# Tagged Questions

The Doppler effect refers to change in the observed frequency of a wave if the observer and source are in relative motion.

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### Doppler Effect, Why Can't We Just Add Speed to Argument?

In considering the doppler effect, why can we not just take the distance between the two as a function of time, divide by the wave velocity, and add this as a shift in our time argument of the ...
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### Doppler effect observed in octaves

I have a question about the interpretation of the Doppler effect, when you look at the results as a change in octaves. Nothing actually changes when you look at the result in octaves instead of ...
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### Questions on redshift [closed]

What are different kinds of redshifts and are they related to each other? If yes, how? Is redshift a measure of distance or time or etc? For example when we say a supernova with a redshift of $z=0.9$ ...
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### What speed should have an alone planet to have a habitable zone due to relic radiation?

What speed should have an alone planet to have a habitable zone due to relic radiation, without any star involvement? How much time the planet will be able to remain in the habitable zone before the ...
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### Relativistic beaming - which power of the Doppler factor

The context is that I'm building a simulation of a starfield, as seen from a relativistic spaceship. (EDIT: the simulation can now be found here.) One reference that I'm using is this paper by John ...
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### At what frequency does the medium of air change from acoustic to light?

I'm calculating the max doppler frequency of a fading channel in an in-room environment and looking at different carrier frequencies. Calculated as follows: F = vf/c Where F -> max doppler shift v ...
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### some questions regarding Doppler shifting versus absorption-emission

I just got in what I thought was a silly exchange where a self-identified physicist states that the difference between "red-shifting" (in the Doppler sense) and the re-emission of light at longer ...
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### Distance to the galaxy which has velocity which is perpendicular to the point of view of the static observer

Let's have the galaxy which has velocity which is perpendicular to the point of view of the static observer in the FLRW universe. It emits the light. There is a given redshift $z_{0}$. It will be easy ...
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### Relation between red filter band and redshift? [closed]

I am interested in knowing what is the significance of red filter band in the study of redshift dependence of spatial orientation of galaxies?
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### Equivalence principle and acceleration vs a gravitational field

I picked this up on the net: Einstein came to realize the principle of equivalence, and it states that an accelerated system is completely physically equivalent to a system inside a gravitational ...
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### Why does the sound of a tuning fork change as we walk away?

This is a question I got for my class. We have the following situation. We take two tuning forks, we hit them, then we take one of the tuning forks and walk away with it. The sound changes as we walk ...
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### About the standard derivation of the gravitational redshift

The objective is to derive the gravitational redshift ONLY from the Einstein's equivalence principle (E.E.P.), without using the whole theory of Relativity. This is the standard "informal" derivation ...
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### is redshift still true if object emitting is not moving away but the observer is

If something is moving away fast it's color would appear redder than its actual color due to phenomenon known as Doppler effect. But what if the object isn't moving but i am moving away faster would ...
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### Doppler effect problem with moving mirror

This was the given question: A light beam of intensity $I$ and frequency $f$, directed along the positive $z$-axis, is reflected perpendicularly from a perfect mirror which itself is moving ...
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### Reduction in gravity from photon exchange

Let's say you have two bodies at are held at rest relative to each other exchanging (real, not virtual) photons back and forth. Then we let them go (maybe push them apart slightly), so they will ...
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### The Equivalence principle of General Relativity and the Doppler Effect

I am studying General Relativity and am trying to understand the Equivalence Principle more thoroughly. Basically, it is said that if you are in a uniformly accelerated frame of reference in free ...
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### Why same formula describing Doppler effect in the case of light in different situations?

I made comparison between Doppler effect in light and sound and one interesting thing that I saw was that in the case of sound there are different formulas to find the apparent frequency in different ...
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### How is mobile communication made possible from airplanes?

I have heard that communication using mobile phones begins to degrade when used on a vehicle moving at speeds above 200 km/hr due to doppler effect as described here (p-22). How is this limitation ...
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### The Doppler effect in a medium like air (sound) versus the electromagnetic Doppler effect

When you have a listener and a source, and when one of the two move relative to the other, the frequency perceived by the listener will be different. Example: If the listener travels toward the ...
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### Dopler Shift for Reflected Sound Waves

If a car was going at near mach-1 speed heading towards a wall and honked its horn. What frequency would the driver measure the reflected sound wave? I trying to think about this reasonably, but the ...
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### Is it better to call the doppler effect a change in wavelength or frequency?

Why is it preferable to say that the doppler effect causes a shift in frequency rather than a shift in wavelength? I often read on websites that they define the doppler effect as a change in ...
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### Radar as range detector between cars-static sender vs dynamic sender

Say I have a radar transmitter on a car,used to detect its distance from a car in front of it by measuring the doppler shift on the reflected wave. i.e Car 1 is the source, its wave gets reflected by ...
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### Why does a doppler red shift prove expansion?

