View source for Doppler effect

[[File:Doppler effect diagrammatic.svg|thumb|upright=1.2|Change of [[wavelength]] caused by motion of the source.]] 
[[File:Dopplerfrequenz.gif|thumb|upright=1.2|An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding. The red circles represent sound waves.]]

{{Listen | filename=Speeding-car-horn_doppler_effect_sample.ogg|title=Passing car horn|format=[[Ogg]]}}

The '''Doppler effect''' (or the '''Doppler shift''') is the change in [[frequency]] of a [[wave]] in relation to an [[observer (physics)|observer]] who is moving relative to the wave source.<ref name="Giordano">{{cite book
 | last1  = Giordano
 | first1 = Nicholas
 | title  = College Physics: Reasoning and Relationships
 | publisher = Cengage Learning
 | date   = 2009
 | pages  = 421–424
 | url    = https://books.google.com/books?id=BwistUlpZ7cC&pg=PA424
 | isbn   = 978-0534424718
 }}</ref> It is named after the [[Austria]]n physicist [[Christian Doppler]], who described the phenomenon in 1842.

A common example of Doppler shift is the change of [[pitch (music)|pitch]] heard when a [[vehicle]] sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.<ref name="Possel">{{cite web
  | last = Possel
  | first = Markus
  | title = Waves, motion and frequency: the Doppler effect
  | website = Einstein Online, Vol. 5
  | publisher = Max Planck Institute for Gravitational Physics, Potsdam, Germany
  | date = 2017
  | url = http://www.einstein-online.info/spotlights/doppler
  | format = 
  | doi = 
  | accessdate = September 4, 2017
  | archive-url = https://web.archive.org/web/20170914003837/http://www.einstein-online.info/spotlights/doppler
  | archive-date = September 14, 2017
  | url-status = dead
  }}</ref>

The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave [[Crest (physics)|crest]] is emitted from a position closer to the observer than the crest of the previous wave.<ref name="Possel" /><ref name="Henderson">{{cite web
  | last = Henderson
  | first = Tom
  | title = The Doppler Effect – Lesson 3, Waves
  | website = Physics tutorial
  | publisher = The Physics Classroom
  | date = 2017
  | url = http://www.physicsclassroom.com/class/waves/Lesson-3/The-Doppler-Effect
  | format = 
  | doi = 
  | accessdate = September 4, 2017}}</ref>  Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wave fronts is reduced, so the waves "bunch together".  Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is then increased, so the waves "spread out".

For waves that propagate in a medium, such as [[sound]] waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted.<ref name="Giordano" /> The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as [[light]] or [[gravity]] in [[general relativity]], only the relative difference in velocity between the observer and the source needs to be considered.

==History==
[[File:Picture of the first 'wall formula' in the city of Utrecht 01.jpg|thumb|Experiment by Buys Ballot (1845) depicted on a wall in Utrecht (2019)]]
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).<ref name="AlecEden">Alec Eden ''The search for Christian Doppler'', Springer-Verlag, Wien 1992. Contains a facsimile edition with an [[English language|English]] translation.</ref> The hypothesis was tested for sound waves by [[C. H. D. Buys Ballot|Buys Ballot]] in 1845.<ref group=p>{{cite journal | last=Buys Ballot | title=Akustische Versuche auf der Niederländischen Eisenbahn, nebst gelegentlichen Bemerkungen zur Theorie des Hrn. Prof. Doppler (in German) | journal=Annalen der Physik und Chemie | date=1845 | volume=142 | issue=11 | pages=321–351 | doi=10.1002/andp.18451421102|bibcode = 1845AnP...142..321B | url=https://zenodo.org/record/1423606 }}</ref> He confirmed that the sound's [[Pitch (music)#Pitch and frequency|pitch]] was higher than the emitted frequency when the sound source approached him, and lower than the emitted frequency when the sound source receded from him. [[Hippolyte Fizeau]] discovered independently the same phenomenon on [[electromagnetic wave]]s 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).<ref group=p>Fizeau: "Acoustique et optique". ''Lecture, [[Philomatic Society|Société Philomathique]] de Paris'', 29 December 1848. According to Becker(pg. 109), this was never published, but recounted by M. Moigno(1850): "Répertoire d'optique moderne" (in French), vol 3. pp 1165–1203 and later in full by Fizeau, "Des effets du mouvement sur le ton des vibrations sonores et sur la longeur d'onde des rayons de lumière"; [Paris, 1870]. ''Annales de Chimie et de Physique'', 19, 211–221.</ref><ref>Becker (2011). Barbara J. Becker, ''Unravelling Starlight: William and Margaret Huggins and the Rise of the New Astronomy'', illustrated Edition, [[Cambridge University Press]], 2011; {{ISBN|110700229X}}, 9781107002296.</ref> In Britain, [[John Scott Russell]] made an experimental study of the Doppler effect (1848).<ref group=p>{{cite journal | last=Scott Russell | first=John | url=http://www.ma.hw.ac.uk/~chris/doppler.html | title=On certain effects produced on sound by the rapid motion of the observer | journal=Report of the Eighteenth Meeting of the British Association for the Advancement of Science | date=1848 | volume=18 | issue=7 | pages=37–38 | accessdate=2008-07-08 }}</ref>

