What is Doppler: Definition and 747 Discussions

The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian 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.The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. 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. 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 electromagnetic waves or gravitational waves, only the relative difference in velocity between the observer and the source needs to be considered, giving rise to the relativistic Doppler effect.

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  1. Isomorphism

    On the definition of width in Doppler broadening problems.

    The following question has stumped me. I am not getting the answer marked. Instead I am getting option a). I have produced my attempt below. Also I have no idea which formula to use when the question says Doppler width. Since frequencies form a distribution, there are multiple notions of width...
  2. J

    I Something I noticed about Doppler asymmetry

    Good day, I was playing a bit with a Doppler effect scenario and tried to combine it a bit with Special Relativity and I concluded something that I don’t quite understand, or that I at least don’t get the meaning of. I am aware that the scenario and my concluded formula don’t describe the full...
  3. T

    I SR Doppler Effect: Differences in Wave VS Momentum Models

    In his seminal paper on Special Relativity: On the Electrodynamics of Moving Bodies, Einstein derives a formula for the Relativistic Doppler effect. See this section: § 7. Theory of Doppler's Principle and of AberrationThe formula is: In this Wiki article, the same formula is derived. The...
  4. Artlav

    I Relativistic doppler shifted blackbody color?

    I'm trying to render the sky as it would appear from a starship moving at some large part of the speed of light. Geometry was straightforward, but colors are the problem. How would the doppler effect change the color of a star? The expectation is that the stars behind are red and the ones ahead...
  5. C

    Help me derive the Doppler effect ratio

    Homework Statement I'm watching this lecture and wanted to attempt to derive the expression for ΔTo/ΔTs Homework Equations ΔTo = ΔTs + VΔTo / U The Attempt at a Solution I have worked out that ΔTs = 1-V / U, however I'm stuck trying to get ΔTs / ΔTo because of the ΔTo on both sides of the...
  6. Amara

    Taylor expansion of the relativistic Doppler effect?

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  7. R

    Doppler effect of a satellite (extended)

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  8. shawn9521

    I Understanding the Doppler Effect: Exploring Sound Waves on a Moving Object

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  9. andrewkirk

    Can we hear a supersonic plane?

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  10. H

    I Is it possible to image individual air molecules?

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  11. QuarkDecay

    B Doppler effect/ both moving same direction

    What happens if a source and the observer move at the same direction? Always hear how only one of them doesnot move, or when they move towards each other. For example. An ambulance moves towards direction A with Va. A biker moves towards direction A as well, with Vb. What happens to the...
  12. penelopa

    I Radar Doppler effect: classical vs relativistic

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  13. S

    Doppler effect with moving medium but no relative motion

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  14. DaTario

    I Explaining Doppler Effect Asymmetry

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  15. S

    Problem on Doppler effect and sound level

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  16. S

    Doppler shift and change in intensity of a sound wave

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  17. P

    Sound waves and speed of it in different reference frames

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  18. S

    B Photon red shifting and Doppler effect

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  19. S

    Bouncing ball and Doppler effect

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  20. D

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  21. F

    Help with Doppler shift galaxy question

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  22. T

    Doppler effect in different scenarios

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  23. BiGyElLoWhAt

    I Doppler shift for an observer in circular motion

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  24. Trec93

    Doppler effect Formula manipulation

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  25. S

    B Help understanding this derivation of relativistic Doppler

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  26. mrsmitten

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  27. R

    Doppler effect and signs of the equation

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  28. T

    Looking for Doppler Solution to Maxwell's Equations?

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  29. R

    A How Does Arm Length Affect Doppler Shift Measurement?

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  30. B

    Ultrasound - Doppler angle vs. beam steering angle

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  31. Y

    Doppler effect train frequency question

    Homework Statement When a train running at a speed of 72 km/h approaches a crossing signal, a passenger in the train hears the siren at 720 Hz. What frequency does the passenger detect after the train passes the crossing signal? Take the speed of sound in air to be 340 m/s. Homework Equations...
  32. Mr Davis 97

    Does the Doppler Effect Affect Wave Velocity in Moving Observers?

    Imagine that there is a stationary source that is propagating waves (such as sound waves). Let's say that the wave speed in the medium is 343 m/s. If I am an observer, and I begin to move towards the waves, will the wave speed increase due to the idea of relative velocities, or will only the...
  33. T

    Distance from source to observe Doppler effect?

    I was just looking at a few animations and something that could be just visualized incorrectly or I may have interpreted correctly is that the wavelengths coming from the sources are initially smaller close to the source, but become larger as you get far away from the source. Is there any...
  34. B

    Deriving E=mc^2 from relativistic doppler effect

    How is E=mc^2 derived from relativistic doppler effect? I looked up on wikipedia but it seems to have some errors. https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalence#Alternative_version
  35. Cookiey

    Asymmetry in the Doppler Effect for sound

    I'm a bit confused about this. So say there's an observer and a source of sound. If the observer moves towards the source of sound, the frequency seems to increase because he encounters more wavelengths in the same amount of time. In a second case, if the source moved towards the observer, the...
  36. Dennydont

    How does the doppler effect resolve the twin paradox?

