CBSE 2026 · Region 4 · Set 3 · Q20 · 2 marks
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.
Ray Optics and Optical InstrumentsRefraction at Spherical Surfaces and by LensesApplyvery_short_answermedium
More from Ray Optics and Optical Instruments
- Assertion: A convex lens, when immersed in a liquid, disappears. Reason ( R ): The refractive indices of…2024 · asked 3×
- A convex lens (n = 1.52) has a focal length of 15.0 cm in air. Find its focal length when it is immersed in…2024 · asked 3×
- (i) Draw a labelled ray diagram showing the formation of the image at infinity by an astronomical telescope.…2022 · asked 3×
- (i) (1) Write two points of difference between an interference pattern and a diffraction pattern. (2) Name…2023 · asked 3×
- Two transparent media of refractive indices n 1 and n 2 are separated by a spherical transparent surface. The…2022 · asked 3×
- Write two necessary conditions for total internal reflection. Two prisms ABC and DBC are arranged as shown in…2022 · asked 3×
- (i) Define SI unit of power of a lens. (ii) A plano convex lens is made of glass of refractive index 1.5. The…2022 · asked 3×
- (i) An object is placed 30 cm from a thin convex lens of focal length 10 cm. The lens forms a sharp image on…2025 · asked 3×
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.