Path difference in Fabry-Perot etalon

[hadoque]

Hi
I'm trying to understand how the Fabry-Perot etalon works for an experiment regarding the Zeeman effect.
Looking at the image could anyone tell me why the path difference in the etalon is $$\Delta l = 2t \cos{\theta}$$? In my head, the path difference is $$\Delta l = 2t/\cos{\theta}$$, in other words, just the length of two more reflections.

 

[preposterous]

You would get that equation if you only accounted for the OPD inside resonator. There is also a little OPD outside you must subtract. See...

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/fabry.html

 

[hadoque]

Wow, I posted that question two years ago. Thanks for taking the time to answer it though.. :)