Doppler Shift, Redshift & Blueshift: Structures & Vocabulary

The Doppler effect changes the observed wavelength of radiation when a source and observer have relative motion along the line of sight. Receding sources shift spectral features toward longer wavelengths, while approaching sources shift them toward shorter wavelengths.

For ordinary Doppler motion, successive wavefronts arrive more widely spaced when the source recedes and more closely spaced when it approaches. At cosmological distances, redshift also reflects the expansion of space and requires a relativistic cosmological interpretation rather than the simple low-speed formula.

Spectrographs measure line positions with high precision. Binary-star velocities, exoplanet reflex motion, galaxy rotation, and cosmic expansion are all inferred from systematic wavelength shifts matched to known spectral features.

Redshift is a measured ratio, commonly z = (λobs - λrest)/λrest. The same observed z can require different physical interpretations depending on regime: classical Doppler motion for modest speeds, special-relativistic Doppler formulas near light speed, gravitational redshift in curved spacetime, and cosmological redshift for expansion over large distances. Using one word for these effects does not mean they share one mechanism.