# A physicist is studying the photoelectric effect using a clean caesium metal surface. They use a source that emits red light of very high intensity and find that no electrons are emitted from the caesium surface. The physicist then replaces the red light with a source that emits blue light of much lower intensity and finds that electrons are emitted immediately. Explain why electrons are emitted with the blue light but not with the red light, even though the red light has a much higher intensity.

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

## Mark scheme (5 marks)

1. Light travels as photons (discrete packets/quanta of energy)
2. The energy of a photon depends on the frequency (or colour) of the light, not the intensity
3. Blue light has a higher frequency than red light, so each blue photon carries more energy
4. A single photon must have enough energy to exceed the work function of caesium to release one electron
5. Red photons have insufficient energy to overcome the work function, so no electrons are emitted regardless of intensity (more red photons cannot help because photons do not combine their energy)

## 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-physicist-is-studying-the-photoelectric-41aa845b) · Published by Druglandscape Ltd.