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.[1][2][3] 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 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.[4]
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.[4][5] 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 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.[3] 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 waves, only the difference in velocity between the observer and the source needs to be considered.
- ^ United States. Navy Department (1969). Principles and Applications of Underwater Sound, Originally Issued as Summary Technical Report of Division 6, NDRC, Vol. 7, 1946, Reprinted...1968. p. 194. Retrieved 2021-03-29.
- ^ Joseph, A. (2013). Measuring Ocean Currents: Tools, Technologies, and Data. Elsevier Science. p. 164. ISBN 978-0-12-391428-6. Retrieved 2021-03-30.
- ^ a b Giordano, Nicholas (2009). College Physics: Reasoning and Relationships. Cengage Learning. pp. 421–424. ISBN 978-0534424718.
- ^ a b Possel, Markus (2017). "Waves, motion and frequency: the Doppler effect". Einstein Online, Vol. 5. Max Planck Institute for Gravitational Physics, Potsdam, Germany. Archived from the original on September 14, 2017. Retrieved September 4, 2017.
- ^ Henderson, Tom (2017). "The Doppler Effect – Lesson 3, Waves". Physics tutorial. The Physics Classroom. Retrieved September 4, 2017.