==General==

In classical physics, where the speeds of source and the receiver relative to the medium are lower than the velocity of waves in the medium, the relationship between observed frequency <math>f</math> and emitted frequency <math>f_\text{0}</math> is given by:<ref name=encphysci>{{cite book
|title=Encyclopedia of Physical Science
|first1=Joe
|last1=Rosen
|first2=Lisa Quinn
|last2=Gothard
|publisher=Infobase Publishing
|date=2009
|isbn=978-0-8160-7011-4
|page=155
|url=https://books.google.com/books?id=avyQ64LIJa0C&pg=PA155}}</ref>
::<math>f = \left( \frac{c \pm v_\text{r}}{c \pm v_\text{s}} \right) f_0 \,</math>
:where
::<math>c \;</math> is the propagation speed of waves in the medium;
::<math>v_\text{r} \,</math> is the speed of the receiver relative to the medium, added to <math>c</math> if the receiver is moving towards the source, subtracted if the receiver is moving away from the source;
::<math>v_\text{s} \,</math> is the speed of the source relative to the medium, added to <math>c</math> if the source is moving away from the receiver, subtracted if the source is moving towards the receiver.

Note this relationship predicts that the frequency will decrease if either source or receiver is moving away from the other.

Equivalent formula, easier to remember:
::  <math>\frac{f}{v_{wr}} = \frac{f_0}{v_{ws}} = \frac{1}{\lambda}</math>
:where 
::<math>{v_{wr}}</math> is the wave's velocity relative to the receiver;
::<math> {v_{ws}}</math> is the wave's velocity relative to the source;
::<math>{\lambda}</math> is the wavelength.

The above formula assumes that the source is either directly approaching or receding from the observer. If the source approaches the observer at an angle (but still with a constant velocity), the observed frequency that is first heard is higher than the object's emitted frequency. Thereafter, there is a [[monotonic]] decrease in the observed frequency as it gets closer to the observer, through equality when it is coming from a direction perpendicular to the relative motion (and was emitted at the point of closest approach; but when the wave is received, the source and observer will no longer be at their closest), and a continued monotonic decrease as it recedes from the observer. When the observer is very close to the path of the object, the transition from high to low frequency is very abrupt. When the observer is far from the path of the object, the transition from high to low frequency is gradual.

If the speeds <math>v_\text{s} \,</math> and <math>v_\text{r} \,</math> are small compared to the speed of the wave, the relationship between observed frequency <math>f</math> and emitted frequency <math>f_\text{0}</math> is approximately<ref name=encphysci />
{|
|-
!Observed frequency||Change in frequency
|-
|width=70%|<center><math>f=\left(1+\frac{\Delta v}{c}\right)f_0</math></center>|||<center><math>\Delta f=\frac{\Delta v}{c}f_0</math></center>
|}

:where
::<math>\Delta f = f - f_0 \,</math>
::<math>\Delta v = -(v_\text{r} - v_\text{s}) \,</math> is the opposite of the velocity of the receiver relative to the source: it is positive when the source and the receiver are moving towards each other.

