Editing Doppler effect (section)

{{short description|Frequency change of a wave for observer relative to its source}}
{{for|the music project|Dopplereffekt}}
{{redirect|Doppler}}
[[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
| type         = 
| filename     = Speeding-car-horn_doppler_effect_sample.ogg
| title        = Passing car horn
| help         = no
| pos          = center
}}
]]The '''Doppler effect''' (also '''Doppler shift''') is the change in the [[frequency]] of a [[wave]] in relation to an observer who is moving relative to the source of the wave.<ref>{{cite book | author=United States. Navy Department | title=Principles and Applications of Underwater Sound, Originally Issued as Summary Technical Report of Division 6, NDRC, Vol. 7, 1946, Reprinted...1968 | year=1969 | url=https://books.google.com/books?id=gjYGC_sc6lcC&pg=PA194 | access-date=2021-03-29 | page=194}}</ref><ref>{{cite book | last=Joseph | first=A. | title=Measuring Ocean Currents: Tools, Technologies, and Data | publisher=Elsevier Science | year=2013 | isbn=978-0-12-391428-6 | url=https://books.google.com/books?id=FRVaNZEQCa4C&pg=PA164 | access-date=2021-03-30 | page=164}}</ref><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> The ''Doppler effect'' is named after the 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
  | access-date = 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>

When the source of the sound wave is moving towards the observer, each successive cycle of the wave is emitted from a position closer to the observer than the previous cycle.<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
  | access-date = September 4, 2017}}</ref> Hence, from the observer's perspective, the time between cycles is reduced, meaning the frequency is increased. Conversely, if the source of the sound wave is moving away from the observer, each cycle of the wave is emitted from a position farther from the observer than the previous cycle, so the arrival time between successive cycles is increased, thus reducing the frequency.

For waves that propagate in a [[transmission medium|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 in such cases may therefore result from motion of the source, motion of the observer, motion of the medium, or any combination thereof. For waves propagating in [[vacuum]], as is possible for [[electromagnetic waves]] or [[gravitational wave]]s, only the difference in velocity between the observer and the source needs to be considered.