Ray Optics Question 6
Question 6 - 2024 (29 Jan Shift 1)
A biconvex lens of refractive index 1.5 has a focal length of $20 \mathrm{~cm}$ in air. Its focal length when immersed in a liquid of refractive index 1.6 will be:
(1) $-16 \mathrm{~cm}$
(2) $-160 \mathrm{~cm}$
(3) $+160 \mathrm{~cm}$
(4) $+16 \mathrm{~cm}$
Show Answer
Answer: (2)
Solution:
$\mu_{1}=1.5$
$\mu_{m}=1.6$
$f_{a}=20 \mathrm{~cm}$
As $\frac{f_{m}}{f_{a}}=\frac{\left(\mu_{1}-1\right) \mu_{m}}{\left(\mu_{1}-\mu_{m}\right)}$
$\frac{f_{m}}{20}=\frac{(1.5-1) 1.6}{(1.5-1.6)}$
$f_{m}=-160 \mathrm{~cm}$
