The wavelength of an He-Ne laser is determined from the change in the interference pattern when moving an interferometer mirror using the shifting distance d of the mirror. During this shift, the interference lines on the observation screen move. In evaluation, either the interference maxima of interference minimum passing a fixed point on the screen while the plane mirror is shifted are counted. For the wavelength, the following equation applies:
               wavelength = 2d/n

1. Rotating the knurled head screws X1 and X2     to bring two beams coincide.
2. Insert the beam expander (a lens) to the front of     laser beam and adjust its position. Lined pattern     should appear on the screen.
3. Slowly twist knobs X1 and X2 again and you     should get a circular pattern on the screen..