Doppler Effect

The Doppler effect is the difference between the frequency emitted by a source and the frequency detected by an observer when the two are in relative motion. A familiar example is a siren: its sound is heard at a higher pitch as it approaches and at a lower pitch after it passes. The siren itself need not change its emitted tone. For sound, a moving source changes the spacing of wavefronts in the medium, while a moving observer encounters wavefronts at a different rate. The medium matters: the frequency shift depends on whether the source, the observer, or the medium is moving, so the equations for moving sources and moving listeners are not interchangeable.

The same general phenomenon occurs with electromagnetic waves, including light. When a light source and observer move closer together, the observed wavelength is shorter; when they separate, it is longer. In visible light, these changes are commonly called blueshift and redshift, respectively. Because light in a vacuum does not travel through a material medium, its Doppler shift is described using relative motion and, at high speeds, the relativistic relationship between frequency, wavelength and velocity. Astronomers identify known spectral lines and measure their displacement to estimate an object's motion toward or away from the observer. This is its line-of-sight, or radial, motion—not by itself a measure of the object's full three-dimensional velocity.

Doppler measurements are useful wherever a frequency shift can reveal motion. Radar systems compare transmitted and returned waves to estimate the speed of vehicles; Doppler weather radar helps track the movement of precipitation. In medicine, Doppler ultrasound analyzes echoes from moving red blood cells to assess blood-flow speed and direction. Astronomers use shifts in stellar spectra to measure motion along the line of sight; tracking changes over time can also reveal the small motion of a star caused by an orbiting planet. These instruments apply the same broad principle to different kinds of waves, while their measurement methods and equations depend on the signal and the situation.

The effect is named for Austrian physicist Christian Doppler, who first described the phenomenon in 1842. Its applications now extend well beyond the sound and starlight examples that made the underlying frequency shift intuitive: the same principle supports measurements in acoustics, medicine, radar and astronomy.