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Total internal reflection occurs when light travels from medium of refractive index n₁ to n₂ (n₁ > n₂) at angle greater than critical angle θ_c. Critical angle satisfies:
Asin θ_c = n₁/n₂
Bsin θ_c = n₂/n₁
Ccos θ_c = n₂/n₁
Dtan θ_c = n₁
Answer & Solution
Correct answer: B. sin θ_c = n₂/n₁
At critical angle, refracted ray grazes interface (θ₂ = 90°). Snell: n₁ sin θ_c = n₂ sin 90° = n₂. So sin θ_c = n₂/n₁. For water-air: sin θ_c = 1/1.33 → θ_c ≈ 48.75°.
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