Doppler principleDoppler's principle

Doppler principle, Doppler's principle /dopˈlər(z) prinˈsi-pl/

noun

The law of change of wave frequency (Doppler effect or shift) when a source of vibrations is moving towards or from the observer, explaining the fall of pitch of a railway whistle when the engine passes, and enabling astronomers to measure the radial velocity of stars by the displacement of known lines of the spectrum

ORIGIN: From Christian Doppler (1803–53), Austrian physicist, who announced it in 1842