AS Daily A Level Physics question

2026-05-02 OCR A Waves (Module 4): Wave optics I — total internal reflection (TIR) Module 4: Waves — 4.4.2 Refraction at boundaries; critical angle and total internal reflection Module 4: Waves — Practical: measuring critical angle with a semicircular block (ray box)

In a school lab, a narrow laser beam travels inside a solid acrylic block (refractive index 1.49) and strikes the top flat surface to air. Inside the block the beam makes an angle of 42° to the surface at the point of incidence. Which statement must be true about what happens at that surface?

A It refracts into the air at about 30° to the normal, because 42° is below the critical angle.
B It undergoes total internal reflection, because the angle to the normal (90°−42°=48°) exceeds the critical angle (≈42° for acrylic–air). (correct)
C It refracts into the air but bends towards the normal, since air is optically less dense than acrylic.
D It splits into reflected and refracted rays, with the refracted ray at about 70° to the normal, because the inside angle is above the critical angle.

Answer

The correct answer is B.

Correct: B — It undergoes total internal reflection, because the angle to the normal (90°−42°=48°) exceeds the critical angle (≈42° for acrylic–air). A uses the 42° given to the surface as if it were to the normal; with the correct angle (48°) and sin c ≈ 1/1.49 ≈ 0.67, c ≈ 42°, so refraction cannot occur. B is consistent: 48° > 42° gives total internal reflection. C is wrong because going from acrylic to air (lower refractive index) would bend away from the normal if refraction occurred, not towards it, and here the incidence exceeds the critical angle. D is wrong because above the critical angle there is no refracted ray at all; suggesting ~70° to the normal also contradicts the required bending away from the normal and the fact that refraction is impossible here.

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