Path difference in Fabry-Perot etalon

1. Jan 13, 2009

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.

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2. Feb 19, 2011

preposterous

3. Feb 20, 2011