Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
4.16P Describe how changes, if any, in velocity, frequency and wavelength,…
Full specification wording
Topic 4 – Waves: 4.16P Describe how changes, if any, in velocity, frequency and wavelength, in the transmission of sound waves from one medium to another are inter-related
When a sound wave crosses from one medium into another, its frequency stays the same because the frequency is set by the source and the boundary does not create or destroy oscillations.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 15 of 15
Study notes
From the checked teaching notes for this specification point.
The idea
When a sound wave crosses from one medium into another, its frequency stays the same because the frequency is set by the source and the boundary does not create or destroy oscillations. What changes is the speed of the wave, which depends on the medium: sound travels faster in a solid than in a gas because particles are closer and transmit vibrations more quickly. Since wave speed equals frequency multiplied by wavelength (v = f × λ), a fixed frequency means wavelength must change in proportion to speed. Entering a faster medium, the wave speeds up and its wavelength increases; entering a slower medium, it slows and its wavelength shortens. Velocity here means the wave's speed through the medium, not a change in direction.
What earns the marks
- States that the frequency of the sound wave remains unchanged when it passes from one medium to another.
- Explains that the frequency is fixed by the source and the boundary does not alter the number of oscillations per second.
- Identifies that the wave speed (velocity) changes because it depends on the properties of the new medium.
- Applies the relationship wave speed = frequency × wavelength to show that wavelength changes when speed changes at constant frequency.
- Deduces that wavelength increases when the wave enters a faster medium and decreases when it enters a slower medium.
Common mistakes and how to put them right
Assumes the frequency changes at the boundary.
Put it right: Keep the frequency constant and change the speed and wavelength instead.
Treats wavelength as fixed and lets frequency absorb the change.
Put it right: Hold frequency constant and adjust wavelength in proportion to speed.
Confuses wave velocity with the speed of the vibrating particles.
Put it right: Describe velocity as the speed at which the wave pattern travels through the medium.
Exam tips
- Anchor the answer on the rule that frequency is unchanged, then reason about speed and wavelength from v = f × λ.
- Name the direction of change explicitly: faster medium gives longer wavelength, slower medium gives shorter wavelength.
- Read the question for the word 'if any' and be ready to state that frequency shows no change at all.
Try the lesson
The start of the Topic 4 – Waves: 4.16P Describe how changes, if any, in velocity, frequency and wavelength, in the transmission of sound waves from one medium to another are inter-related lesson, made from checked teaching notes for this specification point.
Have a go first
A sound wave passes from air into water. Which quantity stays the same as the wave crosses the boundary?
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