergence of light rays achieved by it. It is the reciprocal of the focal length expressed in metres, $P=\frac{1}{f}$. Its SI unit is the dioptre (D): 1 dioptre is the power of a lens whose focal length is 1 metre ($1\text{D}=1{\text{m}}^{-1}$). (b) Lens of A: $f=+50$ cm $=+0.50$ m, so $P=\frac{1}{0.50}=+2$ D; the lens is convex (converging). Lens of B: $f=+100$ cm $=+1.00$ m, so $P=+1$ D; the lens is convex (converging). Lens of C: $f=-100$ cm $=-1.00$ m, so $P=-1$ D; the lens is concave (diverging).