# Explain why light below a certain frequency cannot release electrons from a metal surface, no matter how intense the light is.

> AQA A-Level Physics (7408) — 3.2.2 EM Radiation and Quantum Phenomena · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Each photon interacts with one electron (one-to-one interaction)
2. The energy of a photon depends on its frequency (E = hf / higher frequency means more energy per photon)
3. Electrons need a minimum amount of energy (the work function) to escape the metal surface
4. Below the threshold frequency, each photon does not have enough energy to overcome the work function
5. Increasing intensity increases the number of photons but not the energy of each photon, so electrons are still not released

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