# What is Doppler shift: Definition and 148 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. ### B Highway: Deriving speed from Doppler shift

I passed a motorcycle on the highway going the opposite direction. I know I was doing 125/km/h. I estimated that the frequency of his motor dropped by an entire octave, so that's a doubling of the wavelength. My intuition is telling me that's extremely unlikely. I can't actually calculate how...
2. ### Aberration and Doppler shift in Special Relativity

A large disk rotates at uniform angular speed ##\Omega## in an inertial frame ##S##. Two observers, ##O_1## and ##O_2##, ride on the disk at radial distances ##r_1## and ##r_2##, respectively, from the center (not necessarily on the same radial line). They carry clocks, ##C_1## and ##C_2##...
3. ### Speed of a star in Special Relativity

Hey, I have this problem from the Special Relativity by AP. French . Exercise 3.3, Chapter 3. The figure shows a double-star system with two stars, A and B, in circular orbits of the same period T about their center of mass. The earth is in the plane definied by these orbits at a distance R of...
4. ### I Doppler Shift & Christoffel Symbols Issues

About a month or two ago I started doing simulations of light physics around black holes and yesterday I got a fast Christoffel symbols function for the Schwarzschild metric in cartesian coordinates, but now the photon ring appears flipped. I feel as though it is wrong. But as I am still pretty...
5. ### A Understanding 3D Doppler Shift with Satellites

I am trying to understand a case of doppler shift if I have two satellite circling the earth with different velocities but the same direction and with different altitudes as well. How I will be able to figure out the doppler shift in three dimension between the two satellite if one of the...
6. ### SH2372 General Relativity - Lecture 5

0:00 What is spacetime? 20:20 Kinematics, basic definitions 35:18 Motion of test particles 41:18 Frequency shift of light 1:10:19 Simultaneity
7. ### I Ship Braking: Blue or Red Doppler Shift?

Let's imagine we have a ship heading towards our planet. It fires forward thrusters to brake. The ship is long way away. Because the ship decelerates, (slows down towards us) would we see particles from its exhaust doppler shifted blue or red? On the one hand the gas is flying towards Earth, so...
8. ### I Phase Speed of Wave in non-relativistic Doppler Shift Derivation

Consider the situation where an observer at rest on the ground measures the frequency of a siren which is moving away from the observer at speed ##v_{Ex}##. Let ##v_w## be the speed of the sound wave. Let ##\lambda_0##, ##f_0##, ##\lambda_D##, and ##f_D## be the wavelengths and frequencies...
9. ### Doppler shift between a moving ship on the ocean and a satellite in LEO

Summary:: I would like to calculate the Doppler shift for a signal sent by a moving ship to a moving satellite. I want to calculate the frequency observed by an LEO satellite when a signal is transmitted from a moving ship. The LEO satellite has a velocity of 7120m/s and orbits at a height of...
10. ### B Doppler Shift of Neutron: Energy & Momentum

When a photon emitted by a distant galaxy travels across space and is detected at a later time when the universe has expanded by a factor of 2, its wavelength, frequency, energy and momentum are all changed by a factor of 2. If a neutron is emitted with energy E and momentum p and is also...
11. ### I What is the four-momentum of a photon?

I'm trying to understand the Energy-Momentum relativistic relationships for a light particles. It is commonly said that the Energy of a photon depends on the observer by the relationship ## E = - \mathbf{p} \cdot \mathbf{u}## where p is the 4-momentum of the source emitting light particles and u...
12. ### I How to measure a 21 cm EM wave if it is Doppler shifted?

we know that all emission from asctrophysical context is doppler shifted. So, how to make sure the doppler shifted 21 cm not contaminated by some other emission?
13. ### Hubble's Law and Doppler Shift

(a) How far would a galaxy be whose speed with respect to the Earth is ##c##? Would it be observable from the Earth? r=rα=1.5⋅1026 m =1.6⋅1010 lightyears<4.65⋅1010 lightyearsr=rα=1.5⋅1026 m =1.6⋅1010 lightyears<4.65⋅1010 lightyearsr=rα=1.5⋅1026 m =1.6⋅1010 lightyears<4.65⋅1010 lightyears...
14. ### Beat frequency and Doppler shift

I found the beat frequency to be 4 Hz and the carrier frequency to be 260 Hz, but I'm not sure how to apply them to the solution/integrate the doppler and beat frequency equations?
15. ### I How to differentiate Doppler shift from star emission

I have seen people talking about measuring Doppler shift of stars to know how fast they are receding from us. But if I measured say a particular frequency f, how do I know whether this is shifted frequency or just the emission?
16. ### I Classical waves and the Doppler shift

In the video: The professor mentions how the wavelength will be the same for a moving observer vs a moving observer for a classical wave like a sound wave. However, how does that explain doppler shift? Don't we observe the effect because a moving observer measures a different wavelength than a...
17. ### Doppler shift of a hydrogen line on sun

Homework Statement Homework Equations [/B] delta lambda/lambda = velocity/speed of light radius of the sun= 696 *10^6mThe Attempt at a Solution Hello, the solution states the correct answer for this problem is 5.8*10^-12m. This is exactly 2x my answer but i don't really know what i did...
18. ### B How does the Doppler shift explain changes in photon energy without interaction?

