# 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. ### I Doppler and Searchlight (Headlight)

The images below were taken from the website https://www.bing.com/videos/riverview/relatedvideo?q=ScienceClic+Alessandro+Roussel+What+would+we+see+at+the+speed+of+light%3f&mid=40C52E198C7F4F9D4EE240C52E198C7F4F9D4EE2&FORM=VIRE Starting at about min 11:30. Question: I know that objects that...
2. ### Doppler effect derivation for moving observer and stationary source

For this, Does someone please know where they got that ##f'## is number of waves fronts received per unit time from? Also could we write the equation highlighted as ##f' = \frac{n\lambda}{t}## where ##n## is the number of wavefronts in a time ##t##? I derived that from ##\frac{vt}{\lambda} =...
3. ### Where Can I Find Doppler Effect Sound Samples for Specific Scenarios?

Hi, I am looking for some sound clip examples of the doppler effect for some specific questions. Specifically I would like to find a single tone/horn recorded in the three different ways: 1. Not moving at all 2. Moving past the recorder at a slower speed 3. Moving past the recorder at a higher...
4. ### Doppler effect and acceleration of source

Hi. I need help with part a). I calculated the wavelength of the source by using the formula f_0 = v_phasefront / λ and got λ = (343 m/s) / (520 Hz) = 0.6596 m. And then I set up an equation for the velocity of the source v(t) = a*t (with v(t = 0 )= 0 m/s) and s(t) = 1/2 * at^2 + s_0. But I...
5. ### A Consistency check for Doppler correction calculations

Hello! This should be an easy calculation, but I want to make sure as the result seems a bit weird. Say I have an individual atom, for which I know the energy with a given uncertainty. In my case say I have ##1000 \pm 2.5## eV. Lets say that I perform collinear spectroscopy on this atom (for...
6. ### 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...
7. ### 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...
8. ### B Highschool Student Struggling to Understand Doppler Effect Paper

I'm a highschool student learning about astrophysics, and I'm trying to understand this paper by Dr. Sophia Cisneros because I find it interesting and I want to do an informational interview. The problem is, I just have really bad reading comprehension, especially with all the scientific jargon...
9. ### B Doppler Shifting of Light: Simplification or Reality?

Is red shifting of light from distant stars actually caused from them moving away from us, or is this a simplification. Doesn't this break the rule of light always going the same speed. Could the light rays be possibly lengthened because when the light left many yrs ago space was smaller, now it...
10. ### B Understanding the Doppler Effect at an Angle

hello everyone How is the sound Doppler effect formula in the case where the movement of the source and the observer is at an angle?
11. ### B Doppler effect on electromagnetic waves in a car

Is the doppler effect on electromagnetic waves receive by cell phone in fast car and so somebody talk with cell phone in fast car view this effect
12. ### I Does dividing maximum frequency by sound frequency give accurate velocity?

Hello everyone! I'm watching a Walter Lewin lecture and it seems to me at least that he is dividing maximum frequency of the sound by sound frequency of the transmitter to derive velocity of the transmitter, does this work? It seems that quantity would be dimensionless and velocity obviously...
13. ### I Formula for Doppler effect in moving medium?

Hi, I know the usual formula for both moving source and receiver in a static medium (from wiki): Is ir correct? What about when the medium is moving too? I can't seem to find an answer, and worst, I'm finding contradicting ones. For example, when the source and the receiver are moving at the...

16. ### I Doppler Analysis of Twin Experiment: Valid Argument?

in a discussion on the twin experiment, I read that an author (Darwin) proposed that the non-inertial traveler twin sees the transition between the redshifts and blueshifts at the very moment when he turns around, while the inertial twin sees this transition with a delay, which means that for...
17. ### I Deriving Doppler Effect Frequency w/ Stationary Person & Moving Source

Can you derive the formula for frequency observed from doppler effect with stationary person and moving sound source away from the person like this: ##v_t = v + v_s## where ##v_t## is the total velocity observed by stationary person from moving sound, v is velocity of sound and ##v_s## is...
18. ### I Energy conservation in Doppler (NOT cosmological) redshifts?

