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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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

Model answer (4 marks)

The frequency of a wave is set by the source that generates it and does not depend on the medium.
When the wave enters a different medium the physical properties (e.g. depth, density, elasticity) change, so the wave speed v changes.
The wave equation v = fλ shows that speed, frequency and wavelength are related.
Because f is unchanged and v decreases, λ must decrease proportionally.

Examiner tips

State that frequency is fixed by the source. Explain that speed changes due to different medium properties. Use the wave equation to link v, f and λ. Show that λ must change when v changes while f stays constant.

Common mistakes

Claiming the frequency changes with the medium. Forgetting to mention the wave equation. Saying the wavelength increases instead of decreases.

Mark scheme (4 marks)

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

Key terms in this question

frequency · medium