# Interference (Michelson Interferometer)

1. Oct 20, 2005

### Quasi Particle

Hello,
.
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 be very complicated.
.
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:
.
$$a=k_{c}x - \omega_{c}t$$ is the phase
$$v=\frac{\omega_{c}}{k_c}$$ is the phase speed
$$\omega_{c}=\frac{2 \pi c}{\lambda_{c}}$$ is the angular frequency of the carrier wave
$$k_c=\frac{2 \pi}{\lambda_{c}}$$ 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.
.
Please can you give me a clue where I'm mistaken?
Cheers!

Last edited: Oct 20, 2005