    If you had twin 1 on the earth, and twin 2 fly to a star and back at a speed of v with the Earth and star separated by a distance L, twin 1 sends out flashes at intervals of t seconds (measured in his frame). Taking into consideration the numbers of redshifted and blueshifted flashes that the...
  37. Buckethead

    How Does Doppler Shift Affect Light From a Shuttle to a Mother Ship in Space?

    Imagine a mother ship traveling away from Earth at .5c and a shuttle takes off from the mother ship in the same direction of travel at .5c relative to the mother ship. The shuttle fires a blue laser (400nm) to a transparent screen mounted on top of the mother ship as it is traveling away. Due...
  38. T

    Confusion in Doppler shift equation?

    I'm just trying to build Doppler shift equation for Moving observer and stationary source. I have an problem in a step. I'm attaching a screen-shot please check it and tell me where to this step ( 1 - v/u)^-1 comes from ? Thanks in advance.
  39. P

    Doppler effect for accelerated source

    Homework Statement At the initial moment source and detector are located in same point and are both in rest. Source is released into free fall and detector remains in rest. Source has frequency fs=512 Hz and speed of sound is v=340 m/s . When detector detects frquency f =485 Hz , what...
  40. T

    Spectroscopy and the doppler effect

    So the prof shows us how to tell if an object is red shifted, by comparing two spectra (i assume of the same object). The emission lines had similar gaps but were shifted right, therefore moving away from us. How can you obtain two different spectra of the same object if it's constantly moving...
  41. W

    Understanding the Doppler Effect with Moving Sources and Observers

    Homework Statement Can't figure how to prove when both source and observer are moving that the observed frequency is the product of the two independent cases below.. Homework Equations We know individual cases when source is moving and observer is still, the observed frequency is equal to...
  42. M

    On the usual confusion among what it is and what is measure

    I have found a book from my father epoch of undergraduate student in Physics. It was apperently Physics 101, and was written with a typewriter and the formulae with pen. I have gone straight to Special Relativity section and, to my dismay, and even if all formulae were indeed correct, the...
  43. I

    Relativistic Doppler Effect: Explaining the Equation

    [mentor's note - lightly edited to fix the Latex] Hi there, I was hoping if someone could clear my small misconception for this equation. f' = f \left( \frac{ 1- \beta }{ 1+\beta } \right)^2 I had thought if the numerator is negative and denominator is positive that means the signal of light...
  44. N

    Moving vehicles and Doppler Effect

    Homework Statement Here is the problem: http://faculty.kfupm.edu.sa/PHYS/kuhaili/doppler_problem.htm {Mentor's edit: Here's the text copied from the url: A fire engine moving to the right at 40 m/s sounds its horn ( frequency 500 Hz ) at the two vehicles shown in the figure. The car is...
  45. J

    Exploring the Doppler Effect: Frequency & Distance

    Hi all, I was just thinking about the Doppler effect today and I was wondering why distance between the source and listener does not affect the frequency experienced by the listener. Consider the Doppler formula: fL = fS*(v+vL)/(v+vS), with the positive direction taken from listener to source...
  46. B

    Resolving Phase Ambiguity in Pulsed Doppler Ultrasound

    Hi. I understand the principle behind Pulsed doppler ultrasound. If the echo from a transmit pulse is sampled (gated) during a narrow fixed time period, the echo will correspond to a particular depth. What I do not understand is how we can extract phase information by repeating the process of...
  47. D

    Question concerning Doppler effect

    Homework Statement In the picture below, can you help me understand what do equations 3 and 5 mean. They concern the soppler effect but i can't seem to understand what do they represent. Homework Equations ft + fv/c=ft((c+v)/c fd=ft-fr 3. The attempt at solution
  48. B

    Light and Doppler effect paradox?

    Hello, I just thought of something that looks like a paradox to me. Suppose you have a device which can detect a light source's frequency, and will explode if the frequency exceeds a threshold value f_t. Place this device stationary wrt the light source. Now, suppose there are two observers, one...
  49. Y

    Frequency spectrum in Doppler effect

    Homework Statement I have been investigating the Doppler effect in a circular motion with a stationary source and moving observer (however the main aim is to determine the speed of sound in the end). Using Vernier software - Logger Pro - I have obtained two graphs of the sound pressure against...
  50. G

    Relativistic Doppler and Hubble's law

    I have a quick question about the Special Relativity. Non-relativistically, we expect no doppler shift in the wavelength of the emitted light if the source is moving at right angles to the line of sight to the observer. However there is a transverse doppler shift even in this case, caused by...
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