# Thin Film Interference problem

1. Dec 8, 2007

### sdobbers

Hey guys, I was wondering if someone could explain to me when Thin Film Interference will produce constructive/destructive interference. I think I might understand it, but I'm not 100% sure and I have a test coming up soon. Any help would be great!

Ok, so I know we have the equation:
2t = m$$\lambda$$

where t, is the distance the thickness of the film, and $$\lambda$$ is the wavelength.

Say we have...

-------
n1
-------
n2
-------
n3

Where n1 = air, n2 > n1 and n2 > n3. So then would the part of the ray reflecting off n2, have a 1/2 $$\lambda$$ phase change? But the ray reflecting off of n3 would undergo no phase change?? Would this produce destructive interference?

What about when n2 > n1, and n3 > n2. I'm guessing that both the ray that reflects off n2, and off n3 would both undergo 1/2 $$\lambda$$ phase change, so this would be constructive?

Or is it reversed?

2. Dec 8, 2007

You got it.

3. Dec 8, 2007

Thanks :)