{{hidden begin|title=Proof|toggle=left}}
Given <math>f = \left( \frac{c + v_\text{r}}{c + v_\text{s}} \right) f_0 \,</math>

we divide for <math>c</math>

<math>f = \left( \frac{1 + \frac{v_\text{r}} {c}} {1 + \frac{v_\text{s}} {c}} \right) f_0  = \left( 1 + \frac{v_\text{r}}{c} \right) \left( \frac{1}{1 + \frac{v_\text{s}} {c}} \right) f_0 \,</math>

Since <math>\frac{v_\text{s}}{c} \ll 1</math> we can substitute the geometric expansion:

<math> \frac{1} {1 + \frac{v_\text{s}}{c}} \approx 1 - \frac{v_\text{s}}{c}</math>
{{hidden end}}

<gallery mode="packed" heights="250px">
File:Dopplereffectstationary.gif|alt1=|Stationary sound source produces sound waves at a constant frequency {{math|''f''}}, and the wave-fronts propagate symmetrically away from the source at a constant speed c. The distance between wave-fronts is the wavelength. All observers will hear the same frequency, which will be equal to the actual frequency of the source where {{math|''f'' {{=}} ''f''{{sub|0}} }}.

File:Dopplereffectsourcemovingrightatmach0.7.gif|alt2=|The same sound source is radiating sound waves at a constant frequency in the same medium. However, now the sound source is moving with a speed {{math|''υ''{{sub|s}} {{=}} 0.7 ''c''}}. Since the source is moving, the centre of each new wavefront is now slightly displaced to the right. As a result, the wave-fronts begin to bunch up on the right side (in front of) and spread further apart on the left side (behind) of the source. An observer in front of the source will hear a higher frequency {{math|''f'' {{=}} {{sfrac|''c'' + 0|''c'' – 0.7''c''}} ''f''{{sub|0}} {{=}} 3.33 ''f''{{sub|0}} }} and an observer behind the source will hear a lower frequency {{math|''f'' {{=}} {{sfrac|''c'' – 0|''c'' + 0.7''c''}} ''f''{{sub|0}} {{=}} 0.59 ''f''{{sub|0}} }}.

File:Dopplereffectsourcemovingrightatmach1.0.gif|alt3=|Now the source is moving at the speed of sound in the medium ({{math|''υ''{{sub|s}} {{=}} ''c''}}). The wave fronts in front of the source are now all bunched up at the same point. As a result, an observer in front of the source will detect nothing until the source arrives where {{math|''f'' {{=}} {{sfrac|''c'' + 0|''c'' – ''c''}} ''f''{{sub|0}} {{=}} &infin; }} and an observer behind the source will hear a lower frequency {{math|''f'' {{=}} {{sfrac|''c'' – 0|''c'' + ''c''}} ''f''{{sub|0}} {{=}} 0.5 ''f''{{sub|0}} }}.

File:Dopplereffectsourcemovingrightatmach1.4.gif|alt4=|The sound source has now surpassed the speed of sound in the medium, and is traveling at 1.4 ''c''. Since the source is moving faster than the sound waves it creates, it actually leads the advancing wavefront. The sound source will pass by a stationary observer before the observer hears the sound. As a result, an observer in front of the source will detect {{math|''f'' {{=}} {{sfrac|''c'' + 0|''c'' – 1.4''c''}} ''f''{{sub|0}} {{=}} -2.5 ''f''{{sub|0}} }} and an observer behind the source will hear a lower frequency {{math|''f'' {{=}} {{sfrac|''c'' – 0|''c'' + 1.4''c''}} ''f''{{sub|0}} {{=}} 0.42 ''f''{{sub|0}} }}.
</gallery>

==Analysis==
To understand what happens, consider the following analogy. Someone throws one ball every second at a man. Assume that balls travel with constant velocity. If the thrower is stationary, the man will receive one ball every second. However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out. The inverse is true if the thrower is moving away from the man. So it is actually the ''wavelength'' which is affected; as a consequence, the received frequency is also affected. It may also be said that the velocity of the wave remains constant whereas wavelength changes; hence frequency also changes.

