CBSE 2026 · Region 3 · Set 1 · Q4 · 1 mark
A thin plano-convex lens and a thin equi-concave lens are kept coaxially in contact as shown in the figure. Assuming both the lenses are made of glass of refractive index $\displaystyle \mu$, and R is the radius of curvature of each curved surface, the focal length of the combination is :
Official answer
From CBSE’s own marking scheme for this paper.
(B)
$\displaystyle -\frac{\mathrm{R}}{\mu-1}$
Ray Optics and Optical InstrumentsRefraction at Spherical Surfaces and by LensesApplymcqmedium
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CBSE Class 12 Physics past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.