Fizeau. What is connection between fringe shifts and velocity?

In summary: When Fizeau (and Michelson and Morley) measured the fringe shifts that occur when light travels through a moving medium, how exactly (in words) did they deduce the change in velocity?They counted the fringe shifts that occurred, calculated what the frequency shift must have been based on those fringe shifts, and then calculated what the velocity shift must have been based on the frequency shift.
  • #1
lenfromkits
107
0
When Fizeau (and Michelson and Morley) measured the fringe shifts that occur when light travels through a moving medium, how exactly (in words) did they deduce the change in velocity? My understand is this:

1) Count the fringe shifts that occur.
2) Calculate what the frequency shift must have been based on those fringe shifts.
3) Calculate what the velocity shift must have been based on the frequency shift.

So, my real concern is with #3. What is the relationship that they used between the frequency and the velocity? Does a 1% increase in frequency translate into a 1% increase in velocity? This is puzzling because "normally" in a vacuum, a frequency change does not mean there is a velocity change (c is constant), so how do they manage to figure out what velocity change would be associated with a frequency change?

Thanks for your help.
 
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  • #2
It is not a frequency shift, it is a phase shift. The frequency is determined by the light source and is the same for both paths. The phase shift gives you the time delay, and the time delay combined with the path length gives you a difference in speed.
 
  • #3
DaleSpam said:
It is not a frequency shift, it is a phase shift. The frequency is determined by the light source and is the same for both paths. The phase shift gives you the time delay, and the time delay combined with the path length gives you a difference in speed.

Thanks. I see, that makes sense. But that leads to a new question. Given the what we know about the Doppler shift, shouldn't the moving water have shifted the frequency as well?
 
  • #4
lenfromkits said:
Thanks. I see, that makes sense. But that leads to a new question. Given the what we know about the Doppler shift, shouldn't the moving water have shifted the frequency as well?
No. Why would it?
 
  • #5
DaleSpam said:
No. Why would it?

It seems like it would - for the reason I mentioned. Doppler shift. Is there any documentation about this effect in this scenario?

Normally (in vacuum), adding to the forward motion of light decreases its wavelength and increases its frequency (no change in speed). This could be in essence described as "instead of the forward motion being added to the speed of light, the entire forward amount is translated into a reduction of wavelength and increase in frequency).

In the medium, however, the forward motion 'does' add onto the speed of light - BUT only partially. "Part" of the forward motion results in an increase of light. The rest of the forward amount must then be attributed to a reduction in wavelength and increase in frequency.
 
  • #6
lenfromkits said:
It seems like it would - for the reason I mentioned. Doppler shift.
What Doppler shift? The emitter is not moving relative to the detector.

lenfromkits said:
Normally (in vacuum), adding to the forward motion of light decreases its wavelength and increases its frequency (no change in speed).
Sure, in the case where the emitter is moving towards the detector. That is the usual Doppler effect and does not apply here since the emitter and detector are stationary wrt each other.
 
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  • #7
If a distant planet is moving towards us, then there is a blue shift. If the distant planet is stationary relative to us, then there would be no shift.

If light from a stationary planet is somehow 'boosted' along the way (like in the case of Fizeau) then it would have the same effect as the planet moving towards us, even though it is not.
 
  • #8
lenfromkits said:
If light from a stationary planet is somehow 'boosted' along the way (like in the case of Fizeau) then it would have the same effect as the planet moving towards us, even though it is not.
No, it wouldn't, it would only change the speed and wavelength, not the frequency.

When you are doing EM problems and you have some boundary with incident and transmitted radiation the frequency of the incident and the transmitted radiation must be equal or you will get a phase discontinuity at the boundary. This is not particular to light, but is a feature of all kinds of waves.
 
  • #9
So in plain english, do you mean that: if the frequency in the Fizeau experiment 'HAD' been altered, then the resulting fringe pattern on the screen would have been a mess? Ie, the fringe patterns only show a fringe pattern if the frequencies are the same? Is that right?
 
  • #10
No, in plain English, if the frequency in the Fizeau experiment had been altered then light would not be a wave and would not follow Maxwell's equations. You wouldn't be able to do interferometry at all if it weren't a wave.
 
  • #11
DaleSpam said:
No, in plain English, if the frequency in the Fizeau experiment had been altered then light would not be a wave and would not follow Maxwell's equations. You wouldn't be able to do interferometry at all if it weren't a wave.

Thanks. Still trying to fully understand your comment "e frequency of the incident and the transmitted radiation must be equal or you will get a phase discontinuity at the boundary" in my own words.

The point you just made about waves and Maxwell, however, are not relevant for light - because the forward motion of a body on light DOES increase its frequency - aka Doppler Shift. The Fizeau experiment is kind of a hybrid between normal wave dynamics and light wave dynamics. The refractive index is a "continuum" such that for Fizeau and his water, the index was 1.33. For air it is 1.0003. Given that there is not clear difference between water and air other than they sit at different points on this refractive scale, when light travels through moving air - at least to some degree the speed must increase of the light (given that it is not an exact 1 refractive index). However - when light is boosted when traveling through air, we definitively see a frequency shift, implying that light through water would do the same thing, although just to a smaller amount. The only difference between the two scenarios is that in one case the light is 'boosted' by the emitter whereas in the other case it is 'boosted' along the way by the medium - which really narrows down the whole question to just this point - will that medium shift the frequency in the same way.

I realize that I need to investigate this based on documentation about the experiment but have a hard time finding it -which is really my question - where to find it. I have read several articles on the topic of the doppler shift in Fizeau's case, but they were arguments for Ether and blended in too much theoretical discussion for me to figure out what exactly is observed.
 
