The Doppler effect is a shift in the observed frequency or wavelength of a wave caused by relative motion between an emitter and an observer. It was first described in 1842 by Christian Doppler, and later applied to light by Hippolyte Fizeau in 1848; for electromagnetic waves it is often called the Doppler–Fizeau effect. Applications span astronomy, meteorology, medicine, and navigation. Doppler effect | Definition, Example, & Facts – Encyclopaedia Britannica; Armand-Hippolyte-Louis Fizeau – Britannica.

History

In May 1842 Doppler presented "Über das farbige Licht der Doppelsterne..." proposing that motion shifts the color (frequency) of starlight; this became the conceptual basis for the phenomenon now bearing his name. Dutch scientist C. H. D. Buys Ballot tested the idea with sound in 1845, and Fizeau independently framed the effect for light in 1848, enabling stellar velocity measurements. Doppler effect – Britannica; On the coloured light of the binary stars… – overview; Fizeau biography – Britannica.

Physics and equations

For waves in a material medium (e.g., sound in air), the classical shift depends on the wave speed in that medium and the velocities of source and receiver along their line of sight. With wave speed v, source speed v_s (positive when moving away), receiver speed v_r (positive when moving toward), and emitted frequency f₀, the observed frequency is:

f_obs = ((v ± v_r) / (v ∓ v_s)) · f₀. This model captures the familiar change in a siren’s pitch as a vehicle approaches and recedes. University Physics, OpenStax; Physics – OpenStax.

For electromagnetic waves, no propagation medium is invoked; at significant fractions of light speed, Special relativity modifies the shift via time dilation. For purely longitudinal motion at speed v with β = v/c,

f_obs = f₀ · sqrt((1 ± β)/(1 ∓ β)),

with the plus sign for approach and minus for recession. A purely transverse case (motion perpendicular to the line of sight) yields a redshift by the Lorentz factor alone—the “transverse Doppler effect”—a key relativistic prediction. The Feynman Lectures on Physics, Vol. I, Ch. 34 – Caltech; HyperPhysics: Relativistic Doppler; Relativistic Doppler effect – overview.

Experimental confirmation of these relativistic shifts includes the Ives–Stilwell canal-ray measurements (1938) and Mössbauer rotor experiments in the 1960s, which probed time dilation with high precision. Optica history of JOSA highlights (Ives–Stilwell citation); Mössbauer effect – Britannica.

Spectroscopy and line broadening

In gases, thermal motion produces a distribution of emitter velocities; the superposition of many small Doppler shifts broadens spectral lines with a Gaussian profile (“Doppler broadening”). Measuring this width enables thermometry and velocity diagnostics. NIST provides working formulae for Doppler full width at half maximum (FWHM) and discusses related line-shape effects. Advanced “Doppler-free” laser techniques (e.g., saturation spectroscopy) resolve intrinsic transition frequencies by canceling first-order shifts. NIST Atomic Spectroscopy: Spectral Line Shapes; Spectroscopy: Techniques for obtaining Doppler-free spectra – Britannica; Deployable Doppler Thermometry – NIST.

Astronomical uses

Radial velocities of stars and galaxies are inferred from small displacements of spectral lines (blueshift toward shorter wavelengths for approach, redshift for recession), enabling detection of spectroscopic binaries, stellar rotation, and the discovery of exoplanets via the radial-velocity method. Early exoplanet finds were dominated by such “Doppler wobble” measurements, and modern spectrometers achieve sub–meter-per-second precision. ESA: Detecting exoplanets with radial velocity; NASA NEID technology highlight; NASA Exoplanet radial velocity resource.

On cosmological scales, the dominant redshift for distant galaxies arises from the expansion of space (cosmological redshift). Although it manifests as a wavelength stretch like a Doppler shift, its physical origin is the metric expansion of the universe. Hubble mission: Cosmological redshift – NASA; redshift – Britannica.

Related internal topics: Redshift, Exoplanet.

Meteorology and remote sensing

Weather surveillance networks use Doppler radar to measure not just the position of precipitation but its radial velocity, improving warnings for severe storms and tornadoes. NOAA’s NEXRAD (WSR‑88D) network transmits microwave pulses and retrieves velocity from the phase change between transmitted and returned signals; dual‑polarization upgrades add hydrometeor typing. NWS: How Does the Radar Work?; NOAA/NSSL: NEXRAD and dual-polarization; Scientific American: How Doppler radar works.

Medicine and biology

In Ultrasound imaging, Doppler techniques measure blood-flow direction and velocity by analyzing frequency shifts from moving red blood cells. Clinical variants include color Doppler, spectral Doppler, duplex ultrasound, and transcranial Doppler. Medical organizations describe uses in diagnosing arterial stenosis, venous insufficiency, and cardiac valve disorders. Mayo Clinic: Doppler ultrasound; MedlinePlus: Doppler Ultrasound; Merriam‑Webster Medical Dictionary: Doppler ultrasound.

Navigation and communications

Receivers in the Global Positioning System derive precise velocity by tracking the Doppler shift on satellite carrier signals; the effect is of order v/c ≈ 10⁻⁵ and is exploited by correlators searching frequency–time offsets. This Doppler-based velocity aids code tracking and supports real‑time kinematic applications. Physics Today: Relativity and the GPS; Penn State (GEOG 862): Doppler shift in GNSS; NPS: GPS overview (carrier Doppler for velocity).

Terminology and scope

The term “Doppler shift” is often used interchangeably with “Doppler effect.” For light, “Doppler–Fizeau effect” is common in French sources, reflecting Fizeau’s independent 1848 contribution. Observationally, “redshift” and “blueshift” refer to wavelength displacement toward longer or shorter values, respectively, with context distinguishing kinematic (Doppler), gravitational, and cosmological mechanisms. Doppler effect – Britannica; Hubble cosmological redshift – NASA.

Related internal topics: Special relativity, Ultrasound, Doppler radar, Global Positioning System.