The Doppler effect
2 marks · EasyA stationary ambulance travels past a person. While the ambulance moves away, the person notices a change in the siren’s pitch.
Describe what happens to the pitch of the siren as the ambulance moves away.
Explain why this change happens, using wavefronts in your explanation.
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(a) The pitch decreases (is lower).
(b) As the ambulance moves away, wavefronts are stretched so wavelength increases, frequency decreases and so does pitch.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States pitch is lower/decreases.
B1 (b) · 1 mark · Explains using wavefronts that they are stretched, so frequency and pitch decrease.
Worked solution
a) The pitch is lower as the ambulance moves away. b) The wavefronts are stretched behind the ambulance, so wavelength increases and frequency (and pitch) decrease.