Hi,(adsbygoogle = window.adsbygoogle || []).push({});

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

[tex] \delta = \frac{2 \pi}{\lambda}2nlcos\theta [/tex]

Where n is refractive index inside cavity, l distance between cavity walls, [tex]\theta [/tex] 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 [tex]\frac{2l}{cos\theta} [/tex]. And i don't understand why would we do that to find phase difference.

Can anyone explain what are we actually doing here?

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Fabry Perot Cavity - phase difference?

**Physics Forums | Science Articles, Homework Help, Discussion**