My question boils down to this. If the Universe was contracting the stars closer to the center would move faster to the middle than stars that were further away from the center. That would also ...
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### Relativistic Doppler Effect compared with Classical Doppler Effect

Looking at the classical Doppler effect there is one generalized equation, and they have a velocity of source, and observer. In the relativistic version, there is only one velocity taken into ...
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### Manipulating a formula for a relativistic Doppler shift

Lets say we take the standard configuration when $x'y'$ is moving away from system $xy$ (image 1). By knowing that the phase is constant in all frames $\phi=\phi'$ we can derive the Lorenz ...
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### A Doppler Effect problem with a moving medium

I tried solving the following question and started having multiple doubts: Two cars A and B are moving towards each other with some speed $25$ m/s. Wind is blowing with speed $5$ m/s in the ...
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### Is there a way to create a flickering frequency to be dependent on speed of the person looking at it?

Is there a way to make a screen or a road sign flash at different rates, depending on the velocity of the observer looking at it? I would like to achieve a state where two observers going at ...
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### Relativistic Doppler effect derivation

This is about a step in a derivation of the expression for the relativistic Doppler effect. Consider a source receding from an observer at a velocity $v$ along the line joining the two. Light is ...
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### In terms of the Doppler effect, what happens when the source is moving faster than the wave?

I'm just trying to understand this problem from a qualitative perspective. The Doppler effect is commonly explained in terms of how a siren sounds higher in pitch as it is approaching a particular ...
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### Independence of frequency in sound waves?

Why does the frequency of sound wave depend only on the source? Why is the frequency and not any other "quality" independent of everything but the source? And that said, why is velocity and ...
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### Special Theory of relativity on electromagnetic waves

Since time slows down and length contracts, when we travel almost at speed of light, if the speed of light (or EM waves) remains same and the wavelength of light remains same, do we measure the ...
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### Pound-Rebka-Snider experiment in the inertial frame

In Schutz's book (page 120), Schutz first derives the gravitational redshift in the PRS experiment in a previous paragraph. $\frac{\nu^{\prime}}{\nu}=\frac{m}{m+mgh+O(v^4)}=1-gh+O(v^4)$. Here ...
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### Red/Blue Shift in EM Waves

A Doppler weather radar operates at a frequency of 3.40 GHz. The wave from this radar system reflects from an approaching weather system moving with a speed of 39.0 m/s. What is the difference ...
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### Senior High School work on Doppler measurements

Sorry this question is really vague- as a result I'm very confused. Perhaps you can help me make sense of it. "Radar measurements: Construct a space-time diagram of the radar tracking of an asteroid ...
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### How would I perceive a purple ball when traveling at relativistic speeds

I have a 'thought experiment' which I'd like to ask about. I was thinking what speed I would have to drive towards a crossing to see a red light as a green light - pretty easy, using the doppler ...
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### Blue-shifting as opposed to violet-shifting

A recent XKCD comic implies that the sky is blue as opposed to violet due to human physiology, and that animals more sensitive to shorter wavelengths will perceive the Earth's sky as the shortest ...
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### Meaning of negative frequency of sound wave

Suppose that Alice and Bob are both holding speakers emitting sound at a frequency $f$. Alice is stationary while Bob is moving towards Alice at twice the speed of sound. In the case of Alice, if I ...
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### Doppler cooling limit vs recoil limit

I was discussing laser cooling in class today and I understood that the main principle of the process is to tune a laser to a frequency lower than the absorbtion frequency of the atom and so only the ...
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### Doppler effect “apparent frequency”

In discussing Doppler effect, we use the word "apparent frequency". Does it mean that the frequency of the sound is still that of the source and it is some physiological phenomenon in the listener's ...
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### Relativistic Doppler effect

If i have 2 coordinate systems (CS) which are travelling one towards another. CS $xy$ with an observer in its origin and CS $x'y'$ with a source in its origin. Correct me if I am wrong, but i think ...
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### Are all effects that change color perception examples of redshift/blueshift?

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 ...
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### How does the Hubble Redshift work?

I am a little confused about the workings of the Hubble Redshift. I do understand the classical Doppler-effect, however in special relativity the velocity of light c is a natural velocity limit. So ...
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### How does Doppler effect differ between EM -waves in Electrodynamics and Sound -waves?

I have an electrodynamics -course that contains doupler -effect but unfortunately with little explanations. Is it the same thing as the classical doppler effect for example with sound, more here, or ...
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### Why does heterodyne laser Doppler vibrometry require a modulating frequency shift?

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 ...
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### Sound frequency of dropping bomb

Everyone has seen cartoons of bombs being dropped, accompanied by a whistling sound as they drop. This sound gets lower in frequency as the bomb nears the ground. I've been lucky enough to not be ...
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### Why did we need relativity to derive $E=mc^2$?

Okay, so the way I understand one of the "derivations" of $E=mc^2$ is roughly as follows: We observe a light bulb floating in space. It appears motionless. It gives off a brief flash of light. We ...
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### quantum explanation of doppler effect

How would quantum mechanics explain doppler effect? And just for curiosity, is there any effect similar to doppler effect occuring at quantum level?
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### Is the classical Doopler Effect, for light shift, $1-v/c$, exact? What is it an approximation of?

Is the classical doopler effect for light shift equal to $1-v/c$ exact or an approximation of a classical formula? I know that it is an approximation of the relativistic formula, but what was the ...