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AS Daily A Level Physics question

2026-05-27 OCR A Waves & Optics (M4) OCR A Physics Module 4: Electrons, waves and photons — 4.4 Quantum physics (photon model E = hf; threshold frequency; intensity vs photon rate)

A clean zinc surface in vacuum is illuminated in turn by two monochromatic LEDs, each adjusted to deliver the same power per unit area onto the metal. LED A emits light at 5.0×10^14 Hz; LED B emits at 1.0×10^15 Hz. The threshold frequency for zinc is 8.8×10^14 Hz. Which statement must be true?

  1. A Both LEDs cause emission because they deliver the same power; LED B just gives more energetic electrons.
  2. B Neither LED causes emission because the power is too small to supply the work function in a reasonable time.
  3. C Only LED B causes emission; at the same power its photon arrival rate is lower than LED A’s. (correct)
  4. D Only LED B causes emission; doubling the intensity of LED A would then also cause emission.

Answer

The correct answer is C.

Correct: C — Only LED B causes emission; at the same power its photon arrival rate is lower than LED A’s. A Both cannot cause emission because LED A’s frequency is below threshold, so no electrons are emitted regardless of power. B Emission depends on individual photon energy exceeding the threshold, not on total power building up over time, so LED B will cause emission. C LED B is above threshold so it ejects electrons; with equal power, its higher-energy photons mean fewer photons per second than LED A. D Increasing the intensity of LED A (still below threshold) cannot cause emission because the photon energy remains insufficient.

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