Hi all, My question is about Doppler redshifts, but I'm going to mention cosmological redshifts first because I'm a lay person as far as cosmology's concerned (I'm an amateur astronomer and did a few introductory astrophysics/cosmology courses at university, but my degree focus was planetary...
19. ### 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...
20. ### I Time Dilation vs. Doppler Effect: Similarities & Differences

Does time dilation in Special Relativity relate to the Doppler effect? If you move near the speed of light you experience time differently and the sound is stretched. Are these similar phenomenon?
21. ### I Relativistic Doppler Effect near a Black Hole

Hey everyone, if I were to view a shining person rotating near a black hole at near the speed of light there would be 2 kinds of redshifts: gravitational redshift and relativistic doppler effect redshift. Right? But, say at some point, the person is traveling towards me, then the doppler effect...
22. ### I Sound and apparent wind -- Any Doppler effect?

If two cars are driving side by side at the same speed, their relative speed is zero but do they nonetheless perceive a sound Doppler effect from the other car's siren because of the apparent headwind generated by their speed?
23. ### 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...
24. ### 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...
25. ### 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...
26. ### A How Does the Anomalous Doppler Effect Work?

From what I understand, the anomalous Doppler effect can occur when a charged particle moves through a medium faster than light would move through that medium; however in the paper, The Doppler Effect in a Warm Uniaxial Plasma, it mentions that this effect can occur when a dipole moves faster...
27. ### The relativistic Doppler effect

I found the observed frequency from the energy. Then I used the receding Doppler shift formula to find, the source frequency but after that when i tried to use the Rydberg equation I got a value for the energy level less than one. and I'm pretty sure my work is right, any help is greatly...
28. ### Doppler question - find the observed frequency

So first I calculate the final velocity by multiplying the time by the acceleration, 9.8, to get 88.2 m/s. Now I use the equation. (343/(343-(-88.2))*108.3 = 86.1477. But the answer should be 88.47. What am I doing wrong here?
29. ### B Doppler effect in moving medium?

Hi there, here's the problem: There's a sound, with a certain frequency coming, from a source. Both the listener and the source are fixed in a inertial reference frame. But there's wind blowing from the source to the listener. Now, this situation isn't the same as the listener chasing the...
30. ### Understanding the Doppler Effect: Solving the Bat Problem

I'm struggling a lot with this problem on the Doppler effect. I understand the first step which is to treat the bat as the source of the emitted sound, giving And the second to treat the bat now as the observer, but instead of using f_b on the left the solution involves setting both...
31. ### I Doppler effect - can't imagine how the frequency of light can change

AM/FM radio stations, cell phone towers transmit signals at certain frequencies. How can the frequency of a signal change depedning on whether the receiver is moving towards or away from the source? I thought that the frequency of an electromagnatic wave is determined at the source (the energy...
32. ### Doppler effect in Special relativity -- Solution verification

I am assuming that B is a stationary observer here. For the first part of the trip, using the formula, rocket A is approaching B at velocity $$v_A$$ =0.6 c. The length that A travels is $$L_A = v_A t_1$$ where $$t_1 =$$ 6 hrs. For the first part of the trip, B is receiving signals at a rate...
33. ### 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...
34. ### I Velocity Addition and Doppler Effect: Explained

The Doppler effect of light corresponds to the classical Doppler effect corrected by time dilation, but the first one is obtained with classical velocity additions (c+v at the front of the source and c-v at the back) whereas velocity addition of special relativity gives c at the front and c at...
35. ### Doppler effect and beat frequency

Summary:: Two speakers A and B are at rest, and a listener L stays on the line that connects the two speakers (see picture). The speakers have almost the same frequency. Assume that the speed of sound in air is 340 m/s. When the listener is at rest, he/she hears beats with frequency 6 Hz. The...
36. ### Find the altitude and the speed of two jets with Doppler Effect