With an observer stationary relative to the medium, if a moving source is emitting waves with an actual frequency <math>f_\text{0}</math> (in this case, the wavelength is changed, the transmission velocity of the wave keeps constant <math>\text{--}</math> note that the ''transmission velocity'' of the wave does not depend on the ''velocity of the source''), then the observer detects waves with a frequency <math>f</math> given by

:<math>f = \left ( \frac {c}{c \pm v_\text{s}} \right ) f_0</math>

A similar analysis for a moving ''observer'' and a stationary source (in this case, the wavelength keeps constant, but due to the motion, the rate at which the observer receives waves <math>\text{--}</math> and hence the ''transmission velocity'' of the wave [with respect to the observer] <math>\text{--}</math> is changed) yields the observed frequency:

:<math>f = \left ( \frac {c \pm v_\text{r}}{c} \right ) f_0</math>

These can be generalized into the equation that was presented in the previous section.

:<math>f = \left ( \frac {c\pm v_\text{r}}{c \pm v_\text{s}} \right ) f_0</math>

An interesting effect was predicted by Lord Rayleigh in his classic book on sound: if the source is moving toward the observer at twice the speed of sound, a musical piece emitted by that source would be heard in correct time and tune, but ''backwards''.<ref>{{cite book|last=Strutt (Lord Rayleigh)|first=John William|title=The Theory of Sound|editor=MacMillan & Co|date=1896|edition=2|volume=2|pages=154|url=https://archive.org/stream/theorysound02raylgoog#page/n176/mode/2up|publisher=Macmillan}}</ref> The Doppler effect with sound is only clearly heard with objects moving at high speed, as change in frequency of musical tone involves a speed of around 40 meters per second, and smaller changes in frequency can easily be confused by changes in the amplitude of the sounds from moving emitters.  Neil A Downie has demonstrated <ref>Downie, Neil A, 'Vacuum Bazookas, Electric Rainbow Jelly and 27 other projects for Saturday Science', Princeton (2001) {{ISBN|0-691-00986-4}}</ref> how the Doppler effect can be made much more easily audible by using an ultrasonic (e.g. 40&nbsp;kHz) emitter on the moving object. The observer then uses a heterodyne frequency converter, as used in many bat detectors, to listen to a band around 40&nbsp;kHz. In this case, with the bat detector tuned to give frequency for the stationary emitter of 2000&nbsp;Hz, the observer will perceive a frequency shift of a whole tone, 240&nbsp;Hz, if the emitter travels at 2 meters per second.

==Application==
===Robotics===
Dynamic real-time path planning in robotics to aid the movement of robots in a sophisticated environment with moving obstacles often take help of Doppler effect<ref>{{Cite book |doi = 10.1007/978-3-030-04239-4_19|chapter = Potential and Sampling Based RRT Star for Real-Time Dynamic Motion Planning Accounting for Momentum in Cost Function|title = Neural Information Processing|volume = 11307|pages = 209–221|series = Lecture Notes in Computer Science|year = 2018|last1 = Agarwal|first1 = Saurabh|last2 = Gaurav|first2 = Ashish Kumar|last3 = Nirala|first3 = Mehul Kumar|last4 = Sinha|first4 = Sayan|isbn = 978-3-030-04238-7}}</ref>. Such applications are specially used for competitive robotics where the environment is constantly changing, such as robosoccer.

