# Explain why the speed of a wave changes when it passes from one medium into another, while its frequency remains constant.

> IB DP Physics Standard Level (2023 syllabus) — C.2 Wave model · Explain · 4 marks

> A sinusoidal water wave travels from deep water into a region of shallow water, where it slows down.

## Mark scheme (4 marks)

1. The frequency of a wave is determined by the source that produces it, not by the medium.
2. In the new medium the wave speed changes because the physical properties of the medium (e.g. depth/density/elasticity) are different.
3. The wave equation v = fλ links speed, frequency and wavelength.
4. Since frequency is constant and speed decreases, the wavelength must also decrease proportionally.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](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/explain-why-the-speed-of-a-2727daf3) · Published by Druglandscape Ltd.