NEET 2024 · PhysicsPrevious Year Question
A light ray enters through a right-angled prism at point P with the angle of incidence 30° as shown in the figure. It travels through the prism parallel to its base BC and emerges along the face AC. The refractive index of the prism is:
Answer: (A) √5/2. Answer: (A) √5/2 Solution: At entry face: sin 30° = µ sin θ_c-related angle inside ⇒ the refracted ray inside travels parallel to base BC.
- A.√5/2✓
- B.√3/4
- C.√3/2
- D.√5/4
Correct Answer
(A) √5/2
Solution & Explanation
Answer: (A) √5/2 Solution: At entry face: sin 30° = µ sin θ_c-related angle inside ⇒ the refracted ray inside travels parallel to base BC. The ray then meets face AC and emerges grazing (along AC), i.e. it strikes AC at the critical angle so that the emergent ray runs along the face. Geometry with the entry refraction and the grazing-emergence condition gives: sin 30° = µ cos θ_c and sin θ_c = 1/µ. So 1/2 = µ cos θ_c = µ·√(1 − 1/µ²) = √(µ² − 1). Squaring: 1/4 = µ² − 1 ⇒ µ² = 5/4 ⇒ µ = √5/2.
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