Calculating thin film thickness from wavelength and phase shift

[Marvin94]

A wavelength of incident light on a thin film is given, and also is known how much the rays coming out from the top surfaces are out of phase. How can be the thickness of the film calculate?

 

[RUber]

Do you know the material parameters of the film?
Phase shift = time to pass through the medium, and speed in the medium is determined by the material parameters. Are there other assumptions that should be made?

 

[Marvin94]

Material's parameters are also known.

 

[RUber]

Speed of light in free space c, divided by wavelength $\lambda$ is your free space frequency (cycles per second).
Your phase is the portion of the cycle that has been completed, so phase speed is $\omega = \frac {2\pi c}{\lambda}$ (radians per second).
Speed of light is also $\frac{1}{\sqrt{\varepsilon_0 \mu_0} }$
Speed of light through medium is $\frac{1}{\sqrt{\varepsilon \mu}}$
So the question is saying: given the phase shift, determine the time spent in the medium. With that time, and the speed of travel through the medium, determine the thickness.