A student is investigating the emission of electrons from a metal surface. They find that green light does not cause any electrons to be emitted from the surface, no matter how long the light is shone onto it. However, when violet light is used instead, electrons are emitted immediately. Explain why green light fails to release electrons from this metal surface, and why violet light is able to do so.

WJEC A-Level Physics (Wales) — 3.2 Photon and particle properties · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Light travels as photons, which are packets of energy. The energy of a photon is given by E=hf, where f is the frequency. Violet light has a higher frequency than green light, so violet photons carry more energy. Each photon can interact with only one electron. If a photon does not have enough energy to overcome the metal’s work function, no electron is released. Violet photons have sufficient energy to meet or exceed the work function, so electrons are emitted immediately.

Examiner tips

  • Use the equation E=hf to link frequency and photon energy
  • Explain the work function and its role in electron emission
  • Show the logical sequence: higher frequency → higher energy → enough to overcome work function

Common mistakes

  • Confusing wavelength with frequency; remember E=hf, not E=hc/λ
  • Assuming all photons can release electrons regardless of energy
  • Failing to mention the work function as the threshold energy

Mark scheme (5 marks)

  1. Light travels as photons (packets/quanta of energy)
  2. The energy of a photon depends on the frequency of the light
  3. Violet light has a higher frequency than green light, so violet photons carry more energy
  4. Each photon interacts with one electron — if a single photon does not have enough energy to overcome the work function, no electron is released
  5. Violet photon energy meets or exceeds the work function, so electrons are released immediately

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