Hi all, the question which I'd like to share is the following: if you look at the formal theory of any random optics book for FP interferometer you will get to the formula of the FSR (Free Spectral Range), defined as "The wavelength separation between adjacent transmission peaks" (Wikipedia) and the formula is given by: Δλ ≈ Λ20 / 2nlcos(θ) where n is the refractive index inside the mirrors and l is the mirror separation. Now the question is, if I shine a monochromatic laser inside such a device I'll obtain ring fringes of the same color (suppose I shine the cavity with a red laser). So red circles!! why should I have a FSR defined as a Δλ since the λ of my source is always the same?? I really don't understand this!! Thanks for your help!