# A student shines ultraviolet light onto a negatively charged metal plate. The plate loses its charge. When the student replaces the ultraviolet light with red light of much greater intensity, the plate no longer loses its charge. Explain these observations in terms of photons and the photoelectric effect.

> WJEC A-Level Physics (Wales) — 3.2 Photon and particle properties · Explain · 5 marks

## Mark scheme (5 marks)

1. Light (electromagnetic radiation) is composed of discrete packets of energy called photons
2. Each photon has a specific energy that depends on the frequency (not intensity) of the light
3. Ultraviolet photons have sufficient energy to exceed the work function of the metal, so electrons are released
4. Red light has a lower frequency than ultraviolet, so each red photon has less energy than a UV photon
5. Red photons cannot release electrons regardless of intensity because no single photon has enough energy to overcome the work function / electrons cannot accumulate energy from multiple photons

## Key terms

- [photon](https://www.gradenine.co.uk/glossary/photon)
- [photoelectric effect](https://www.gradenine.co.uk/glossary/photoelectric-effect)
- [intensity](https://www.gradenine.co.uk/glossary/intensity)

## 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-ultraviolet-light-onto-fad1e252) · Published by Druglandscape Ltd.