Doppler effect (redirected from Doppler equations)

Doppler effect, change in the wavelength (or frequency) of energy in the form of waves, e.g., sound or light, as a result of motion of either the source or the receiver of the waves; the effect is named for the Austrian scientist Christian Doppler, who demonstrated the effect for sound. If the source of the waves and the receiver are approaching each other (because of the motion of either or both), the frequency of the waves will increase and the wavelength will be shortened—sounds will become higher pitched and light will appear bluer. If the sender and receiver are moving apart, sounds will become lower pitched and light will appear redder. A common example is the sudden drop in the pitch of a train whistle as the train passes a stationary listener. The Doppler effect in reflected radio waves is employed in radar to sense the velocity of the object under surveillance. In astronomy, the Doppler effect for light is used to measure the velocity (and indirectly distance) and rotation of stars and galaxies along the direction of sight. In the spectrum of nearly every star there are wavelengths, characteristic of atoms, that lie near but not quite coincident to the same wavelengths as measured in the laboratory. The small deviations or shifts are generally due to the relative motion of the celestial object and the earth. Both blue shifts and red shifts are observed for various objects, indicating relative motion both toward and away from the earth. Such shifts have been used to measure the orbital velocity of the earth, to detect binary stars and variable stars, and to detect rotation of other galaxies. The Doppler effect is responsible for the red shifts of distant galaxies, and also of quasars, and thus provides the best evidence for the expansion of the universe, as described by Hubble's law. In addition to observations of visible light, the Doppler effect for radio waves is utilized by astronomers to determine the velocities of dust clouds in the spiral arms of the Milky Way galaxy. These observations provided the first direct proof that our own galaxy is rotating. The Doppler shift in radar pulses reflected from the surfaces of Venus and Mercury have been analyzed to obtain new values for their periods of rotation about their axes.

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Doppler effect

Apparent difference between the frequency at which waves—including light, sound, and radio waves—leave a source and that at which they reach an observer. The effect, first described by the Austrian physicist Christian Doppler (1803–1853), is caused by the relative motion of the observer and the wave source. It can be observed by listening to the blowing horn or siren of an approaching vehicle, whose pitch rises as the vehicle approaches the observer and falls as it recedes. It is used in radar and to calculate the speed of stars by observing the change in frequency of their light.

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Doppler effect

The change in electromagnetic frequency that occurs when the source of the radiation and its observer move toward or away from each other. The faster they come together, the higher the frequency. The faster they move away, the lower the frequency. Discovered by Austrian physicist Christian Doppler (1803-1853), this condition has a great effect on low-earth orbit (LEO) satellites as they weave towards and away from the earth. See Doppler radar.

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Doppler effect

a phenomenon, observed for sound waves and electromagnetic radiation, characterized by a change in the apparent frequency of a wave as a result of relative motion between the observer and the source

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Doppler effect

The change in the frequency of a wave observed at a receiver whenever the source or the receiver of the wave is moving relative to the other or to the carrier of the wave (the medium). The effect was predicted in 1842 by C. Doppler, and first verified for sound waves in 1845 from experiments conducted on a moving train.

The Doppler effect for sound waves is now a commonplace experience: If one is passed by a fast car or a plane, the pitch of its noise is considerably higher in approaching than in parting. The same phenomenon is observed if the source is at rest and the receiver is passing it. The linear optical Doppler effect was first observed in 1905 from a shift of spectral lines emitted by a beam of fast ions (canal rays) emerging from a hole in the cathode of a gas discharge tube run at high voltage. Still, their velocity was several orders of magnitude below that of light in vacuum. The precise interferometric experiments of A. A. Michelson and E. W. Morley (1887) showed clearly that the velocity of light is not bound to any ether, but is measured to be the same in any moving system. This result was a crucial check for A. Einstein's theory of special relativity (1905), which also makes a clear prediction for the optical Doppler effect.

The Doppler effect has important applications in remote-sensing, high-energy physics, astrophysics, and spectroscopy.

Let a wave from a sound source or radar source, or from a laser, be reflected from a moving object back to the source, which may itself move as well. Then a frequency shift is observed by a receiver connected to the source. The measurement provides an excellent means for the remote sensing of velocities of any kind of object, including cars, ships, planes, satellites, flows of fluids, or winds.

The light from distant stars and galaxies shows a strong Doppler shift to the red, indicating that the universe is rapidly expanding. However, this effect can be mixed up with the gravitational redshift that results from the energy loss which a light quantum suffers when it emerges from a strong gravitational field.

The Doppler width and Doppler shift of spectral lines in sunlight (Fraunhofer lines) are important diagnostic tools for the dynamics of the Sun's atmosphere, indicating its temperature and turbulence.