Doppler effect



  • The shifting of the wavelength of wave energy caused by relative motion between source and observer. Observed on the largest possible scale in the expansion of the universe, the Doppler effect lengthens the wavelength of light if source and observer are moving away from each other and shortens it if they are approaching. Astronomers call this a redshift for recession and a blueshift for approach, since red light has the longest wavelengths in the visible wavebands and blue light the shortest. The Doppler effect can be used to unravel many astronomical problems, e.g. to measure the rotation of stars and planets where it is possible to observe one limb approaching the Earth and another receding from it, and to sort out the orbits of multiple star systems.


  • A change in the observed frequency of a wave, when there is relative motion between the source and the observer. As the distance between the source and the observer decreases, the observed frequency increases, thus the wavelength decreases. In the same manner, as the distance between the source and the observer increases, the observed frequency decreases, thus the wavelength increases. This effect occurs in sound and electromagnetic waves, and an example is the manner in which the pitch of a blowing horn of a car is perceived as higher as the car approaches, and then becomes lower as it moves away. Another example is the way in which the light of a star, as observed from the earth, shifts toward the violet end of the spectrum, or higher frequencies, when it is approaching. The opposite is true for stars that are receding from the Earth, that is, its light shifts towards the red end of the spectrum. Also called Doppler shift (2), or Doppler principle.
  • synonymDoppler principle
  • synonymDoppler shift
  • The extent of the change in the observed frequency of a wave, due to the Doppler effect. Also called Doppler frequency, or Doppler displacement.