A student is investigating the photoelectric effect using two different metals. Metal A releases electrons when exposed to violet light but not when exposed to green light. Metal B releases electrons when exposed to ultraviolet light but not when exposed to violet light. Explain why electrons are released from Metal A by violet light but not by green light, and why Metal B requires ultraviolet light to release electrons.

Eduqas A-Level Physics — 2.7 Photons · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In the photoelectric effect, light shining on a metal surface can cause electrons to be emitted. Whether electrons are emitted depends on the frequency of the light used and the metal being investigated.

Model answer (5 marks)

Metal A has a work function that can be overcome by the energy of a violet photon but not by a green photon. Light consists of photons; the energy of each photon is E=hf, so violet light (higher frequency) carries more energy than green light. An electron can absorb only one photon, so the photon energy must be at least equal to the metal’s work function. For Metal A the violet photon energy exceeds this threshold, releasing electrons, whereas the green photon energy is below the threshold, so no electrons are emitted.

Metal B has a larger work function than Metal A. The energy of a violet photon is insufficient to reach this higher threshold, but ultraviolet light has a higher frequency and therefore a higher photon energy that exceeds Metal B’s work function, allowing electrons to be emitted.

Examiner tips

  • Use the photon energy equation E=hf to link frequency to energy.
  • Show the comparison of photon energy with each metal’s work function.
  • Explain the one‑photon absorption rule.
  • Mention the higher work function of Metal B as the reason for needing UV light.

Common mistakes

  • Claiming electrons absorb multiple photons to reach the work function.
  • Using the wrong direction of comparison (e.g., saying green has higher energy than violet).
  • Ignoring the work function concept and only citing colour differences.

Mark scheme (5 marks)

  1. Light exists as discrete packets of energy called photons
  2. The energy of a photon depends on its frequency — higher frequency means higher energy photon
  3. Each electron can only absorb one photon, so the energy of a single photon must be sufficient to release an electron
  4. Green light photons do not have enough energy to release electrons from Metal A / violet photons have sufficient energy to overcome the energy needed to free an electron from Metal A
  5. Metal B requires a higher minimum energy to release electrons than Metal A, so only the higher frequency ultraviolet photons have sufficient energy

Key terms in this question

photoelectric effect

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