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I was discussing the reason why we see beating from a Michelson interferometer, and one of my friend said it 's because the light have different frequencies, therefore, they would be out of phase.

But my argument is, Because two frequencies travel at the same speed, their phase difference remain the same. But when you keep adjusting the movable mirror of Michelson interferometer, you can shift their phase difference more quickly, and that's why you get beating phenomena.

What is the actual source of beating?

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    $\begingroup$ Could you define what you mean by "beating" in this context. In time? In some other variable? $\endgroup$ – dmckee --- ex-moderator kitten Dec 7 '13 at 0:03
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    $\begingroup$ By "beating" do you mean interference, i.e. the sinusoidal oscillation of the output as a function of the arm length? The word "beating" to me is wont to imply a variation with time, in analogy with the throbbing or beating sound from two slightly different frequencies as their linearly varying with time relative phase repeatedly shifts them from being in to out of phase. $\endgroup$ – Selene Routley Dec 7 '13 at 0:12
  • $\begingroup$ If it was an electron then any beating could be zitterbewegung, do photons have dirac vibrations? $\endgroup$ – Jitter Dec 7 '13 at 0:22
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    $\begingroup$ @Jitter, that is something else entirely. $\endgroup$ – lionelbrits Dec 7 '13 at 0:30
  • $\begingroup$ @Jitter as Lionel says off topic: but photons do not have Dirac vibrations: you can think of the Dirac equation as two coupled Weyl equations with the mass being the coupling term that converts left and right helicities back and forth in the zitterbewegung. Maxwell's equations can be written in the same form (the spinor is rank 2, though) and so there is no mass, thus no oscillation back and forth between left and right hand circularly polarised photons. $\endgroup$ – Selene Routley Dec 7 '13 at 13:14
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When you move the mirror, you change the way the light has to go along, thus changing the time it needs to get to the interference surface. This means you change the phase shift of both light beams, too. It has nothing to do with different frequencies (you use only one light source, as I assume?).

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