Air-gap optical path difference in a Fabry–Pérot interferometer
= Air-gap optical path difference in a Fabry–Pérot interferometer
{title2=$\Delta=2d\cos\theta$}
For an air gap of thickness $d$, an extra internal round trip has length $2d/\cos\theta$, but consecutive transmitted rays emerge at different positions. Comparing them at the same outgoing <wavefront> subtracts $2d\tan\theta\sin\theta$, leaving <optical path difference> $2d\cos\theta$. Reflection-coating <wave phases> are separate from this geometric path calculation.