8.2.1 Fabry-Perot

1. Align the laser and interferometer in Fabry-Perot mode.
2. Adjust the micrometer knob to a medium reading (approximately 50  m). In this position, the relationship between the micrometer reading and the mirror movement is most nearly linear.
3. Turn the micrometer knob one full turn counterclockwise. Continue turning counterclockwise until the zero on the knob is aligned with the index mark. Record the micrometer reading.
4. Adjust the position of the viewing screen so that one of the marks on the millimeter scale is aligned with one of the fringes in your interference pattern. You will find it easier to count the fringes if the reference mark is one or two fringes out from the center of the pattern.
5. Rotate the micrometer knob slowly counterclockwise. Count the fringes as they pass your reference mark. Continue until some predetermined number of fringes have passed your mark. Count at least 20 fringes. As you finish your count, the fringes should be in the same position with respect to your reference mark as they were when to count. Record the final reading of the micrometer dial.
6. Record , the distance that the movable mirror moved toward the beam splitter according to your readings of the micrometer knob. Remember, each small division on the micrometer knob corresponds to 1  m (10  m) of mirror movement.
7. Record m, the number of fringe transitions that you counted.
8. Repeat the above steps several steps, recording your results several times.
9. For each trial, calculate the wavelength of the light, then average your results.