Doppler effect

The change in a wave’s observed frequency caused by relative motion between its source and observer. Approaching motion raises the observed frequency; receding motion lowers it.

Wavefront: A surface or line joining points on a wave that share the same phase. Motion changes the spacing between successive wavefronts reaching an observer.
Redshift: An increase in the wavelength of electromagnetic radiation, shifting it toward the red end of the spectrum. Astronomers use Doppler shifts to infer motion away from an observer.
Christian Doppler: An Austrian physicist who proposed in 1842 that relative motion changes the observed frequency of waves. The effect takes its name from his theoretical account of frequency shifts.
Relativistic Doppler effect: The frequency shift produced by relative motion when special relativity must be accounted for. It replaces classical Doppler formulas when relative speeds approach light speed.
Radial velocity: The component of an object's velocity along the line of sight from an observer. A frequency shift measures motion along the line of sight, not transverse motion alone.
Wave velocity: The speed and direction at which a disturbance propagates through a medium or space. The observed shift depends on wave speed relative to source and observer motion.
Blueshift: A decrease in the wavelength of electromagnetic radiation, shifting it toward the blue end of the spectrum. It marks motion toward an observer in the line-of-sight direction.
Doppler's principle: Christian Doppler’s 1842 proposal that relative motion changes the observed frequency of waves. It introduced the central claim later tested for sound and extended to light.
Gravitational redshift: A change in light frequency caused by differences in gravitational potential. It shifts light without requiring relative motion between source and observer.
Proper motion: The apparent angular change in a star’s position across the sky over time. Unlike Doppler measurements, it tracks motion across the line of sight.

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