Athin film with index of refraction n = 1.40 is placed in one arm of a michelson interferometer, perpendicular to the optical path.
if this causes a shift of 9.0 bright fringes of the pattern produced by the light of wavelength 693 nm, what is the film thickness?

The thickness of the film is

Explanation:

The relation between the refraction index, and a wavelength is given by

where is the number of fringes, is the wavelength of the incident light, and is the thickness of the film.

From this relation, we have that

wich gives us that

is the thickness of the film.

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