# Fabry Perot Cavity - phase difference?

1. Apr 13, 2010

### trelek2

Hi,

I'm having trouble understanding where the formula (appearing in my notes or Wikipedia) comes from.
One can read on Wikipedia: The phase difference between each succeeding reflection is given by
$$\delta = \frac{2 \pi}{\lambda}2nlcos\theta$$
Where n is refractive index inside cavity, l distance between cavity walls, $$\theta$$ angle of the ray.

What are we actually finding?
It would seem we are multiplying wavenumber k by refractive index n and distance traveled by light. But as I look at it, it seems to me that distance traveled is $$\frac{2l}{cos\theta}$$. And i don't understand why would we do that to find phase difference.

Can anyone explain what are we actually doing here?

2. Apr 13, 2010