# Explain why the photoelectric effect cannot be explained by the wave model of light, and how the photon model successfully accounts for the existence of a threshold frequency.

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

> When ultraviolet radiation is shone onto a zinc plate, electrons are emitted immediately. However, visible light of any intensity, no matter how long it is left shining on the zinc, fails to release any electrons at all.

## Mark scheme (5 marks)

1. The wave model predicts that energy is spread continuously across the wavefront, so any frequency of light should eventually supply enough energy to release an electron given sufficient time or intensity.
2. The immediate emission of electrons cannot be explained by the wave model, which would predict a delay as energy builds up.
3. In the photon model, light is quantised into photons, each carrying a discrete packet of energy equal to hf.
4. A single photon interacts with a single electron; if the photon's energy is less than the work function of the metal, the electron cannot be released regardless of how many photons (intensity) arrive.
5. The threshold frequency is the minimum frequency at which a photon has exactly enough energy to equal the work function, so below this frequency emission is impossible no matter the intensity.

## Key terms

- [photoelectric effect](https://www.gradenine.co.uk/glossary/photoelectric-effect)
- [photon](https://www.gradenine.co.uk/glossary/photon)
- [threshold frequency](https://www.gradenine.co.uk/glossary/threshold-frequency)
- [wave model](https://www.gradenine.co.uk/glossary/wave-model)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-photoelectric-effect-cannot-e7e24900) · Published by Druglandscape Ltd.