Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
7.11P Describe that if a wave source is moving relative to an observer…
Full specification wording
Topic 7 – Astronomy: 7.11P Describe that if a wave source is moving relative to an observer there will be a change in the observed frequency and wavelength
When a wave source moves relative to an observer, the waves reaching the observer are compressed or stretched, so the observed frequency and wavelength differ from those emitted.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 10 of 18
Study notes
From the checked teaching notes for this specification point.
The idea
When a wave source moves relative to an observer, the waves reaching the observer are compressed or stretched, so the observed frequency and wavelength differ from those emitted. If the source moves towards the observer, each successive wavefront starts closer, so the observed wavelength is shorter and the observed frequency is higher. If the source moves away, successive wavefronts start further apart, so the observed wavelength is longer and the observed frequency is lower. The wave speed in the medium stays the same, so frequency and wavelength change in opposite directions. For light from a receding galaxy the observed spectrum shifts towards the red (longer wavelength, lower frequency).
What earns the marks
- States that a source moving towards an observer produces a shorter observed wavelength and a higher observed frequency.
- States that a source moving away from an observer produces a longer observed wavelength and a lower observed frequency.
- Explains that the observed change arises because successive wavefronts are crowded together or spread apart as the source moves.
- Recognises that frequency and wavelength change in opposite directions while the wave speed stays constant.
Common mistakes and how to put them right
Claims that the observed frequency rises when the source moves away.
Put it right by linking motion away to longer wavelength and lower frequency.
Assumes the wave speed changes with the source motion.
Put it right: Keep the speed fixed and change frequency and wavelength instead.
Treats the effect as a change in the source's emitted frequency.
Put it right: Describe it as a change in the frequency observed because of relative motion.
Exam tips
- Decide the direction of relative motion first, then apply the rule: towards means shorter wavelength and higher frequency.
- Check that your chosen frequency and wavelength move in opposite directions, since speed is constant.
- Link the answer to a real case such as the red-shift of light from a receding galaxy to show understanding.
Try the lesson
The start of the Topic 7 – Astronomy: 7.11P Describe that if a wave source is moving relative to an observer there will be a change in the observed frequency and wavelength lesson, made from checked teaching notes for this specification point.
Have a go first
A wave source moves towards a stationary observer. What happens to the frequency and wavelength observed?
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