JEE Main 2025 · PhysicsMultiple choiceSingle correctEasyCalculation

JEE Main 7 April 2025, Shift 1, Physics Q42

Question 42 of 75 in this shift, Physics question 17 of 25, Section A.

A lens having refractive index 1.6 has focal length of 12 cm, when it is in air. Find the focal length of the lens when it is placed in water. (Take refractive index of water as 1.28)

(1) $555$ mm
(2) $288$ mmOfficial answer
(3) $355$ mm
(4) $655$ mm

Official answer

Option 2

NTA final key.

Chapter: Ray Optics and Optical Instruments
Topic: Lens Maker's Formula
Idea tested: Lens Immersed in a Liquid

Same idea in other shifts

Asked 5× in all

8 Apr 2025, Shift 2 · Q41A concave-convex lens of refractive index 1.5 and the radii of curvature of its surfaces are 30 cm and 20 cm, respectively. The concave…HardSingle correct
29 Jan 2024, Shift 1 · Q45A biconvex lens of refractive index 1.5 has a focal length of 20 cm in air. Its focal length when immersed in a liquid of refractive index…MediumSingle correct
11 Apr 2023, Shift 1 · Q52The radius of curvature of each surface of a convex lens having refractive index 1.8 is 20 cm. The lens is now immersed in a liquid of…MediumNumerical value
16 Mar 2021, Shift 2 · Q3The refractive index of a converging lens is 1.4. What will be the focal length of this lens if it is placed in a medium of same…EasySingle correct

Question text from the official JEE Main paper published by NTA; answer from the final answer key. Chapter, topic and idea tags, difficulty and skill are Lumi’s. Lumi analyses 42 of the 174 JEE Main shifts held from 2017 to 2026.