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  • #12
lenfromkits said:
The point you just made about waves and Maxwell, however, are not relevant for light
:rofl: You may want to rephrase that.

lenfromkits said:
- because the forward motion of a body on light DOES increase its frequency - aka Doppler Shift.
Sure, but the emitter and the detector are not moving towards each other in the Fizeau experiment. You really need to forget about the Doppler effect, it is completely irrelevant here.

lenfromkits said:
The Fizeau experiment is kind of a hybrid between normal wave dynamics and light wave dynamics. The refractive index is a "continuum" such that for Fizeau and his water, the index was 1.33. For air it is 1.0003. Given that there is not clear difference between water and air other than they sit at different points on this refractive scale, when light travels through moving air - at least to some degree the speed must increase of the light (given that it is not an exact 1 refractive index).
No, the speed of light is always slower in air or water than in vacuum. It is never increased.
 
  • #13
DaleSpam said:
:rofl: You may want to rephrase that.

Sure, but the emitter and the detector are not moving towards each other in the Fizeau experiment. You really need to forget about the Doppler effect, it is completely irrelevant here.

No, the speed of light is always slower in air or water than in vacuum. It is never increased.

I'm not sure what you mean by "it is never increased" The whole result of the Fizeau experiment shows that light speed increases. Speed of light when in a medium increases. My point is that speed of light in air also increases the frequency when the emitter moves, therefore implying that if the emitter moves in a medium then also the frequency would increase. You suggested I forget about the doppler effect but that's the whole point of my question. The question stems around one issue - does a moving medium have a similar effect on the frequency of light as a moving emitter? Clearly the answer is not clear. I appreciate your opinion, but what I need is actual experimental documentation to either refute or confirm that.

So, can anyone in the forum provide any reference material to show if light traveling through a moving medium is affected at all in the same way as light being emitted from a moving body?

Thanks.
 
  • #14
lenfromkits said:
I'm not sure what you mean by "it is never increased" The whole result of the Fizeau experiment shows that light speed increases. Speed of light when in a medium increases.
No, the speed of light in a medium never increases relative to the speed of light in vacuum. The speed of light is slowed down less if the medium is moving parallel to the direction of propagation than if it is moving anti-parallel, but it slows down either way.

lenfromkits said:
The question stems around one issue - does a moving medium have a similar effect on the frequency of light as a moving emitter?
No.

lenfromkits said:
Clearly the answer is not clear. I appreciate your opinion, but what I need is actual experimental documentation to either refute or confirm that.
The Fizeau experiment does just that.
 
  • #15
DaleSpam said:
lenfromkits said:
I'm not sure what you mean by "it is never increased" The whole result of the Fizeau experiment shows that light speed increases. Speed of light when in a medium increases..
No, the speed of light in a medium never increases relative to the speed of light in vacuum. The speed of light is slowed down less if the medium is moving parallel to the direction of propagation than if it is moving anti-parallel, but it slows down either way.
I think the two of you are talking at cross-purposes. Light in a moving medium can go faster than light in the same stationary medium (if the medium moves in the same direction) but always slower than light in vacuum.

(All motion measured relative to the same observer, of course.)
 
  • #16
DrGreg said:
I think the two of you are talking at cross-purposes. Light in a moving medium can go faster than light in the same stationary medium (if the medium moves in the same direction) but always slower than light in vacuum.
Yes, which is why I said:
DaleSpam said:
the speed of light is always slower in air or water than in vacuum.
and
DaleSpam said:
the speed of light in a medium never increases relative to the speed of light in vacuum.
I don't know how I could have been more clear.
 
  • #17
When the two paths of an interferometer have different frequencies the resulting interference is a beat pattern in time. This is called heterodyne interferometry. The Fizeau experiment was a homodyne experiment and had a spatial interference pattern, not a beat frequency. If the frequencies had shifted at all it would have been obvious as a flashing of the interference fringes at the beat frequency.
 

1. What is the Fizeau experiment and how does it work?

The Fizeau experiment is a classic optical experiment used to measure the speed of light. It involves splitting a beam of light into two parts, one of which is reflected off a mirror and the other is transmitted through a spinning wheel. By measuring the interference pattern between the two beams, the speed of light can be calculated.

2. How does the Fizeau experiment demonstrate the connection between fringe shifts and velocity?

The Fizeau experiment demonstrates the connection between fringe shifts and velocity by showing how the speed of light affects the interference pattern. As the spinning wheel changes the distance traveled by the light beam, the interference pattern shifts. This shift is directly related to the velocity of the spinning wheel, which in turn is related to the speed of light.

3. Why is the Fizeau experiment important in the study of optics and physics?

The Fizeau experiment is important because it provided the first direct measurement of the speed of light, which was a crucial step in understanding the nature of light and its role in the laws of physics. It also demonstrated the wave-like nature of light and paved the way for further experiments and discoveries in the field of optics.

4. What other applications does the Fizeau experiment have besides measuring the speed of light?

The Fizeau experiment has been used in various applications such as testing the properties of different materials and measuring the refractive index of liquids. It has also been used to study the properties of electromagnetic waves and to verify theories such as special relativity.

5. Has the Fizeau experiment been improved upon or replaced by newer methods?

While the Fizeau experiment is still a valid method for measuring the speed of light, newer and more precise methods have been developed, such as the Michelson-Morley experiment and the modern definition of the meter. However, the Fizeau experiment remains an important and influential experiment in the history of science.

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