Being honest, I couldn't do any works for this problem... (even couldn't start...) Does anyone can help me for solving this...? Thank you!
37. ### Doppler effect and hydrogen alpha distributions

https://www.asi.edu.au/wp-content/uploads/2015/03/PhysicsASOE2014solutions.pdf q 14b) i) Assuming that the planet is rotating at a constant rate, shouldn't the distribution be even across all wavelengths, or do I have something very wrong with my model. I take the graph as the summation of...
38. ### Changing of frequencies in the Doppler effect

I am trying to figure out why the relativity don't get in the doppler effect, that is, why can't we apply the "galileo transformations" in doppler effect? OBS: i am not saying about doppler effect relativistic (using Lorentz transformation) In another words, i am moving in direction to a source...
39. ### Doppler radar measurement by an aircraft

u is the aircraft speed. c is light speed f is the initial frequency λ is the initial wavelength λ' is the apparent wavelength λƒ = u +λ'ƒ λƒ = u + (c/ƒ')*ƒ c = u + (c/ƒ')*ƒ u = c(1-(ƒ/ƒ')) u = 1500m/s The answer is half of it, where is my error?
40. E

### B Is the Doppler Effect Formula Different for Sources in Different Regions?

If the source is at ##(t,0)## in ##S## and the receiver is at ##(t',0)## in ##S'## which moves at ##\beta_x## w.r.t. ##S##, then by considering two crests at ##(0,0)## and ##(T_s, 0)## in the source frame ##S## and transforming these events into ##S'## we can derive that ##\lambda_{r} =...
41. ### Doppler effect - two airplanes flying towards each other

I found it confusing since there is only "mutual" speed of both aircrafts and hence I do not know how to correctly put it into the common Dopplers formula...
42. ### 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?
43. ### 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...
44. ### Doppler effect and how it affects the light spectrum

Looking for a beginners explanation to the following question: How is Doppler effect separated from the original spectrum of light emanating from a moving body (in astronomy or other physics branches)? If the question does not make sense, here is the reasoning to ask it: If a certain color is...
45. ### Deriving the Relativistic Transverse Doppler Effect (Circular Motion)

**I realize some of my inline math delimiters '$$' and '$$' are not acting on the text for some reason, and it looks clunky. I spend 20-30 minutes trying to understand why this is, but I can't. My limited LaTeX experience is in Overleaf, and these delimiters work fine in that compiler. My...
46. ### How to link frequency and speed? Doppler effect

I had many attempts on trying to solve this one, but I got always stuck in the problem-solving part: how do I manage to find the source-speed from the Doppler formula, in an analytical way, and then reach to the result-formula? Anyway, I'm pretty sure the only formula needed to solve this...
47. ### B Some questions about the Doppler Effect at high velocities

Hey all. Question 1: Let's say we start with a red wavelength photon, that is absorbed by an atom and raises the atom to an excited state. The atom is then accelerated. After reaching a high velocity (say 0.1c), the photon is emitted. I would have thought that the energy of the emitted photon...
48. ### B Help with understanding the Doppler effect

My answer on this question for now is that producing a waves in the medium is an event which is basically must be invariant in the any of frame of references. For an example: a brick is freely falling, then the brick suddenly splinted into two pieces — no matter from which frame will we observe...
49. ### How to Solve a Doppler Effect Problem with a Moving Vehicle and Siren at Rest

Pretending the siren is at rest in air: Wavelength = velocity/frequence --> (343 m/s) / 10,000 Hz = .0343m. I don't believe this is the correct way to go about solving the problem, since the vehicle is moving at the start and the siren is not at rest.
50. ### B Doppler Effect: Light Blueshifted When Source Moving Towards Observer

If I and the light source are both moving toward one another, I see light blueshifted. What if I'm at rest and the source moving towards me?