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AS Daily A Level Physics question

2026-03-10 OCR A Waves (M4) OCR-A Module 4.4.2 Wave behaviour: refraction, Snell’s law and total internal reflection; critical angle; applications to optical fibres

A step-index optical fibre has a core of refractive index 1.48 and a cladding of refractive index 1.44. A light ray inside the core strikes the core–cladding boundary at 75° to the normal. Which statement must be true?

  1. A It totally internally reflects because 75° is greater than 45°, so the ray cannot escape.
  2. B It refracts into the cladding; using sin c = 1.44/1.48 ≈ 0.97 gives c ≈ 77°, and 75° is below this. (correct)
  3. C It totally internally reflects because the core has a higher refractive index than the cladding, whatever the angle.
  4. D It totally internally reflects because the critical angle is about 42° if the outside were air, and 75° exceeds this.

Answer

The correct answer is B.

Correct: B — It refracts into the cladding; using sin c = 1.44/1.48 ≈ 0.97 gives c ≈ 77°, and 75° is below this. A The 45° idea is a common myth; the critical angle depends on the refractive-index ratio, not a fixed number. B This matches the calculated critical angle and the condition for total internal reflection, so it is correct. C A higher core index is necessary but not sufficient; TIR only occurs when the incidence angle exceeds the critical angle. D This wrongly treats the outside as air; using n = 1 for air gives c ≈ 43°, but the boundary here is with cladding (n = 1.44), so that reasoning does not apply.

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