Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Fringe shifts - Michelson Morely Experiment

  1. Mar 19, 2012 #1
    I would love to get my head around this; I did a quick search for this, but only came across one thread with fringe shift in the title (in the first 5 pages of the search results).

    The OP will probably sound like me "thinking out loud", because it largely will be. I'm not entirely sure of what the right question to ask is, but I'm hoping that a point of departure might materialise. I suppose, in short, I'm trying to understand the conclusions drawn from the experiment, and the justification for them.

    The speed of light, in the MMX, is not measured in the sense of distance/time; is it the fringe shifts that are used to determine if there is a change in speed along either of the arms?

    Some of the questions I have:
    - what is a fringe shift?
    - what causes a fringe shift?
    - does frequency play a part?
    - can anything about wavelength be inferred?
  2. jcsd
  3. Mar 19, 2012 #2


    User Avatar
    Gold Member

  4. Mar 19, 2012 #3


    User Avatar
    Science Advisor
    Gold Member

    Like the name suggests, the Michelson Morely interferometer experiment is an interference experiment. You have 2 arms of the same length, and part of the light goes along one arm while the other part goes along the other arm (the light beam is split with a half-silvered mirror or some other beam splitter). They then come back to the observer and is recombined. If the light moving along one arm moves at a different speed as the other arm, then you would not expect the light to come back in synch (crest to crest, trough to trough), and so you would expect to see interference effects. The fact that you don't see interference effects suggests the light traveled with the same speed along both arms.
  5. Mar 22, 2012 #4
    thanks guys.

    I've done some googling before, but find it more helpful to engage with people on such issues, to try and pose a few questions that might demonstrate where my lack of understanding lies.

    my basic understanding is that a single "beam" of light is split, it travels along two different arms of an interferometer and reconvenes, at slightly different angles to each other; the interference patterns (together with the angles) result in a fringe; if the speed of light along either arm changed, it would result in a fringe shift.

    Is that in the ball park at all?
  6. Mar 22, 2012 #5


    User Avatar
    Science Advisor
    Gold Member

    That's exactly right.

    Also, if a length of a path changes, it will result in a fringe shift
  7. Mar 22, 2012 #6
    Am I correct in thinking that, because the actual speed, in terms of distance/time, is not measured, that it is the wavelength that gives us the information about speed? That is, the wavelength of the reconvening beams is the same?
  8. Mar 22, 2012 #7
  9. Mar 22, 2012 #8
    Wavelength is the same but the phase (how the peaks and troughs line up) is not.
  10. Mar 23, 2012 #9
    Thanks NotAName.

    I'm trying to get from there to how the speed of light is invariant for all observers, irrespective of their motion relative to the source.

    My inital thoughts are that this demonstrates that the wavelength of light, in a vacuum, is constant, but not necessarily the measured speed. I can imagine that light in a vacuum would have an absolute wavelength i.e. the distance between peaks is always the same, but that if I was traveling relative to the light, and measured its speed in terms of distance/speed, that I would get a value less than c.

    There's probably something I'm missing though.
  11. Mar 23, 2012 #10
    cheers gh!
  12. Mar 23, 2012 #11
    btw, thanks guys!
  13. Mar 23, 2012 #12


    User Avatar
    Gold Member

    Remember that frequency x wavelength = speed, and when the light source is moving wrt to you, you'll measure a different wavelength and frequency ( because of the relative velocity) but the same speed.
  14. Mar 26, 2012 #13
    Just remember that it's utterly different than you would expect from a wave travelling in a medium so your intuitive ideas will not apply to light -as explained by relativity- as they would for, say, sound waves.

    The Doppler effect in sound waves is easily understood to have the explanation that, when you run into waves more frequently, the "frequency" goes up. This is because you can change the relationship between your speed and the speed of the wave in the medium. This is not true in relativity.

    Secondarily, a moving sound source writes waves into the medium at higher and lower frequencies because the waves escape from the source faster or slower thus creating shorter or greater distances between the peaks. They stack up or stretch out. This is also not true with light since it's speed is constant with respect to the emitter.

    These intuitive mechanical effects for sound simply cannot exist for light, therefore the Doppler effect in light is currently regarded as a basic truth of reality that has no deeper explanation other than the fact that time and light is different between frames.

    Any attempt to explain doppler mechanically requires the total pulse length of a beam of light to be changed improperly and would violate light constancy.
  15. Apr 10, 2012 #14
    How is the wavelength of light measured in an interferometer for example?

    Also, would you actually measure a different wavelenght; that is a part I struggle with. I can see how the frequency would change, but not necessarily the wavelength.
  16. Apr 10, 2012 #15

    I find it more intuitive to think about a light wave traveling than a sound wave, because I don't need to take the medium into account with light.

    I'm picturing a light wave traveling with a constant speed, and with an absolute wavelenght i.e. the same distance between peaks. If an observer is at rest relative to the source I imagine that they would detect a certain frequency of the light wave.

    If an observer moves towards the light source, I imagine that the frequency would increase, but the wavelength wouldn't necessarily change.

    I try to apply this to an interferometer and I don't expect that there would be any fringe shift in such an experiment. I wouldn't conclude that the speed of light is constant in all directions though.

    Where am I going wrong?
  17. Apr 10, 2012 #16
    The speed of light is independent of the velocity of the emitter yes . And the speed is invariant as measured in any inertial system . But the same mechanical explanation that applies to sound also applies to light. I.e. the motion of the receptor in between wave peaks results in either higher or lower wave lengths /frequencies
    The difference is the classic wave equations need a relativistic adjustment , transformation.
  18. Apr 10, 2012 #17
    The wave length would also change. The motion between the peaks would necessarily mean a shorter wave length just as the motion would result in a shorter time interval i.e. higher frequency.
    I am afraid it would be exceedingly difficult to get from the interferometer to an understanding of the measured invariance of light. I think it really requires an understanding of length contraction,time dilation and most importantly the relativity of simultaneity.
  19. Apr 13, 2012 #18
    I'm sorry, I have to disagree. A mechanical explanation makes no sense in this context. If you can "run into" peaks more frequently, then you have changed the relationship between your speed and the speed of light, thus breaking light constancy.

    No typical physical or intuitive ideas can be used with light according to modern theory.
  20. Apr 13, 2012 #19


    Staff: Mentor

    Austin0 was correct.

    This is not correct. I believe that you are mistakenly assuming that the frequency changes but the wavelength does not.

    Last edited: Apr 14, 2012
  21. Apr 14, 2012 #20
    That's an often seen misunderstanding which could derail Mangaroosh. As most people here know, an MMX discussion is 100% compatible with a wave traveling in a medium (such as expressed by Maxwell's equations); special relativity emphasizes that the intuitive ideas that do not apply are some specific concepts of Newtonian mechanics.
    Instead, according to SR (as well as Maxwell of course, on which SR was based), the speed of light is independent of that of the emitter. That is the essential characteristic of wave models as opposed to ballistic emission models.
    Last edited: Apr 14, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook