# A scientist is investigating the photoelectric effect using a clean zinc metal surface. When ultraviolet light is directed at the zinc surface, electrons are emitted immediately. The scientist then reduces the intensity of the ultraviolet light to a very low level. Explain why electrons are still emitted immediately, even at very low intensity, and why a beam of visible light of much greater intensity produces no electron emission at all from the same surface.

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

## Mark scheme (5 marks)

1. Light (electromagnetic radiation) consists of discrete packets of energy called photons
2. The energy of a photon depends on its frequency (higher frequency = higher energy photon)
3. Each electron interacts with a single photon, so if that one photon has enough energy to overcome the work function, the electron is emitted immediately
4. Ultraviolet photons have sufficient energy to exceed the work function of zinc, so electrons are released
5. Visible light photons have lower frequency than ultraviolet, so each photon has insufficient energy to overcome the work function, and no electrons are emitted regardless of intensity

## Key terms

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

## 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-scientist-is-investigating-the-photoelectric-fbb4e383) · Published by Druglandscape Ltd.