The Doppler effect
3 marks · NormalExplain why the frequency of a siren heard by a person changes when a fire engine moves past them at a constant speed.
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Common mistakes: Confusing pitch and loudness; Saying only approaching changes frequency
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Marking points
B1 · 1 mark · States that when the fire engine is approaching, the frequency is higher.
B2 · 1 mark · States that when it moves away, the frequency is lower.
B3 · 1 mark · Describes this is caused by waves compressing ahead and stretching out behind due to the source's motion.
Worked solution
When the fire engine moves towards the person, the sound waves are compressed, causing the frequency (pitch) heard to increase.
As the fire engine moves away, the waves are stretched out and the person hears a lower frequency (pitch).
This change is due to the Doppler effect.