Doppler Effect: Velocity, Frequency and Wavelength Explained

[Stevay]

I've tried searching the forums, but to no real avail.

Here's my question (it is not really related to the Doppler Effect formulas/equations, or derivations of them):

Why is the observed frequency different when you have the source moving towards the observer at a given speed, and when you have the observer moving towards the source at the same speed? Unfortunately, you'll have to explain this to me in terms of velocity, frequency, wavelength, and fairly simple logic (as in my Physics class, we have not yet learned about blueshifts and redshifts).

 

[Janus]

Matbe these will help:

This first image shows lightwaves radiating out from a source and passing over two observers. (the red and blue dots} The source is stationary to both observers. Each observer sees the same frequency of light.

http://home.teleport.com/~parvey/doppler1.gif


The second image shows the same lightsource if it is moving to the right relative to the observers.

http://home.teleport.com/~parvey/doppler2.gif

As each part of the wave is emitted, it expands in a spherical wavefront from the point of emission. By the time the next part of the wave is emitted, the source has moved with respect to the observers, so this part of the wave is emitted a little to the right of where the first was emitted this sqeezes the wavelength to the right and stretches the wavelength to the left. Since the waves are still moving at c relative to the tewo observers, the observer on the right sees a higher frquency of light and the one on the left sees a lower frequency.

This is also true if you consider the observers as moving and the source as stationary.

 

[Stevay]

Well, I get that part.

However, what I meant was this:

The source is moving towards the observer at, let's say, 20 m/s.
The observer is moving towards the source at the same speed, 20 m/s.

In both cases, the observer notes a frequency different from that of the source's frequency (an observed frequency). In both cases, the frequency observed is higher. However, the frequency increase observed for each case is different. Why?

 

[Janus]

Originally posted by Stevay
Well, I get that part.

However, what I meant was this:

The source is moving towards the observer at, let's say, 20 m/s.
The observer is moving towards the source at the same speed, 20 m/s.

In both cases, the observer notes a frequency different from that of the source's frequency (an observed frequency). In both cases, the frequency observed is higher. However, the frequency increase observed for each case is different. Why?

The answer is that they don't. In both cases the observers see the same frequency.

 

[Poy]

Well, time is relative, so as you move faster the waves seem to come more often, wouldn't they? Should probably get a second opinion on this.

 

[Janus]

Originally posted by Poy
Well, time is relative, so as you move faster the waves seem to come more often, wouldn't they? Should probably get a second opinion on this.

Even when you take Relativity into account it is only the relative velocity between source and observer that counts, it doesn't matter which one you consider as moving,

 

[Stevay]

Well, my high school Physics teacher mentioned that, and from Google, the Doppler effect equations for a stationary source, moving observer, and a stationary observer, moving source are different (hence, making a difference in the frequency change in the scenario I mentioned).

 

[HallsofIvy]

Could you give us a URL on that? I googled on "Doppler Effect" and got several pages that gave a formula but they all used "relative velocity"- it didn't matter if it was the source or the observer that was moving.

 

[Stevay]

http://paul.wad.homepage.dk/doppler_theory/doc.html

 

[HallsofIvy]

For sound waves, it is the motion relative to air that is important. In that case, you can get different results if it is the transmitter moving (relative to the air) rather than the receiver moving (again, relative to the air).

 

[vu3ogi]

After reading this discussion, I remember one beautiful imaginary situation (read long back in some book) where you have to scratch your head if you have not understood relativity properly.


Assume that you and your friend are sitting in a train opposite to each other. Your facing is along the train movement and that of your friends' is opposite. What is the maximum speed of the train which doesn't disturb a conversation (sound -> 343 m/s, light -> 300 km/s) between you and your friend? ( Well, to continue conversation both of you should 'see' and 'listen' to each other

Does this have anything to do with doppler?

Take these cases and start thinking.
a. Train speed < 343 m/s
b. 343 m/s < Train speed < 300 km/s
c. Train speed > 300 km/s

(One more problem like this is - can we see the glowing of a bulb in the first compartment from the last compartment when the train is at a spped > speed of light? I had studied this in one of my MIR publisher collection on relativity.)


/*****************************/
VU3OGI is my HAM callsign
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[turin]

Originally posted by vu3ogi
Does this have anything to do with doppler?

It comes down to whether or not you are on an open platform, open box car, have the windows open, or are completely enclosed.

How fast do you think you are moving right now while you sit there reading the computer screen? Do you think that you are stationary?