Q 12-09-028NEETJEE MainTop questionMedium
A convex lens of glass ($\mu = 1.5$) has focal length $20$ cm in air. When immersed in water ($\mu = \dfrac{4}{3}$), its focal length becomes
Answer: (C) $80$ cm
$\dfrac{1}{f} \propto \left(\dfrac{\mu_g}{\mu_m} - 1\right)$: in air $0.5$, in water $\dfrac{1.5}{4/3} - 1 = \dfrac{1}{8}$.
$$f_w = 20 \times \frac{0.5}{1/8} = 80\ \text{cm}$$
Solution by Sreeraj P, M.Sc Physics