===Sirens===
[[File:Juli 2016 - Spoedtransport, Huisarts, Brandweer, Politie en Ambulances met spoed in Rotterdam -451.webm|start=7:14|end=8:30|thumb|Sirens on passing emergency vehicles.]]
A [[siren (alarm)|siren]] on a passing [[emergency vehicle]] will start out  higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer. Astronomer [[John Dobson (astronomer)|John Dobson]] explained the effect thus:

{{quote|The reason the siren slides is because it doesn't hit you.}}

In other words, if the siren approached the observer directly, the pitch would remain constant, at a higher than stationary pitch, until the vehicle hit him, and then immediately jump to a new lower pitch. Because the vehicle passes by the observer, the radial velocity does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity:

:<math>v_\text{radial}=v_\text{s}\cdot \cos{\theta}</math>

where <math>\theta</math> is the angle between the object's forward velocity and the line of sight from the object to the observer.

===Astronomy===
[[Image:Redshift.svg|thumb|upright|[[Redshift]] of [[spectral line]]s in the [[optical spectrum]] of a supercluster of distant galaxies (right), as compared to that of the Sun (left)]]

The [[Relativistic Doppler effect|Doppler effect for electromagnetic waves]] such as light is of great use in [[astronomy]] and results in either a so-called [[redshift]] or [[blueshift]]. It has been used to measure the speed at which [[star]]s and [[galaxy|galaxies]] are approaching or receding from us; that is, their [[radial velocity|radial velocities]]. This may be used to detect if an apparently single star is, in reality, a close [[Binary star|binary]], to measure the rotational speed of stars and galaxies, or to [[Doppler spectroscopy|detect exoplanets]]. This redshift and blueshift happens on a very small scale. If an object was moving toward earth, there would not be a noticeable difference in visible light, to the unaided eye.<ref>{{cite web|url=http://www.astro.ucla.edu/~wright/doppler.htm|title=Doppler Shift|website=www.astro.ucla.edu}}</ref>

Note that redshift is also used to measure the [[Redshift#Expansion of space|expansion of space]], but that this is not truly a Doppler effect.<ref>The distinction is made clear in {{cite book |last=Harrison |first=Edward Robert  |date= 2000 |title=Cosmology: The Science of the Universe |publisher=Cambridge University Press |edition=2nd |url=https://books.google.com/books?id=-8PJbcA2lLoC&pg=PA315|pages=306''ff'' |isbn=978-0-521-66148-5 |ref=harv}}</ref> Rather, redshifting due to the expansion of space is known as [[cosmological redshift]], which can be derived purely from the [[Robertson-Walker metric]] under the formalism of [[General Relativity]]. Having said this, it also happens that there ''are'' detectable Doppler effects on cosmological scales, which, if incorrectly interpreted as cosmological in origin, lead to the observation of [[redshift-space distortions]].<ref>An excellent review of the topic in technical detail is given here: {{cite journal|last1=Percival|first1=Will|last2=Samushia|first2=Lado|last3=Ross|first3=Ashley|last4=Shapiro|first4=Charles|last5=Raccanelli|first5=Alvise|date= 2011 |title=Review article: Redshift-space distortions|journal=Philosophical Transactions of the Royal Society|volume=369|issue=1957|pages=5058–67|doi=10.1098/rsta.2011.0370|pmid=22084293|bibcode=2011RSPTA.369.5058P}}</ref>

The use of the Doppler effect for light in [[astronomy]] depends on our knowledge that the [[electromagnetic spectroscopy|spectra]] of stars are not homogeneous. They exhibit [[spectral line|absorption lines]] at well defined frequencies that are correlated with the energies required to excite [[electron]]s in various [[Chemical element|elements]] from one level to another. The Doppler effect is recognizable in the fact that the absorption lines are not always at the frequencies that are obtained from the spectrum of a stationary light source. Since blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blueshift and those of a receding astronomical light source exhibit a redshift.

Among the [[List of nearest stars|nearby stars]], the largest radial velocities with respect to the [[Sun]] are +308&nbsp;km/s ([[BD-15°4041]], also known as LHS 52, 81.7 light-years away) and −260&nbsp;km/s ([[Woolley 9722]], also known as Wolf 1106 and LHS 64, 78.2 light-years away). Positive radial velocity means the star is receding from the Sun, negative that it is approaching.

