You should determine f by counting the number of fringes passing a point in the field of view as the
micrometer drum is slowly turned. A click counter is provided to help you keep count. Each fringe
represents a mirror shift of λ/2, and it will be necessary to count at least 200(!) fringes to obtain an
accurate value of f. Repeat the calibration for at least 3 well-spaced positions along the micrometer
screw, taking care to avoid errors due to backlash. Unless the results differ significantly, the mean
value of f can be taken to apply to all micrometer readings. You should, of course, estimate an
error on the value of f.
When counting fringes it may help to have a fine pointer (a wire or pin) mounted so that it is
visible in the field along with the fringes or use can be made of the central fringe since it is so
distinctive in appearance. Either method will assist as they will serve as a reference point in space
for the eye. (Confirmation that you have performed the calibration correctly in measuring a value
for this factor f can be made by calculating the ratio of the distance from the pivot to the point
where the rod from M1 touches the lever and the distance from the pivot to the point of application
of the micrometer drive onto the lever. This will provide a check that your value is about right.)