JEE Main 2025 — 29 January, Evening Shift — Question 29

$\frac{1}{8}=\left(\frac{\mu_{\ell}}{\mu_{\mathrm{s}}}-1\right)\left[\frac{1}{\mathrm{R}_{1}}-\frac{1}{\mathrm{R}_{2}}\right]$

$\frac{1}{24}=(1.5-1)\left[\frac{2}{\mathrm{R}}\right]$

$\frac{1}{\mathrm{f}^{\prime}}=\left(\frac{1.5}{1.33}-1\right)\left(\frac{2}{\mathrm{R}}\right)$

$\frac{1}{\mathrm{f}^{\prime}}=\left(\frac{1.5 \times 3}{4}-1\right) \frac{2}{\mathrm{R}}$

(i) divided by (ii)

$\frac{\mathrm{f}^{\prime}}{24}=4$ $\mathrm{f}^{\prime}=96 \mathrm{~cm}$