The Doppler Effect
3 marks · NormalA train travels towards a platform at high speed.
Explain how the frequency of the sound heard by a person standing on the platform compares to the frequency emitted by the train.
If the train then stops, what happens to the sound's frequency for the person on the platform?
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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(a) The frequency heard is higher than the emitted frequency because the wavefronts are compressed as the train approaches.
(b) The sound frequency returns to its normal (emitted) frequency.
Common mistakes: Assuming frequency decreases when train approaches.; Not relating answer to observer's perspective.
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Marking points
M1 (a) · 1 mark · Mentions that frequency increases (or pitch is higher) as train approaches.
A1 (a) · 1 mark · Explanation: wavefronts are compressed / source moving towards observer causes higher frequency.
B1 (b) · 1 mark · States frequency returns to emitted value (original frequency) when train stops.
Worked solution
(a) The frequency of the sound heard by the person on the platform is higher than the actual frequency emitted by the train. This is because as the train moves towards the person, the wavefronts are compressed, resulting in a higher frequency.
(b) If the train stops, the sound waves are no longer compressed, so the observer now hears the actual emitted frequency of the train.