===Radar===
{{Main|Doppler radar}}

The Doppler effect is used in some types of [[radar]], to measure the velocity of detected objects. A radar beam is fired at a moving target — e.g. a motor car, as police use radar to detect speeding motorists — as it approaches or recedes from the radar source. Each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source. As each wave has to move farther, the gap between each wave increases, increasing the wavelength. 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 velocity. 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|date=December 2009}}

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 velocity]] <math>\Delta v</math> is twice that from the same target emitting a wave:

:<math>\Delta f=\frac{2\Delta v}{c}f_0</math>.<ref>{{cite web|url=http://www.radartutorial.eu/11.coherent/co06.en.html|title=Radar Basics|first=Dipl.-Ing. (FH) Christian|last=Wolff|date=|website=www.radartutorial.eu|accessdate=14 April 2018}}</ref>

===Medical===
{{Main|Doppler ultrasonography}}
[[File:CarotidDoppler1.jpg|thumb|upright|Colour flow ultrasonography (Doppler) of a [[carotid artery]] – scanner and screen]]
An [[echocardiogram]] can, within certain limits, produce an accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. One of the limitations is that the [[ultrasound]] beam should be as parallel to the blood flow as possible. 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.<ref>{{cite journal|last1=Davies|first1=MJ|last2=Newton|first2=JD|title=Non-invasive imaging in cardiology for the generalist|journal=British Journal of Hospital Medicine (London, England : 2005)|date=2 July 2017|volume=78|issue=7|pages=392–398|doi=10.12968/hmed.2017.78.7.392|pmid=28692375}}</ref><ref>{{cite journal|last1=Appis|first1=AW|last2=Tracy|first2=MJ|last3=Feinstein|first3=SB|title=Update on the safety and efficacy of commercial ultrasound contrast agents in cardiac applications|journal=Echo Research and Practice|date=1 June 2015|volume=2|issue=2|pages=R55–62|doi=10.1530/ERP-15-0018|pmid=26693339|pmc=4676450}}</ref>

Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it is not the frequency shift (Doppler shift) of the received signal that is measured, but the phase shift (''when'' the received signal arrives).<ref group=p>{{Cite journal|last=Petrescu|first=Florian Ion T|year=2015|title=Improving Medical Imaging and Blood Flow Measurement by using a New Doppler Effect Relationship|url=http://search.proquest.com/openview/cec7b768b14887621e9494261e122a4c/1?pq-origsite=gscholar&cbl=1226369|journal=American Journal of Engineering and Applied Sciences|volume=8|issue=4|pages=582–588|via=Proquest|doi=10.3844/ajeassp.2015.582.588}}</ref>

Velocity measurements of blood flow are also used in other fields of [[medical ultrasonography]], such as [[obstetric ultrasonography]] and [[neurology]]. Velocity measurement of blood flow in arteries and veins based on Doppler effect is an effective tool for diagnosis of vascular problems like [[stenosis]].<ref>{{cite book |first=D. H. |last=Evans |first2=W. N. |last2=McDicken |title=Doppler Ultrasound |edition=2nd |publisher=John Wiley and Sons |location=New York |date=2000 |isbn=978-0-471-97001-9 }}{{page needed|date=June 2015}}</ref>

===Flow measurement===
Instruments such as the [[laser Doppler velocimetry|laser Doppler velocimeter]] (LDV), and [[acoustic Doppler velocimetry|acoustic Doppler velocimeter]] (ADV) have been developed to measure [[velocity|velocities]] in a fluid flow. The LDV emits a light beam and the ADV emits an ultrasonic acoustic burst, and measure the Doppler shift in wavelengths of reflections from particles moving with the flow. The actual flow is computed as a function of the water velocity and phase. This technique allows non-intrusive flow measurements, at high precision and high frequency.

===Velocity profile measurement===
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. Flows can be pulsating, oscillating, laminar or turbulent, stationary or transient. This technique is fully non-invasive.

