My friend asked me earlier about this homework he had. He said, there's a thin film on a piece of glass, and when you look at it, it looks green. Is this contructive interference? So, I was thinking, This would be kind of an iridesence? Depending on what angle you were looking at the film, you would see different colors right? So I was trying to think up an equation to explain this, and the best I can come up with is [tex]\lambda = 2 cos\theta h [/tex] where [tex] \lambda [/tex] is the wavelength of the light, theta is the angle of depression from parallel to the surface being observed and h is the thickness of the film itself. So I figure if the light reflects off the surface of the glass and the film, you'll get some sort of constructive interference. And that is why you see green, right?