A sound wave travels through air and then enters water. Describe and explain what happens to the speed, frequency, and wavelength of the sound wave as it moves from air into water.

Pearson Edexcel International GCSE Physics (4PH1) — 3.1 Properties of waves · Describe and explain · 4 marks · View as Markdown

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

Sound waves are longitudinal waves that can be refracted when they pass from one medium into another. Water is a denser medium than air.

Model answer (4 marks)

The speed of the sound wave increases as it enters the denser medium of water.
The frequency remains unchanged.
The wavelength increases.
The wavelength increases because speed = frequency × wavelength; with frequency constant and speed higher, wavelength must be longer.

Examiner tips

Use the formula v= fλ to link speed, frequency and wavelength.
State that frequency is fixed by the source.
Show the direction of change for each quantity.
Mention the denser medium causes higher speed.

Common mistakes

Saying the frequency changes.
Confusing wavelength with frequency.
Not linking the change in speed to the change in wavelength using the equation.

Mark scheme (4 marks)

The speed of the sound wave increases (as it enters the denser medium of water)
The frequency of the sound wave remains unchanged / stays the same
The wavelength of the sound wave increases
The wavelength increases because speed = frequency × wavelength, and since frequency is constant but speed increases, wavelength must increase

Key terms in this question

frequency · wavelength