Why does an ambulance siren change pitch as it passes you?

An ambulance siren sounds higher as it races toward you and drops lower the moment it passes, even though the siren itself never changes. The effect is named after physicist Christian Doppler, who described it in 1842: it's the change in the frequency of a wave for an observer moving relative to the wave's source. Compared to the sound actually emitted, what you hear is higher-pitched while the source approaches, exactly matched at the instant it passes you, and lower-pitched as it recedes.

The reason is purely geometric. When the siren is moving toward you, each new sound wave is emitted a little closer to you than the last one, so the waves bunch up and arrive more often — a higher frequency. When it moves away, each wave starts a little farther out, so they arrive more spread out — a lower frequency. Doppler proposed this for starlight in 1842; three years later the Dutch meteorologist Buys Ballot tested it with sound and confirmed pitch really does rise on approach and fall on retreat.

The same shift works for light, not just sound, and that's where it gets powerful. Astronomers use the Doppler shift in starlight to measure how fast stars and galaxies are approaching or receding from us, producing a blueshift or redshift. It can reveal that what looks like one star is really a close binary pair, measure how fast a galaxy spins, or even detect exoplanets. One caution: the redshift caused by the expansion of the universe is not simply this kind of Doppler shift, and cosmologists treat it as a separate effect.

Police speed radar runs on the same idea, with radio waves instead of sound: as a car moves away, each successive radar wave has to travel farther before bouncing back, which stretches the returning wavelength and lets the radar work out the car's speed. Doctors use the effect too, to measure the speed of blood during ultrasound scans in obstetrics or cardiology.