IGCSE Physics / Properties of waves
The Doppler effect revision notes
Edexcel International GCSE Physics (4PH1), chapter 10: Properties of waves, specification point 3.8. Part of the full course.
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The Doppler effect is the change in the observed frequency and wavelength of a wave when its source moves towards or away from an observer. It explains why a siren sounds higher as it approaches and lower as it moves away, even though the wave speed stays the same.