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Saturday, 31 October

JEE Main 2021 · PhysicsMultiple choiceSingle correctMediumApplication

JEE Main 27 August 2021, Shift 2, Physics Q14

Question 14 of 90 in this shift, Physics question 14 of 30, Section A.

Curved surfaces of a plano-convex lens of refractive index μ1\mu_1 and a plano-concave lens of refractive index μ2\mu_2 have equal radius of curvature as shown in figure. Find the ratio of radius of curvature to the focal length of the combined lenses.

The figure, in words

A rectangle with a curved line running from its top-left corner to its bottom-left corner, bulging to the right; the region to the left of the curve is labelled mu1 and the region to the right of the curve is labelled mu2.
  1. (1)μ1−μ2\mu_1-\mu_2Official answer
  2. (2)1μ1−μ2\frac{1}{\mu_1-\mu_2}
  3. (3)μ2−μ1\mu_2-\mu_1
  4. (4)1μ2−μ1\frac{1}{\mu_2-\mu_1}

Official answer

Option 1

NTA final key.

Chapter
Ray Optics and Optical Instruments
Topic
Lens Maker's Formula
Idea tested
Lenses in Contact

Same idea in other shifts

Asked 5× in all
  1. 5 Apr 2026, Shift 2 · Q40A thin convex lens and a thin concave lens are kept in contact and are co-axial. Which of the following statements is correct for this…EasySingle correct
  2. 8 Apr 2025, Shift 2 · Q43A convex lens of focal length 30 cm is placed in contact with a concave lens of focal length 20 cm. An object is placed at 20 cm to the…MediumSingle correct
  3. 4 Apr 2024, Shift 1 · Q45An effective power of a combination of 5 identical convex lenses which are kept in contact along the principal axis is 25 D. Focal length…EasySingle correct
  4. 24 Jun 2022, Shift 1 · Q52Two identical thin biconvex lenses of focal length 15 cm and refractive index 1.5 are in contact with each other. The space between the…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. This question needs a figure we do not reproduce; it is described in words above. Lumi analyses 108 of the 174 JEE Main shifts held from 2017 to 2026.