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Friday, 9 October

JEE Main 2025 · PhysicsMultiple choiceSingle correctMediumMulti-step

JEE Main 7 April 2025, Shift 2, Physics Q42

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

A transparent block A having refractive index μ=1.25\mu=1.25 is surrounded by another medium of refractive index μ=1.0\mu=1.0 as shown in figure. A light ray is incident on the flat face of the block with incident angle θ\theta as shown in figure. What is the maximum value of θ\theta for which light suffers total internal reflection at the top surface of the block ?

The figure, in words

Ray enters the vertical flat face of Block A (refractive index μ2=1.25\mu_2=1.25) from the surrounding medium (μ1=1.0\mu_1=1.0) at incident angle θ\theta measured from the horizontal normal, travels inside the block and strikes the top horizontal surface at the critical angle θC\theta_C (measured from the vertical normal), then grazes along the top surface.
  1. (1)tan⁡−1(3/4)\tan^{-1}(3/4)
  2. (2)cos⁡−1(3/4)\cos^{-1}(3/4)
  3. (3)sin⁡−1(3/4)\sin^{-1}(3/4)Official answer
  4. (4)tan⁡−1(4/3)\tan^{-1}(4/3)

Official answer

Option 3

NTA final key.

Chapter
Ray Optics and Optical Instruments
Topic
Refraction and Snell's Law
Idea tested
Total Internal Reflection

Same idea in other shifts

Asked 6× in all
  1. 8 Apr 2024, Shift 1 · Q43Critical angle of incidence for a pair of optical media is 45∘45^\circ. The refractive indices of first and second media are in the ratio:EasySingle correct
  2. 11 Apr 2023, Shift 1 · Q35The critical angle for a denser-rarer interface is 45∘45^\circ. The speed of light in rarer medium is 3×1083 \times 10^8 m/s. The speed of…EasySingle correct
  3. 25 Jul 2021, Shift 1 · Q9A ray of laser of a wavelength 630 nm is incident at an angle of 30∘30^\circ at the diamond-air interface. It is going from diamond to air.…EasySingle correct
  4. 8 Jan 2020, Shift 1 · Q15The critical angle of a medium for a specific wavelength, if the medium has relative permittivity 3 and relative permeability…MediumSingle correct
  5. 9 Jan 2020, Shift 2 · Q17There is a small source of light at some depth below the surface of water (refractive index =43= \frac{4}{3}) in a tank of large cross…HardSingle 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. This question needs a figure we do not reproduce; it is described in words above. Lumi analyses 42 of the 174 JEE Main shifts held from 2017 to 2026.