AS Daily A Level Physics question
A narrow laser beam travels inside a glass block (refractive index 1.50) and strikes the glass–air boundary from the glass side. As the angle of incidence inside the glass is slowly increased, at roughly what angle to the normal will total internal reflection just begin?
Answer
The correct answer is D.
Correct: D — about 42° to the normal. At the onset of total internal reflection from glass to air, sin(critical angle) ≈ n_air/n_glass = 1.00/1.50 ≈ 0.67, so the angle is arcsin(0.67) ≈ 42°. A is wrong because 30° would require sinθ ≈ 0.5, implying n_air/n_glass ≈ 0.5, which is too small for glass–air. B is wrong because ~48° is close to the water–air critical angle (n≈1.33), not glass. C is wrong because 60° (sin60° ≈ 0.87) would imply n_air/n_glass ≈ 0.87, inconsistent with glass being much denser optically than air.