# Thickness of a film given wavelength and phaseshift...

1. Jul 17, 2015

### 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?

2. Jul 17, 2015

### 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?

3. Jul 17, 2015

### Marvin94

Material's parameters are also known.

4. Jul 17, 2015

### 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.