# Thin film interference formulae

1. Aug 1, 2011

### khaos89

Hello, I am trying to understand thin film (in air) interference but I have a problem:

I know we have destructive interference when $\delta=(2m+1)\pi$.

Now i can try to calculate the thickness of the film to get it, so

since $\delta =\frac{4nt\pi}{\lambda} - \pi$ where t is the thickness,

I come to $\delta=(2m+1)\pi=\frac{4nt\pi}{\lambda} - \pi$

that leads me to $t=\frac{\lambda(m+1)}{2n}$ instead of
$t=\frac{m\lambda}{2n}$

(n is the refractive index and m is the integer parameter)

Where am I wrong?

2. Aug 1, 2011

### Staff: Mentor

Why do you think you're wrong?

3. Aug 1, 2011

### khaos89

Thank you for your answer, I think I am wrong because my book's solution is $t=\frac{m\lambda}{2n}$

4. Aug 1, 2011