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

> IB DP Physics Higher Level (2023 syllabus) — E.2 Quantum physics (HL only) · Explain · 4 marks

> When light is incident on a metal surface, electrons are emitted only if the frequency of the light exceeds a certain minimum value known as the threshold frequency. This is true regardless of the intensity of the incident light.

## Mark scheme (4 marks)

1. In the classical wave model, energy is delivered continuously to the surface, so electrons should eventually be emitted at any frequency given sufficient time/intensity.
2. The classical model therefore cannot explain why, at low frequencies, no electrons are emitted even with high-intensity illumination over long periods.
3. In the photon model, light consists of discrete quanta (photons), each carrying energy E = hf, so a single photon must have sufficient energy to liberate one electron.
4. The threshold frequency corresponds to the minimum photon energy equal to the work function (hf₀ = φ), so below this frequency no individual photon can supply enough energy to overcome the surface binding energy, regardless of intensity.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](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-existence-of-a-b6e75e07) · Published by Druglandscape Ltd.