**Question:**

How are power and focal length of a lens related ? You are provided with two lenses of focal length 20 cm and 40 cm respectively. Which lens will you

use to obtain more convergent light ? (CBSE 2012)

**Solution:**

$\mathrm{P}=\frac{1}{f(\text { in metre })}=\frac{100}{f(\text { in } \mathrm{cm})}$

$f_{1}=20 \mathrm{~cm} \therefore \mathrm{P}_{1}=\frac{100}{20}=5.0 \mathrm{D}$

$f_{2}=40 \mathrm{~cm} \therefore \mathrm{P}_{2}=\frac{100}{40}=2.5 \mathrm{D}$

The lens of focal length 20 cm or power 5.0 D will be used to have more convergent light. This is because lens of small focal length or large power

strongly converges the parallel beam of light.