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A2 Daily A Level Physics question

2026-03-09 OCR A Medical imaging: Ultrasound (M6.3) OCR-A H556 Module 6.3 Medical imaging OCR-A 6.3.2 Ultrasound: acoustic impedance, reflection/transmission at boundaries, matching

During a diagnostic ultrasound exam, a pulse at normal incidence encounters a boundary between muscle (acoustic impedance 1.6 Mrayl) and bone (7.8 Mrayl). Neglecting absorption and scattering, which is the best estimate of the fraction of incident intensity reflected at the boundary?

  1. A About 56%
  2. B About 66%
  3. C About 44% (correct)
  4. D About 21%

Answer

The correct answer is C.

Correct: C — About 44%. A … This is the approximate transmitted fraction (1 − 0.44 ≈ 0.56), not the reflected part. B … This equals the magnitude of the amplitude ratio (|ΔZ|/(Z1+Z2) ≈ 0.66) without squaring to get intensity. C … For normal incidence the reflected intensity fraction is (|Z2 − Z1|/(Z1 + Z2))^2 = (6.2/9.4)^2 ≈ 0.44. D … This confuses impedance ratio with reflection; Z1/Z2 ≈ 0.21 is not the intensity reflection coefficient.

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