Q 12-09-181JEE MainJEE Main 2020 (7 Jan, Shift 2)Medium
A thin lens made of glass (refractive index $= 1.5$) of focal length $f = 16$ cm is immersed in a liquid of refractive index $1.42$. If its focal length in the liquid is $f_l$, then the ratio $f_l/f$ is closest to the integer:
Answer: (B) $9$
By the lens maker's formula, $\dfrac1f \propto \left(\dfrac{\mu_{\text{lens}}}{\mu_{\text{medium}}} - 1\right)$:
$$\frac{f_l}{f} = \frac{1.5 - 1}{\frac{1.5}{1.42} - 1} = \frac{0.5}{0.0563} \approx 8.9 \approx 9$$
Solution by Sreeraj P, M.Sc Physics