Practice question · Multiple choice
An ambulance siren drops in pitch as it passes. Nothing about the siren changed. What did?
Hints
- The source emits a crest every fixed interval. Ask where the source is when it emits the next one.
- Wave speed in air is set by the air, not by the source. What is left to change?
Show the answer
C. The rate at which wavefronts reach you, bunched or stretched
Why
The source chases its own wavefronts, bunching them ahead and stretching them behind, so the received frequency differs from the emitted one while the wave speed is unchanged. The same effect red-shifts distant galaxies, which is how we know the universe is expanding.
Practise Waves
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