According to doppler shift you can measure low energy photons at one side of a star and higher energy photons at the other side of a star. How is this possible? I know the classical explanation but what happens on an atomic level?
19. ### B Why Light Experienced a Doppler Shift?

It's already known in physical communities that the velocity of light always constant and not depend on motion of observer or source. But it is also teached to us that there are aslo Doppler shift experienced by light wave like that when measuring CMB radiation. But for me it is makes a...
20. ### B Why do photons allow Doppler shift

If we (a detector) are moving toward a star that emits a single photon (due to its distance) and that photon hits our detector, it will be blue shifted. My question is why. If the color of a photon is a reflection of its energy level and since the speed of the photon is always coming at us at...
21. ### Relativistic Doppler Shift for Transverse Movement

Homework Statement A sodium light source moves in a horizontal circle at a constant speed of 0.100c while emitting light at the proper wavelength of λ0=589 nm. Wavelength l is measured for that light by a detector fixed at the center of the circle. What is the wavelength shift λ-λ0? Homework...
22. ### How to calculate the Doppler Shift Frequency

I would like to know the formula for calculating the Doppler shifted frequency received by someone on Earth that is received from a spaceship traveling at 85 percent the speed of light away from Earth. Please show example of the formula being used. Let's say the spacecraft sent a radio signal...
23. ### I How can I identify the appropriate spectral line for Doppler calculation?

Hi, I was looking into Doppler shift calculations, and I came across this blog post. It gives a very simple and straight forward account of calculating the velocity of a star. Following is the summary the blog post: Spectrum of Alpha Centauri (A. Cen.) is obtained through Lhires III...
24. ### Doppler shift source velocity components

how does the doppler effect work when the observer and source are not moving in a straight line, what do we take in as the velocity of the observer and the source in the doppler effect equation?
25. ### Doppler shift source velocity components

Homework Statement Why does a source moving perpendicular to the line between it and the observer produce no doppler effect?Homework Equations The Attempt at a Solution I have heard the the radial velocity of the source relative to the observer must be zero but i don't understand why, if the...
26. ### I Photon number state and Doppler shift

If a mode of light is in the single photon state in the reference frame of the emitter, what will the state look like in a reference frame where the wavelength is, say, 5% less or more? How about a state with, say, 5 photons? I saw some online discussions and some papers on arXiv (not...
27. ### I Relativistic doppler shift and radar

Consider at stationnary radar at the origin ##z=0## and a target (speed ##v##) moving along the ##z## axis and away from the radar. The radar is sending plane waves (frequency ##f_i##) to the target and they come back to the radar (the radar is then both an emitter and a receiver). I am...
28. ### Can Light Wavelength Remain Unchanged at High Velocities?

Homework Statement A probe is launched with velocity v=0.8c. A beacon emits a light with wavelength \lambda=500nm in its rest frame. Years later the probe is located by NASA using a telescope, When they measure the light they find the wavelength \lambda=500nm in their rest frame. Is this...
29. ### I Doppler Shift and Light Quanta: Questions on the Nature of Light and Observation

Dear All, I have a couple of (noob) questions regarding Doppler Shift and light from a quantum physics perspective: a) Since different observers will see the light at different frequencies depending on their reference frame / velocity thus resulting in Doppler Shift, does that mean that any...
30. ### Doppler shift derivation from Lorentz Transformations

Homework Statement Show how one can obtain the Doppler transformation for the frequency of a receding source just using the Lorentz transformations for the energy (where E=h). Homework Equations Relativistic transformations for momentum and energy: E = γ(E' + vp'x) pc/E = v/c = β The Attempt...
31. ### Doppler shift and change in intensity of a sound wave

How are the intensity of a sound wave and the Doppler shift of frequency related togheter? That is, if the source or the observer are in relative motion, how does the intensity change? For a sound wave $$I=\frac{1}{2} \rho \omega^2 A^2 c=2 \pi^2 \rho f^2 A^2c$$ (##c## is sound speed, ##\rho##...
32. ### Bouncing ball and Doppler effect

