The Doppler effect
2 marks · NormalState what happens to the frequency of sound waves from a moving source as it moves towards a stationary observer. Explain your answer.
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Common mistakes: Saying frequency decreases; Mixing up observer/source movement
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Marking points
B1 · 1 mark · States that frequency increases as the source moves towards the observer.
B2 · 1 mark · Explains that this is because the wavefronts are compressed/shorter wavelength/arrive more frequently.
Worked solution
As the source of sound moves towards the observer, the frequency of the sound waves increases.
This is because the wavefronts are compressed in front of the moving source so they arrive at the observer more frequently (shorter wavelength, higher frequency).