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 · View as Markdown

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

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.

Model answer (4 marks)

Frequency is set by the source and must be the same on both sides of the boundary, because the same number of wave crests reach the interface each second.
When the wave enters the new medium its speed changes (either increases or decreases).
Since wave speed v = fλ, a change in v with f constant forces λ to change.
If the wave slows, λ decreases; if it speeds up, λ increases.

Examiner tips

State that frequency is fixed by the source. Explain the speed change at the boundary. Use v = fλ to link speed, frequency and wavelength. Show the direction of change for λ when v changes.

Common mistakes

Claiming frequency changes. Forgetting to mention the speed change. Using the wrong equation (e.g. v = λ/f).

Mark scheme (4 marks)

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.
When the wave enters a new medium, its speed changes (increases or decreases).
Because wave velocity equals frequency multiplied by wavelength (v = fλ), if speed changes but frequency stays constant, the wavelength must change.
If the wave slows down, the wavelength decreases; if the wave speeds up, the wavelength increases.

Key terms in this question

frequency · wavelength · medium