# A metal surface is illuminated with visible light of high intensity, but no electrons are emitted. When the same surface is illuminated with ultraviolet light of much lower intensity, electrons are emitted immediately. Explain why electrons are emitted with ultraviolet light but not with visible light, and why the emission is immediate even at low intensity.

> Eduqas A-Level Physics — 3.2 Photon and particle properties · Explain · 5 marks

## Mark scheme (5 marks)

1. Light is made up of photons (packets/quanta of energy)
2. The energy of a photon depends on its frequency (higher frequency = higher energy photon)
3. Each electron can only absorb one photon at a time, so the photon must have enough energy to overcome the work function
4. Ultraviolet photons have enough energy to exceed the work function / visible light photons do not have enough energy to release electrons regardless of intensity
5. Emission is immediate because a single ultraviolet photon can instantly transfer all its energy to one electron, so there is no delay

## Key terms

- [intensity](https://www.gradenine.co.uk/glossary/intensity)
- [emission](https://www.gradenine.co.uk/glossary/emission)

## Related

- [Revision notes for Eduqas A-Level Physics](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-metal-surface-is-illuminated-with-32cdde02) · Published by Druglandscape Ltd.