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AS Daily A Level Physics question

2026-04-05 OCR A Waves & Optics (M4) OCR Physics A (H156/H556) Module 4: Electrons, Waves and Photons 4.5 Quantum physics: photon model; E = hf; intensity related to photon number rate

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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