Doppler Effect

The Doppler effect is the change in a wave's observed frequency caused by relative motion between its source and its observer. A source moving toward an observer compresses each successive wave crest closer to the one before it, raising the frequency the observer receives; a source moving away stretches that spacing, lowering it. The wave's actual speed through the medium doesn't change — only how often crests arrive at a given point does.

The everyday example is a passing siren: its pitch drops noticeably as it passes, not because the siren changes what it emits but because the observer's position relative to the source flips from closing to receding at the moment of passing. Police radar guns use the same effect in reverse — a radio wave reflected off a moving car returns shifted in frequency, and the size of the shift gives the car's speed directly.

The effect scales with the ratio of relative velocity to wave speed, which is why it is imperceptible for slow-moving sound sources but dramatic for light from receding galaxies: redshift from distant galaxies is better described as light stretched to longer (redder) wavelengths along with expanding space during its transit — Doppler's close cousin rather than the identical mechanism — and it is the primary evidence for cosmic expansion. Light's version requires relativistic correction at high speeds — because there is no medium for light to move "through," the shift depends only on relative velocity between source and observer, unlike sound, where the medium's own rest frame matters and produces a subtly asymmetric formula depending on which side (source or observer) is moving.