===Satellite communication===

{|style="margin: 0 auto;"
| [[File:SatDoppler.png|thumb|300px|upright|Possible Doppler shifts in dependence of the elevation angle ([[Low Earth orbit|LEO]]: orbit altitude <math>h</math> = 750 km). Fixed ground station.<ref>Otilia Popescuy, Jason S. Harrisz and Dimitrie C. Popescuz, Designing the Communica- tion Sub-System for Nanosatellite CubeSat Missions: Operational and Implementation Perspectives, 2016, IEEE</ref>]]
| [[File:DopplerSatScheme.png|thumb|300px|upright|Geometry for Doppler effects. Variables: <math>v_{mob}</math> is the velocity of the mobile station, <math>v_{Sat}</math> is the velocity of the satellite, <math>v_{rel,sat}</math> is the relative speed of the satellite, <math>\phi</math> is the elevation angle of the satellite and <math>\theta</math> is the driving direction with respect to the satellite.]]
| [[File:SatDopplerSpectrum.png|thumb|300px|upright|Doppler effect on the mobile channel. Variables: <math>f_c = \frac{c}{\lambda_{\rm c}}</math> is the carrier frequency, <math>f_{\rm D,max}=\frac{v_{\rm mob}}{\lambda_{\rm c}}</math> is the maximum Doppler shift due to the mobile station moving and <math>f_{\rm D,Sat}</math> is the additional Doppler shift due to the satellite moving.]] 
|}

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. Dynamic Doppler compensation, where the frequency of a signal is changed progressively during transmission, is used so the satellite receives a constant frequency signal.<ref>{{Citation |last=Qingchong |first=Liu |title=Doppler measurement and compensation in mobile satellite communications systems |journal=Military Communications Conference Proceedings / MILCOM |volume=1 |date=1999 |pages=316–320 |url=https://ieeexplore.ieee.org/document/822695 |doi=10.1109/milcom.1999.822695|isbn=978-0-7803-5538-5 |citeseerx=10.1.1.674.3987 }}</ref>

Doppler shift of the direct path can be estimated by the following formula:<ref>Arndt, D. (2015). On Channel Modelling for Land Mobile Satellite Reception (Doctoral dissertation).</ref>

<math>f_{\rm D, dir} = \frac{v_{\rm mob}}{\lambda_{\rm c}}\cos\phi \cos\theta</math>

where <math>v_{mob}</math> is the velocity of the mobile station, <math>\lambda_{\rm c}</math> is the wavelength of the carrier, <math>\phi</math> is the elevation angle of the satellite and <math>\theta</math> is the driving direction with respect to the satellite.

The additional Doppler shift due to the satellite moving can be described as:

<math>f_{\rm D,sat} = \frac{v_{\rm rel,sat}}{\lambda_{\rm c}}</math>

where <math>v_{\rm rel,sat}</math> is the relative speed of the satellite.

===Audio===
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. This results at the listener's ear in rapidly fluctuating frequencies of a keyboard note.

===Vibration measurement===
A [[laser Doppler vibrometer]] (LDV) is a non-contact instrument for measuring vibration. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface.

===Developmental biology===
During the [[Segmentation (biology)|segmentation]] of [[vertebrate]] [[embryo]]s, waves of [[gene expression]] sweep across the presomitic [[mesoderm]], the tissue from which the precursors of the [[vertebra]]e ([[somite]]s) are formed. A new somite is formed upon arrival of a wave at the [[Anatomical terms of location|anterior]] end of the presomitic mesoderm. In [[zebrafish]], it has been shown that the shortening of the presomitic mesoderm during segmentation leads to a Doppler effect as the anterior end of the tissue moves into the waves. This Doppler effect contributes to the period of segmentation.<ref group=p>{{cite journal |last1=Soroldoni |first1=D. |last2=Jörg |first2=D. J. |last3=Morelli |first3=L. G. |last4=Richmond |first4=D. L. |last5=Schindelin |first5=J. |last6=Jülicher |first6=F. |last7=Oates |first7=A. C. |title=A Doppler Effect in Embryonic Pattern Formation |journal=Science |volume=345 |issue=6193 |pages=222–225 |year=2014 |pmid=25013078 |doi=10.1126/science.1253089|bibcode = 2014Sci...345..222S }}</ref>

