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AS Daily A Level Physics question

2026-04-04 OCR A Quantum (M4) OCR-A Module 4.3.1 Photoelectric effect: threshold frequency, work function, stopping potential; intensity effects

In a darkened lab, a vacuum photocell with a clean sodium cathode (work function 2.3 eV) is illuminated by monochromatic light of wavelength 400 nm. The light intensity is then doubled while the wavelength is unchanged. Which statement must be true?

  1. A Stopping potential rises to about 1.6 V because greater intensity gives more energy per electron; photocurrent stays about the same.
  2. B Stopping potential falls (to roughly 0.4 V) since the same light energy is shared between more emitted electrons at higher intensity.
  3. C Stopping potential remains near 0.8 V (set by photon energy minus work function), while the saturation photocurrent roughly doubles. (correct)
  4. D Both stopping potential and photocurrent are unchanged when intensity is doubled at fixed wavelength.

Answer

The correct answer is C.

Correct: C — Stopping potential remains near 0.8 V (set by photon energy minus work function), while the saturation photocurrent roughly doubles. A Light intensity does not increase photon energy, so the stopping potential does not double to 1.6 V; claiming the photocurrent is unchanged contradicts the intensity–current relationship. B Electrons do not share photon energy; each photon ejects at most one electron, so reducing the stopping potential to ~0.4 V is incorrect. C For 400 nm, photon energy ≈ 1240/400 ≈ 3.1 eV, so KE_max ≈ 3.1 − 2.3 = 0.8 eV giving a stopping potential ≈ 0.8 V, and doubling intensity increases the emission rate, so the saturation photocurrent roughly doubles. D Intensity affects the photocurrent, so stating no change to either quantity is wrong.

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