Doppler effect
The change in pitch or frequency you notice when a source of waves moves towards or away from you.
The Doppler effect is the change in frequency of a wave for an observer who is moving relative to its source. When a source approaches, each wave crest is sent out from a little closer than the last, so the crests arrive bunched together and the frequency rises. As the source moves away, the crests are stretched apart and the frequency falls.
The everyday example is a passing ambulance: its siren sounds higher as it comes towards you, drops as it passes and stays lower as it drives off. The driver hears no change at all, because the siren is not moving relative to them.
Christian Doppler described the idea in 1842, writing about the colours of double stars. In 1845 the Dutch scientist Buys Ballot tested it with sound and found the pitch was higher than normal when the source approached and lower when it moved away. The same effect works for light: objects moving away from us look slightly redder (a redshift) and approaching ones slightly bluer.
A common misconception is that the source itself changes its note. It does not. The speed of the wave is set by the air or other medium, not by the source; only the spacing of the crests reaching you changes. Radar speed guns, weather radar and satellite tracking all make use of this shift.
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