Doppler effect

A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer.

Doppler first proposed this effect in 1842 in his treatise "Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels" (On the coloured light of the binary stars and some other stars of the heavens).

Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848 (in France, the effect is sometimes called "effet Doppler-Fizeau" but that name was not adopted by the rest of the world as Fizeau's discovery was six years after Doppler's proposal).

[p 2][7] In Britain, John Scott Russell made an experimental study of the Doppler effect (1848).

When the observer is very close to the path of the object, the transition from high to low frequency is very abrupt.

Lord Rayleigh predicted the following effect in his classic book on sound: if the observer were moving from the (stationary) source at twice the speed of sound, a musical piece previously emitted by that source would be heard in correct tempo and pitch, but as if played backwards.

Because the vehicle passes by the observer, the radial speed does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity:

The Doppler effect for electromagnetic waves such as light is of widespread use in astronomy to measure the speed at which stars and galaxies are approaching or receding from us, resulting in so called blueshift or redshift, respectively.

This effect typically happens on a very small scale; there would not be a noticeable difference in visible light to the unaided eye.

[10] The use of the Doppler effect in astronomy depends on knowledge of precise frequencies of discrete lines in the spectra of stars.

Positive radial speed means the star is receding from the Sun, negative that it is approaching.

The relationship between the expansion of the universe and the Doppler effect is not simple matter of the source moving away from the observer.

[13] Distant galaxies also exhibit peculiar motion distinct from their cosmological recession speeds.

If redshifts are used to determine distances in accordance with Hubble's law, then these peculiar motions give rise to redshift-space distortions.

Each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source.

In some situations, the radar beam is fired at the moving car as it approaches, in which case each successive wave travels a lesser distance, decreasing the wavelength.

In either situation, calculations from the Doppler effect accurately determine the car's speed.

Moreover, the proximity fuze, developed during World War II, relies upon Doppler radar to detonate explosives at the correct time, height, distance, etc.

[citation needed] Because the Doppler shift affects the wave incident upon the target as well as the wave reflected back to the radar, the change in frequency observed by a radar due to a target moving at relative speed

Velocity measurements allow assessment of cardiac valve areas and function, abnormal communications between the left and right side of the heart, leaking of blood through the valves (valvular regurgitation), and calculation of the cardiac output.

Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements.

Developed originally for velocity measurements in medical applications (blood flow), Ultrasonic Doppler Velocimetry (UDV) can measure in real time complete velocity profile in almost any liquids containing particles in suspension such as dust, gas bubbles, emulsions.

Fast moving satellites can have a Doppler shift of dozens of kilohertz relative to a ground station.

The speed, thus magnitude of Doppler effect, changes due to earth curvature.

[20] After realizing that the Doppler shift had not been considered before launch of the Huygens probe of the 2005 Cassini–Huygens mission, the probe trajectory was altered to approach Titan in such a way that its transmissions traveled perpendicular to its direction of motion relative to Cassini, greatly reducing the Doppler shift.

The Leslie speaker, most commonly associated with and predominantly used with the famous Hammond organ, takes advantage of the Doppler effect by using an electric motor to rotate an acoustic horn around a loudspeaker, sending its sound in a circle.

Since 1968 scientists such as Victor Veselago have speculated about the possibility of an inverse Doppler effect.

The size of the Doppler shift depends on the refractive index of the medium a wave is traveling through.

[24] The first experiment that detected this effect was conducted by Nigel Seddon and Trevor Bearpark in Bristol, United Kingdom in 2003.

[p 5] Later, the inverse Doppler effect was observed in some inhomogeneous materials, and predicted inside a Vavilov–Cherenkov cone.