JEE Main 2021 · PhysicsMultiple choiceSingle correctEasyConceptual

JEE Main 16 March 2021, Shift 2, Physics Q3

Question 3 of 90 in this shift, Physics question 3 of 30, Section A.

The 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 refractive index ? Assume the radii of curvature of the faces of lens are

R_1

and

R_2

respectively.

(1)Zero
(2)1
(3)InfiniteOfficial answer
(4) $\frac{R_1 R_2}{R_1-R_2}$

Official answer

Option 3

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

7 Apr 2025, Shift 1 · Q42A 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…EasySingle correct
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

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.