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I have got a Michelson interferometer and measure a laser beam (HeNe @ 633nm). I move the mirror with a step motor of unknown step length and measure the intensity of the beam. I get a nice beat pattern out of that, with "wavelength" of the carrier wave being 14 steps and of the modulation wave being 800 steps of the step motor. Now I want to determine the step length but that suddenly seems to beverycomplicated.

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The two interfering beams do have the same wavelength, so I would think that this is the wavelength of the carrier wave. But then, there would be no modulation.

Then I remembered that one of the waves "moves", i.e. there is a varying phase difference, and thought that it would be quite interesting to know the phase speed, but here are my formulae:

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[tex]a=k_{c}x - \omega_{c}t[/tex] is the phase

[tex]v=\frac{\omega_{c}}{k_c}[/tex] is the phase speed

[tex]\omega_{c}=\frac{2 \pi c}{\lambda_{c}}[/tex] is the angular frequency of the carrier wave

[tex] k_c=\frac{2 \pi}{\lambda_{c}}[/tex] is the wave number of the carrier wave

which gives the velocity v=c and I'm quite sure I didn't move the mirror at light speed.

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Please can you give me a clue where I'm mistaken?

Cheers!

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# Homework Help: Interference (Michelson Interferometer)

Can you offer guidance or do you also need help?

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