CBSE 2025 · Region 2 · Set 3 · Q9 · 1 mark
The plane face of a planoconvex lens is silvered. The refractive index of material and radius of curvature of the curved surface of the lens are $\displaystyle \mathrm{n}$ and $\displaystyle \mathrm{R}$ respectively. This lens will behave as a concave mirror of focal length
Official answer
From CBSE’s own marking scheme for this paper.
(D)
$\displaystyle \frac{\mathrm{R}}{2(\mathrm{n}-1)}$
Ray Optics and Optical InstrumentsRefraction at Spherical Surfaces and by LensesApplymcqhard
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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.