Homework Statement Consider two parallel walls perfectly reflective placed at the distance ##d = 0.8 m ##. A ball, provided with a device through which are emitted continuously frequency sound waves equal to ##f_0=430 Hz##, is launched from one wall to another. It moves with constant velocity...
33. ### Help with Doppler shift galaxy question

Homework Statement Hi, I'd really appreciate being pointed in the right direction with this past exam question, as I just have no idea how to approach it and can't find similar examples in my notes anywhere. A nearby star has a spectrum similar to the sun, suggesting it has the same mass. The...
34. ### I Doppler shift for an observer in circular motion

Say we have an observer in perfectly circular motion around a source, like a star. Is it reasonable to apply the angle change formula ##cos \theta_o = \frac{cos \theta_s - \frac{v}{c}}{1-\frac{v}{c}cos \theta_s}## and then take the component of the motion parallel to the light wave in the...
35. ### A Effect of Blue-Shifted GWs on Particles

I assume that the energy transported by a gravitational wave is dependent on its frequency, just like EM waves. If that is the case, let's imagine a particle traveling at relativistic speeds relative to the lab frame. Let's imagine the interaction of this relativistic particle with a...
36. ### 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...
37. ### 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.
38. ### Does Length Contraction affect measured velocity?

First, let me clarify if my understanding of length contraction is correct. Is it accurate to say that relativistic velocities not only affect the measured length of an object in the direction of motion, but also the distance to the object from an observer in the direction of motion? For...
39. ### Need help with physics experiment (Speed of sound)

< Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > Last week we had to conduct an experiment with the aim of determining the speed of sound. I decided to play with doppler effect. I was more or less sure what I had to do but then the teacher came...
40. ### 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...
41. ### Expansion is a compounded error based on a wrong assumption?

I accept that what I write below will be unpopular and argued with vehemently, however I think it should be considered with an open mind and recognised for the sense that it has. Originally the concept of an expanding universe (and therefore by extrapolation the concept of the Big Bang that...
42. ### Why Is My Calculation of Doppler Shift Different from the Standard Formula?

Good day to you all, I wanted to share a struggle with you all that I can't seem to get out of. It's about calculating, in the case of sound, the decrease or increase in frequency (thus increase or decrease in wavelength) when a sound source is respectively moving away or towards an observer...
43. ### Swinging fans & Doppler's shift

Homework Statement http://postimage.org/][/PLAIN] free picture upload Homework EquationsThe Attempt at a Solution I consider that if the fan is swinging, the source of sound (fan) will have an speed that lead to doppler's shift. but i can't calculate for this. i think speed of source will be...
44. ### Derivation of Relativistic Doppler Shift: Step-by-Step Guide

i need thee the derivation of relativistic doppler shift. i found its derivation in the book of Serway, but it is not in detail and step by step. in the book there is given that: can anyone give me the step after eq#1.13? how eq#1.13 converted in the form of eq#1.14?
45. ### Cosmological vs. Relativistic Doppler Shift

Does anybody know a good read/primer that compares the observed cosmological redshift values to predictions made by the relativistic (gravitational) redshift? Thank you!
46. ### Sound wave propagation and doppler shift wrt the medium

In contemplation and research regarding another thread's question about the asymmetry of doppler shift wrt the medium, I find myself trying to imagine two people talking... or perhaps better yet matching pitch. In still air with both people facing each other at ten paces of course they would...
47. ### Deriving the doppler shift equation for sound

I am looking at derivations of the Doppler effect for sound and light, and I am very confused about what stays the same when it comes to sound. In this video, at 5:10, it is said that the wavelength will be the same. I'm not sure I agree with this because I'm pretty sure both the observed...
48. ### Exploring Doppler Shift: How Motion Affects Wave Frequency

Hello to everyone! First I would like to go through one simple example: We have two bodies. One is wave source, other is wave receiver. In this example they are positioned like this: (source) O=====> (receiver) O=====> Distance between them is always 1000 m. Let's say that the waves are...
49. ### Fourier transform power dependent on frequency

Homework Statement this is something i noticed doing homework rather than homework itself. I plot fft output from different frequency signals, i am not sure why power changes with increasing frequency? Homework Equations if i take (with MATLAB notation): time = 0:0.01:10 y =...
50. ### Doppler Shift of Light: Visible-to-UV Properties?

If an observer and a visible light source are in relative motion such that the visible light source is blueshifted to ultraviolet frequencies, then would this visible-light-turned-ultraviolet-light being received by the observer have the same properties as that of actual ultraviolet light i.e...