Q 12-09-081JEE MainJEE Main 2025 (29 Jan, Shift 2)Easy
A convex lens made of glass (refractive index $= 1.5$) has focal length $24\ \text{cm}$ in air. When it is totally immersed in water (refractive index $= 1.33$), its focal length changes to
Answer: (B) $96\ \text{cm}$
By the lens maker's formula, $\dfrac{1}{f} \propto (\mu_{rel} - 1)$:
$$\frac{f_w}{f_a} = \frac{1.5 - 1}{\frac{1.5}{1.33} - 1} = \frac{0.5}{0.125} = 4$$
(taking $1.33 \approx 4/3$, so $\mu_{rel} = 9/8$).
$f_w = 4\times24 = 96\ \text{cm}$
Solution by Sreeraj P, M.Sc Physics