The Doppler effect: why a siren’s pitch changes

The Doppler effect is a change in the frequency of a wave heard or measured by an observer when the source, the observer, or both move relative to one another. As a car sounding a siren approaches a stationary listener, the sound has a higher pitch; after it passes, the pitch is lower. The siren can keep emitting the same tone: motion changes the spacing of the sound waves reaching the listener. Sound depends on motion relative to the air, while light in vacuum depends on the relative motion of source and observer as described by special relativity. [1–3]