The Doppler effect
3 marks · NormalA stationary ambulance is sounding its siren at a constant frequency. Describe and explain what happens to the pitch of the siren for an observer as the ambulance starts to move away from them.
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.
Show answer
Common mistakes: Saying pitch increases; Talking about loudness not frequency
Show mark scheme and worked solution
Marking points
B1 · 1 mark · States that the pitch decreases (or frequency gets lower).
B2 · 1 mark · Explains this is because the waves are stretched out behind the moving source.
B3 · 1 mark · Links this to a longer wavelength/lower frequency being heard.
Worked solution
The pitch of the siren decreases as the ambulance moves away.
This is because the sound waves are stretched out behind the ambulance, increasing their wavelength.
A longer wavelength means a lower frequency (so a lower pitch is heard by the observer).