==Inverse Doppler effect==
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. But some materials are capable of [[negative refraction]], which should lead to a Doppler shift that works in a direction opposite that of a conventional Doppler shift.<ref>{{cite web|url=https://physicsworld.com/a/doppler-shift-is-seen-in-reverse/|title=Doppler shift is seen in reverse|date=10 March 2011|website=Physics World}}</ref> First experiment that detected this effect was conducted by Nigel Seddon and Trevor Bearpark in [[Bristol]], [[United Kingdom]] in 2003.<ref group=p>{{cite journal |last1=Kozyrev |first1=Alexander B. |last2=van der Weide |first2=Daniel W. |title=Explanation of the Inverse Doppler Effect Observed in Nonlinear Transmission Lines |journal=Physical Review Letters |volume=94 |issue=20 |pages=203902 |year=2005 |pmid=16090248 |doi=10.1103/PhysRevLett.94.203902 |bibcode=2005PhRvL..94t3902K}}</ref> Later inverse Doppler effect was observed in some inhomogeneous materials and predicted inside Vavilov–Cherenkov cone.<ref>{{Cite journal|last=Shi|first=Xihang|last2=Lin|first2=Xiao|last3=Kaminer|first3=Ido|last4=Gao|first4=Fei|last5=Yang|first5=Zhaoju|last6=Joannopoulos|first6=John D.|last7=Soljačić|first7=Marin|last8=Zhang|first8=Baile|date=October 2018|title=Superlight inverse Doppler effect|journal=Nature Physics|volume=14|issue=10|pages=1001–1005|doi=10.1038/s41567-018-0209-6|issn=1745-2473|arxiv=1805.12427|bibcode=2018arXiv180512427S}}</ref>

==See also==
{{col div|colwidth=30em}}
* [[Differential Doppler effect]]
* [[Doppler cooling]]
* [[Dopplergraph]]
* [[Fading]]
* [[Fizeau experiment]]
* [[Photoacoustic Doppler effect]]
* [[Rayleigh fading]]
* [[Redshift]]
* [[Laser Doppler imaging]]
* [[Relativistic Doppler effect]]
{{colend}}

== Primary sources ==
{{reflist|group=p|30em}}

== References ==
{{reflist}}

==Further reading==
* Doppler, C. (1842). ''[[Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels|Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels (About the coloured light of the binary stars and some other stars of the heavens)]]''. Publisher: Abhandlungen der Königl. Böhm. Gesellschaft der Wissenschaften (V. Folge, Bd. 2, S. 465–482) [Proceedings of the Royal Bohemian Society of Sciences (Part V, Vol 2)]; Prague: 1842 (Reissued 1903). Some sources mention 1843 as year of publication because in that year the article was published in the Proceedings of the Bohemian Society of Sciences. Doppler himself referred to the publication as "Prag 1842 bei Borrosch und André", because in 1842 he had a preliminary edition printed that he distributed independently.
* "Doppler and the Doppler effect", E. N. da C. Andrade, ''Endeavour'' Vol. XVIII No. 69, January 1959 (published by ICI London). Historical account of Doppler's original paper and subsequent developments.
* {{cite web | url = http://archive.ncsa.uiuc.edu/Cyberia/Bima/doppler.html | title = Doppler Effect | first = Eleni | last = Adrian | publisher = [[National Center for Supercomputing Applications|NCSA]] | date = 24 June 1995 | accessdate = 2008-07-13 | archive-url = https://web.archive.org/web/20090512192731/http://archive.ncsa.uiuc.edu/Cyberia/Bima/doppler.html | archive-date = 12 May 2009 | url-status = dead }}

==External links==
{{Commonscatinline}}
* [http://scienceworld.wolfram.com/physics/DopplerEffect.html Doppler Effect], ScienceWorld

{{Authority control}}

{{DEFAULTSORT:Doppler Effect}}
[[Category:Doppler effects| ]]
[[Category:Wave mechanics]]
[[Category:Radio frequency propagation]]
[[Category:Radar signal processing]]
[[Category:Sound]]