People often use interference between the reflected light from front and back surface to measure the thickness of a film. However, there is this a very important formula:

d=nu/(2*n)*), where nu is the frequency difference (difference of 1/lambda ), n is the refractive index, and d is the film thickness.

The caveat is that, we should watch out for the absorption (or the extinction coefficient k).

The formula comes from interference between reflection of from and back surface of a material. However, it does not consider the phase shift of the reflectance, and besides, it does not consider the phaseshift of the transmission of the back-surface reflection.

The problem only goes away when k is very small.

Because the phaseshift dependce on

tan(theta)=epsilon2/epsilon1=2nk/(n^2-k^2),

where theta is the phase shift.

If k<<n, theta~0. Then there is no problem, fortunately, it is the case most of the time.