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AS Daily A Level Physics question

2025-08-25 OCR A A Level Physics 4.5.2(c) 4.5.3(c)

A clean metal photocathode in vacuum is illuminated first by a violet LED (Source 1), then by a green LED (Source 2). For this metal, both LEDs have frequencies above the threshold. The experimenter adjusts the intensity for each source so that the measured photocurrent is the same in both cases (well below saturation). Which statement must be true when switching from Source 1 to Source 2?

  1. A The stopping potential decreases and the de Broglie wavelength of the emitted electrons increases. (correct)
  2. B The stopping potential increases because the intensity is higher for Source 2 to keep the current the same.
  3. C Both the stopping potential and the de Broglie wavelength decrease when the light frequency is reduced.
  4. D If the green source were below threshold, increasing intensity would still eject electrons, so the stopping potential would be unchanged.

Answer

The correct answer is A.

Correct: A — The stopping potential decreases and the de Broglie wavelength of the emitted electrons increases. Lower frequency (still above threshold) means lower maximum photoelectron kinetic energy, so a smaller stopping potential; lower speed also means a larger de Broglie wavelength. A … correct. B … wrongly links stopping potential to light intensity; intensity changes the emission rate (current), not the energy per photon or max electron KE. C … reverses the de Broglie dependence: reducing frequency lowers electron speed, so wavelength increases, not decreases; stopping potential also does not increase with lower frequency. D … misunderstands threshold frequency: if the light were below threshold, no electrons are emitted no matter how intense the light, so there would be no stopping potential to measure.

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