The Doppler effect
3 marks · NormalDescribe a simple experiment you could carry out to demonstrate the Doppler effect for sound. Explain what you would expect to observe.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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Common mistakes: Not referencing motion; Describing echo, not Doppler
Show mark scheme and worked solution
Marking points
B1 · 1 mark · Describes a set-up with a sound source moving past an observer (e.g., bell on string, tuning fork on moving toy, phone in cup).
B2 · 1 mark · States the observer listens for change in pitch/frequency as it approaches and then moves away.
B3 · 1 mark · Explains pitch is higher as the source approaches, lower as it moves away, due to compressed/stretch out waves.
Worked solution
Tie an electric bell or phone in a cup to a string. Ask a person to swing it in a circle or walk past an observer while the bell is ringing.
The observer should notice that as the bell moves towards them, the pitch sounds higher, and as it moves away, the pitch is lower.
This is because the motion causes the wavefronts to be compressed (higher frequency) when coming closer and stretched out (lower frequency) when moving away.