CBSE 2025 · Region 1 · Set 3 · Q19 · 2 marks
A double convex lens of glass has both faces of the same radius of curvature $\displaystyle 17$ cm. Find its focal length if it is immersed in water. The refractive indices of glass and water are $\displaystyle 1.5$ and $\displaystyle 1.33$ respectively.
Marking-scheme solution
$\displaystyle \frac{1}{f}=\left(\frac{n_{g}}{n_{w}}-1\right)\left(\frac{1}{R_{1}}-\frac{1}{R_{2}}\right)$
For double convex lens $\displaystyle R_{1}=R$ and $\displaystyle R_{2}=-R$:
$\displaystyle \frac{1}{f}=\left(\frac{1.5}{1.33}-1\right)\left(\frac{2}{R}\right)$
$\displaystyle =\left(\frac{1.5-1.33}{1.33}\right)\left(\frac{2}{17}\right)$
$\displaystyle f=66.5 \mathrm{~cm}$
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CBSE Class 12 Physics past-paper question from the 2025board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.