# doppler-effect's questions - English 1answer

288 doppler-effect questions.

### 2 Why is the derivation of the Doppler effect different for electromagnetic waves and mechanical waves?

I have read that the Doppler effect occurs for both mechanical and electromagnetic waves, the reason being that mechanical waves require a medium while electromagnetic waves don't. The formula for ...

### 5 What is the evidence that distant galaxies are moving away from us with speeds greater than $c$, due to space expansion?

I came up with this query after @Rob Jeffries's answer to a previous question of mine. So, is there any evidence that distant galaxies are moving away from us with speeds greater than $c$, due to ...

### 2 How does the relativistic Doppler effect model change if I want to include general relativistic effects also?

Derivations for the relativistic Doppler effect abound, which extends the description of the Doppler effect to include the effects of relativity in cases where the source or observer is moving at ...

### 3 Proof for photon energy loss due to cosmological redshift

According to Doppler redshift, the frequency of the EM radiation decreases if the emitting object is receding from the observer - this produces the problem in the quantisation of energy - the observed ...

### How do you calculate the recession velocity?

1 answers, 36 views doppler-effect absorption redshift
\begin{alignat}{7} && \frac{\Delta \lambda}{\lambda} & = \frac{v}{c} \\[2.5px] &\therefore & v & \approx \frac{\Delta \lambda}{\lambda}c \end{alignat} If you want to find ...

### How does emitting light while moving not break the law of conservation of energy?

Say you have 2 identical LED monochromatic flashlights. The battery lasts exactly 1 hour. We are able to calculate the energy emitted as photons from the LED by Planck's Law: $E = h\nu$. Now if we ...

### Redshift - How can Z be greater than 1?

I'm having trouble understanding the equation for redshift: z = Δλ/λ ≈ Δf/f ≈ v/c If z = v/c and c = speed of light, how can z>1 (as nothing can exceed the speed of light)?

### How does conservation of energy explain the Doppler effect?

According to modern physics (although I don't know exactly what the theories that back it up are,) the energy of a system is conserved within a frame of reference, such as in the example ...

### Do high speed particles see light as a different part of the EM spectrum due to the Doppler effect?

If a particle was travelling fast enough and had visible light shot at it, would the partice experience the light as if it was higher or lower wavelength? If so, would the particle see the incident ...

### 3 How is it possible to have red shift along one axis and blue shift on another in a spiral galaxy arm?

I don't understand this image: If the stars are moving in roughly circular orbits why would red shift occur in the direction of the orbit and how would it be possible to have blue shift in two ...

### Doppler shift of light in uniform circular motion of source

2 answers, 40 views special-relativity doppler-effect
Suppose, a monochromatic light source is undergoing uniform circular motion, and the observer is at the center of the circle. When the velocity of the source is perpendicular to the line joining the ...

### Meaning of $v_\mathrm{source}$ in the Doppler effect

The Doppler equation is given by $$f_{observed} = \frac{c+v_r}{c+v_s} \cdot{ f_{emitted}}$$ Does $v_s$ refer to the velocity of the source, relative to the receiver at the time the wave (now at the ...

### 1 Doppler effect equation derivation

2 answers, 716 views special-relativity doppler-effect
I am after to model the Doppler shift of an LEO satellite. At [1] (pp 183) I have found the following formula: $$\frac{f_r}{f_s}=\frac{1-\frac{u}{c}\cos\theta}{\sqrt{1-\frac{u^2}{c^2}}}$$ $f_s$ ...

### Doppler effect in accelerated motion [duplicate]

We know the formula for apparent frequency when source or observer or both are moving with uniform velocity. But what is the apparent frequency when they are accelerating with respect to each other.

### 2 Does the gravitational Doppler effect affect charge?

0 answers, 52 views gravity charge doppler-effect
I know that the charge of objects large enough to have distinct gravity is for the most part irrelevant, but large bodies do have (at a given instant) a fixed charge, which, granted, might be hard to ...

### Spectral lines and the Doppler effect

We can detect the presence of an element in, say, a star because of the frequency of light that it absorbs which produces a line on an absorption spectrum. And we can detect relative motion towards or ...

### How do astronomers know what wavelength the body will emit when it is at rest? [duplicate]

In the book of The First Three Minutes by Weinberg, at page 21, he talks about how do astronomers measure the speed of a luminous body along the line of sight by Doppler affect, i.e. the fractional ...

### Gravitational time dilation - Kip Thorne example

2 answers, 100 views general-relativity doppler-effect
Please find in the images an example from Kip Thorne’s book ‘black holes & time warps’ on Gravitational Time Dilation. I do not understand the last sentences where the doppler effect results in ...

### The Doppler effect from a uniformly moving mirror

0 answers, 41 views optics doppler-effect
Regarding this paper by Aleksandar Gjurchinovski, The Doppler effect from a uniformly moving mirror on page 4 he says that eq. (8) can be transformed into eq. (12) by the use of "some simple algebra"...

### -2 Is length contraction in Special Relativity the same as the Doppler Effect?

In my further reading of Special Relativity, the idea of length contraction when travelling at the speed of light is such that the length gets "squished" in the direction of travel. This immediately ...

### Perpendicular doppler effect photons

I have a photon with frequency $f$ in the lab frame and I observe it in a inertial frame moving perpendicular to the photon. The direction of the velocity of the photon must change, but does this ...

