A2 Daily A Level Physics question

2026-01-10 OCR A Quantum I: photon model; E = hf; intensity vs photon rate; threshold frequency (conceptual) 5.5.2(c)

Two monochromatic LEDs (red at 4.0×10^14 Hz and green at 6.0×10^14 Hz) are adjusted so that a calibrated light meter reads the same intensity at a photodiode for each LED. The photodiode absorbs all incident photons in both cases. Which statement must be true about the photon arrival rates at the photodiode?

A They are the same because equal intensity means the same number of photons per second.
B The green photons arrive at two-thirds the rate of the red photons. (correct)
C The green photons arrive 1.5 times faster than the red photons.
D No green photons are detected because their higher energy prevents absorption.

Answer

The correct answer is B.

Correct: B — The green photons arrive at two-thirds the rate of the red photons. A equal intensity does not imply equal photon count rate when photon energies differ; higher-frequency photons carry more energy each, so fewer are needed for the same intensity. B is correct because for the same intensity the photon rate is inversely proportional to photon energy, so rate ratio = f_red/f_green = 4.0/6.0 = 2/3. C inverts the dependence, wrongly assuming the photon rate increases with frequency for fixed intensity. D confuses energy per photon with detectability; higher-energy photons still arrive and are absorbed by the photodiode, so they are not excluded.

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