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AS Daily A Level Physics question

2026-06-12 OCR A Quantum (M4) OCR-A Module 4.5.1 Quantum physics — photon model; E = hf; intensity vs photon rate (qualitative)

In a darkened lab, a student shines a monochromatic green LED onto a small photodiode. They increase the LED’s intensity using its driver, while keeping the colour exactly the same. Ignoring heating, which statement must be true at the photodiode?

  1. A Each photon carries more energy because the light is brighter.
  2. B The light’s frequency increases, so each photon has greater energy.
  3. C The colour shifts towards blue while the photon arrival rate stays the same.
  4. D More photons per second reach the detector, but energy per photon is unchanged. (correct)

Answer

The correct answer is D.

Correct: D — More photons per second reach the detector, but energy per photon is unchanged. A says photon energy rises with brightness, but photon energy depends only on frequency, which is fixed here. B claims the frequency changes with intensity, which is false for a source with fixed colour. C suggests a colour shift (towards blue) at constant photon rate, but monochromatic colour is held fixed; changing intensity alters rate, not colour. D states the correct link: at fixed frequency, increasing intensity increases photon flux without changing energy per photon.

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