Doppler Effect
Definition
The change in observed pitch (or color) of waves when the source and observer move toward or away from each other — the reason sirens drop in pitch as they pass.
Unit: hertz (Hz) — the shifted frequency
Technical definition
The shift in observed frequency when there is relative motion between wave source and observer: approaching motion compresses wavefronts (higher observed frequency); receding motion stretches them (lower observed frequency).
Example
An ambulance siren at 700 Hz sounds higher-pitched as it races toward you (wavefronts bunch up) and suddenly lower the instant it passes (wavefronts stretch out). Police radar guns use the same physics to clock speeding cars.
The Doppler effect does not change the wave's speed or the source's actual frequency — it changes only what the moving observer receives. The siren still blares 700 Hz; your motion through its wavefronts is what shifts the pitch. And it works for light too: receding galaxies are redshifted, which is how we know the universe is expanding.