The Doppler effect
2 marks · EasyExplain, using the idea of wavefronts, why the pitch of an approaching ambulance siren is higher than when it is stationary.
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Show answer
As the ambulance approaches, the wavefronts are closer together in front, making the wavelength shorter and frequency higher, so the pitch increases.
Also accept: Wavefronts are closer together when moving towards you, increasing frequency and raising pitch.; The moving source compresses the wavefronts in front, increasing frequency and pitch.
Common mistakes: Pitch gets lower; No change in wavefronts
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Marking points
M1 · 1 mark · Mentions wavefronts are closer together/compressed or distance between wavefronts decreases.
A1 · 1 mark · Links this to increase in frequency or pitch.
Worked solution
When the ambulance moves towards you, the wavefronts in front are closer together, meaning a shorter wavelength. This causes the frequency to increase, so the pitch is higher.