Explain why light below a certain frequency cannot release electrons from a metal surface, no matter how intense the light is.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The photoelectric effect cannot be explained using wave theory. Instead, physicists use a model in which light travels as discrete packets of energy called photons.
Model answer (5 marks)
Each photon interacts with one electron (one‑to‑one interaction). The energy of a photon is E = hf, so a higher frequency gives more energy per photon. Electrons in a metal require a minimum energy, the work function φ, to escape. If the light frequency is below the threshold frequency ν₀, then E = hν < φ for every photon. Increasing the intensity raises the number of photons but not their individual energy, so none have enough energy to overcome φ and no electrons are ejected.
Examiner tips
- Use the one‑to‑one photon–electron interaction point
- State E = hf and explain the frequency dependence
- Mention the work function φ as the minimum energy needed
- Explain why below ν₀ photons lack sufficient energy
Common mistakes
- Confusing intensity with frequency, claiming higher intensity gives higher energy
- Saying photons can share energy with multiple electrons
- Forgetting to mention the work function φ
Mark scheme (5 marks)
- Each photon interacts with one electron (one-to-one interaction)
- The energy of a photon depends on its frequency (E = hf / higher frequency means more energy per photon)
- Electrons need a minimum amount of energy (the work function) to escape the metal surface
- Below the threshold frequency, each photon does not have enough energy to overcome the work function
- Increasing intensity increases the number of photons but not the energy of each photon, so electrons are still not released
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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