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AS Daily A Level Physics question

2026-04-05 OCR A Waves & Optics (M4) 4.5.1(c)

Two monochromatic LEDs (A and B) each shine steadily on the same small photodiode. The photodiode receives the same power from each LED (3.0 mW). LED B emits light of frequency 1.5 times that of LED A. Which statement must be true about the rate at which photons from each LED reach the photodiode?

  1. A LED B delivers about two-thirds as many photons per second as LED A. (correct)
  2. B LED B delivers about one and a half times as many photons per second as LED A.
  3. C They deliver the same number of photons per second because the received power is the same.
  4. D It cannot be determined without knowing the photodiode’s area.

Answer

The correct answer is A.

Correct: A — LED B delivers about two-thirds as many photons per second as LED A. A The photon rate is inversely proportional to photon energy, so with the same received power and f_B = 1.5 f_A, N_B/N_A = f_A/f_B = 1/1.5 ≈ 2/3. B This reverses the ratio: higher frequency means higher energy per photon, so fewer photons at the same power, not more. C Equal power does not imply equal photon counts when the photon energies differ. D The area is irrelevant here because the power arriving at the photodiode is already specified as equal for both LEDs.

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