What Is the Doppler Effect?

When a sound, light, or other wave source moves toward an observer, the waves are compressed, increasing their frequency (higher pitch for sound, bluer shift for light). When it moves away, waves are stretched, decreasing frequency (lower pitch, redder shift). Named after Austrian physicist Christian Doppler, who proposed it in 1842, the effect is heard in the changing pitch of a passing ambulance siren and seen in the redshift of galaxies moving away from us. The Doppler effect is used in radar guns (measuring vehicle speed), weather radar (detecting storm motion), and medical ultrasound (measuring blood flow velocity).