### What does it mean by universe is expanding? I know that the universe is expanding because spacetime is expanding but again what does it mean?

I am aware about the balloon expanding analogue, where the galaxies are stuck on the balloon surface and here the balloon is our space time and as the balloon expand our galaxies also go apart. But ...

### 1 What is this connection between clocks and the Doppler effect?

2 answers, 62 views special-relativity doppler-effect
I derived an equation for the relativistic Doppler effect in the course of a routine problem: the frequency change of a photon moving in the $x$ direction, when considered a frame S' which has a ...

### Does Doppler Effect work this way?

If i am looking at an object which is moving towards me fast enough while i am at rest, then i would see the object's color shift towards blue in the Electromagnetic spectrum. But would the same ...

### A joke on the Doppler effect: does this make sense? [closed]

2 answers, 146 views stars doppler-effect
I'm currently writing something that explores the phenomenon of inside jokes, in which I use an astrophysicist joke that is meant to be undecipherable to the average reader. It was recommended to me ...

### 1 Relativistic Doppler effect and Quantum Mechanics

Consider the situation below. I’m at a point in space along with a bunch of detector away from any gravitational field. I see a star moving away from me at a relative speed of $$v$$by measuring the ...

### how to work out conservation of energy and doppler shifts?

Let's modify a bit this question so that we use a single reference frame Let's imagine a laser shining light on an object. Let's put the reference frame at the object. In one instance the laser ...

### 1 Doppler shift and relative velocity

I have come across Doppler shift equation. If we consider the source to be moving toward the observer at a speed $V$ and observer moving toward the source with a speed $U$, for the calculation of the ...

### Mass changes by absorbing light energy with Doppler shift

Suppose you have a mass that is moving at relativistic speed wrt (with respect to) a powerful laser pointed at the mass. As the laser shines on the mass, assume it absorbs all the energy (and momentum)...

### Doppler effect for moving receiver and moving transmitter

0 answers, 58 views vectors doppler-effect
For the case that receiver and transmitter are moving, the shift in frequency is given with $$f_{receiver} = f_{transmitter} \frac{c + v_{receiver}}{c - v_{transmitter}}$$ if they approach each ...

### 1 Can the Doppler effect account for the increase in an object's energy with velocity?

I am aware that the relativistic mass can be expressed in terms of rest mass and momentum, which seems to be the canonical explanation, but I am looking at an alternative way of visualising the ...

### 1 Can sound waves be used as simple explanation of relativity effects in STR?

2 answers, 275 views special-relativity waves doppler-effect
There are so many similarities (Doppler Effect, independence of wave velocity from source speed etc..). Try moving in your car with music and ask you friend outside record it while you moving towards ...

### 1 Moving sound sources and wavelength

3 answers, 83 views acoustics doppler-effect
How does a moving sound source physically compress the waves emitted in front of it and lengthen the waves emitted behind it? Virtually all descriptions of the doppler effect provide nice visuals of ...

### Are all effects that change color perception examples of redshift/blueshift?

3 answers, 548 views visible-light doppler-effect
As defined by Wikipedia: In physics (especially astrophysics), redshift happens when light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to ...

### 33 Can gravitational waves be red-shifted?

1 answers, 3.417 views gravitational-waves doppler-effect
Whenever the Doppler effect is mentioned, it's typically in the context of sound waves or electromagnetic radiation. On the cosmological scale, red-shifting is also important because of the enormous ...

### 4 Why doesn't the speed of the wind have an effect on the apparent frequency?

3 answers, 3.209 views waves frequency doppler-effect
A boy is standing in front of stationary train. The train blows a horn of $400Hz$ frequency . If the wind is blowing from train to boy at speed at $30m/s$, the apparent frequency of sound heard by the ...

### Doppler effect and shift in visible region and constant velocity of waves

I have learned that due to doppler effect the wavelength of light increases when source is moving away from observer. Also this proves big bang theory and also universe is expanding. Like red shift , ...

### Doppler effect and constant velocity of a wave

Why doesn't the velocity of a photon change when it is emitted from a source moving with certain velocity? If a photon is a particle then it should have a lower velocity than one which is emitted from ...

### A doubt on Doppler effect [duplicate]

As I learnt freq. observed by a observer from a moving source is given by $$f=f_0(1+v_s/v)^{-1}$$ If $$v_s=-v$$ f is infinity.Whats going on? Will Doppler effect will be observed if the source say is ...

### 2 Why does heterodyne laser Doppler vibrometry require a modulating frequency shift?

3 answers, 584 views optics laser doppler-effect interferometry
On the wikipedia article (and other texts such as Optical Inspections of Microsystems) for laser Doppler vibrometry, it states that a modulating frequency must be added such that the detector can ...

### Laser / doppler cooling setup

0 answers, 22 views laser doppler-effect cooling
Would it be practical to use doppler / laser cooling to cool a small surface (metal conductor) of 10 x 10 cm from 30 C to 20 C? Let's say using a setup like below? Where can I learn more about the ...

### 1 Doppler Effect and Relativity

The equation for the Doppler effect is given by $$f_L = \frac{v+v_L}{v+v_S}f_S$$ where the velocities of both the source and the listener matter. My question is, how does this fit into Galilean ...

### Doppler effect of sound waves in blood

When the sound waves that are produced by a ultrasound machine reflect there happens an increase into the frequency of the sounds. Why is that so? I think it should be the other way around because the ...