Hi all. I am trying to understand how a tunable Fabry-Perot interferometer works. This is what I have understood so far: An etalon is placed inside a pressure vessel, and the pressure inside this chamber is changed. Apparently, this changes the "effective optical path length" between the two mirrors of the etalon, and thus scans the light over a frequency interval. The change in pressure should alter the index of refraction, but I cannot see how this changes the frequency of the outputted light. What do you say about this?