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Physics · 2026 · 2 marks
Ray Optics and Optical InstrumentsRefraction at Spherical Surfaces and by Lenses2 marksApplyvery short answer
CBSE 2026 · Region 4 · Set 3 · Q20
A convex lens of refractive index 1.5 has a focal length of 20 cm in air. Find its nature and focal length when it is immersed in a transparent…
A convex lens of refractive index $\displaystyle 1.5$ has a focal length of $\displaystyle 20$ cm in air. Find its nature and focal length when it is immersed in a transparent liquid of refractive index 1.25.
Marking-scheme solution
$\displaystyle \dfrac{1}{f}=\left({ }^{a} \mu_{g}-1\right)\left(\dfrac{1}{R_{1}}-\dfrac{1}{R_{2}}\right)$
$\displaystyle \dfrac{1}{f^{\prime}}=\left({ }^{\ell} \mu_{g}-1\right)\left(\dfrac{1}{R_{1}}-\dfrac{1}{R_{2}}\right)$
$\displaystyle \dfrac{f^{\prime}}{f}=\dfrac{\left({ }^{a} \mu_{g}-1\right)}{\left({ }^{\ell} \mu_{g}-1\right)}=\dfrac{1.5-1}{\left(\dfrac{1.5}{1.25}-1\right)}=\dfrac{0.5}{0.25} \times 1.25$
$\displaystyle \dfrac{f^{\prime}}{f}=2.5$
or $\displaystyle f^{\prime}=2.5 \times 20=50 \mathrm{~cm}$
Nature $\displaystyle \rightarrow$ converging.
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CBSE Class 12 Physics past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.