# A student shines a beam of red light onto a metal surface. No electrons are emitted. The student then replaces the red light with ultraviolet radiation of the same intensity. This time, electrons are emitted from the surface. Explain why electrons are emitted when ultraviolet radiation is used but not when red light is used.

> WJEC A-Level Physics (Wales) — 2.7 Photons · Explain · 5 marks

## Mark scheme (5 marks)

1. Light (electromagnetic radiation) travels as packets of energy called photons
2. The energy of a photon depends on the frequency of the radiation
3. Ultraviolet radiation has a higher frequency than red light, so ultraviolet photons have more energy
4. Each electron can only absorb one photon at a time, so the electron needs a single photon with enough energy to escape the surface (the threshold/work function energy)
5. Red light photons do not have enough energy to release an electron, but ultraviolet photons do have sufficient energy to free an electron from the surface

## Key terms

- [ultraviolet radiation](https://www.gradenine.co.uk/glossary/ultraviolet-radiation)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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-student-shines-a-beam-of-c4b59905) · Published by Druglandscape Ltd.