Carefully adjust M1 so that the central dark fringe is in the centre of the field of view. A reference
marker may be placed close to M1 to help define the fringe position. Place the thin Mylar film in
one arm of the interferometer and note the shift in M1 necessary to bring the dark fringe back to
its original position. Repeat this measurement with the film in the other arm of the interferometer.
Calculate the additional optical path introduced by the film and, given that that the refractive
index of Mylar is 1.64, work out the film thickness. Repeat the measurement a few times to allow
for an uncertainty calculation to be made.
[Insertion of Mylar into a beam path increases the optical path light must travel, but remember,
when doing this, that the Mylar is replacing air which was originally in the beam path. This needs
to be considered when doing the calculation.]
In making these measurements you may notice that the central fringe becomes tinted when the
film is in one arm of the interferometer. This is because the refractive index of the film varies with
wavelength slightly so that its optical path is not identical for all wavelengths.