A thin film is put on the surface of glass, with indexes of refraction of x and y respectively. If light, with wavelength L in air, is reflected from the 2 sides of the film, to interfere constructively, what is the minimum thickness required. The question is stated just like that, so I think it is vague in some parts. I can't think of a formula to work this out. I know for con. interference, 2t=mL, where m is a whole number. But this isn't enough, and doesn't relate the indexes of r. Any hints????