Question

A biconvex lens of refractive index 1.5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index 1.6 will be: 

(1) -16 cm

(2) -160 cm

(3) +160 cm

(4) +16 cm

Answer 1

The correct option is (2) -160 cm.

\(f = \cfrac{R}{2(\cfrac{\mu_L}{\mu_S}-1)}\)

\(20 = \frac{R}{2(1.5-1)} .....(1)\)

\(f' = \cfrac{R}{2(\cfrac{1.5}{1.6}-1)} .......(2)\)

\(\frac{20}{f'} = \frac{(1.5-1.6)/1.6}{1.5-1}\)

\(\frac{20}{f'} = \frac{-0.1}{1.6} \times \frac{1}{0.5}\)

\(f' = -160 cm\)