# Explain why the wavelength of a wave changes when it crosses from one medium into another, but its frequency does not.

> Edexcel GCSE Physics (1PH0) — 4.1 Waves · Explain · 4 marks

> When waves travel from air into water, they undergo refraction. Students often find it difficult to explain why only some wave properties change at the boundary.

## Mark scheme (4 marks)

1. Frequency is determined by the source of the wave and remains constant because the number of waves arriving at the boundary per second must equal the number leaving it.
2. When the wave enters a new medium, its speed changes (increases or decreases).
3. Because wave velocity equals frequency multiplied by wavelength (v = fλ), if speed changes but frequency stays constant, the wavelength must change.
4. If the wave slows down, the wavelength decreases; if the wave speeds up, the wavelength increases.

## Key terms

- [frequency](https://www.gradenine.co.uk/glossary/frequency)
- [wavelength](https://www.gradenine.co.uk/glossary/wavelength)
- [medium](https://www.gradenine.co.uk/glossary/medium)

## Related

- [Revision notes for Edexcel GCSE Physics (1PH0)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-wavelength-of-a-f87b8a47) · Published by Druglandscape Ltd.