What is the Formula for Thin Film Interference?

[Precursor]

Homework Statement

[PLAIN]http://img403.imageshack.us/img403/2696/33095583.jpg

The attempt at a solution

I simply took 425 nm and divided it by 4 by using the destructive reflection from thin film. I got 106 nm as my answer, which is c. Is this correct?

 

[Doc Al]

I simply took 425 nm and divided it by 4 by using the destructive reflection from thin film.

What's your reasoning behind this?

 

[Liquidxlax]

remember your rules of reflection. when an electromagnetic wave is in incidence of a medium of higher optical density the light will have a phase shift of pi. If an electromagnetic wave is incidence on a less optically dense medium there is no shift.

contructive 2mpi = (4(pi)ntcos(x))/lambda -k + b

destructive (2q+1)pi = (4(pi)ntcos(x))/lambda -k + b

where is the phase shift due to the intial and main medium and b is the phase shift due to the main medium and final. x is the angle of incidence which would be 0 in your case. t is film thickness of main medium. n is the index. lambda is wavelength.