A discharge lamp emits two lines near the green mercury line at nm
PHYSICS · Subjective · Fermi University Physics · Optics & Photonics · Interference and diffraction · Interference and diffraction
Optics and Photonics, Ch Multiple-beam interference: thin films, multilayers and the Fabry-Perot etalon
Problem
A discharge lamp emits two lines near the green mercury line at λ=546.1 nm, separated in frequency by Δν=8.0 GHz. You have a pair of flat, parallel, lossless mirrors of power reflectance R=0.85 that can be set facing each other in air (n=1) at any spacing d, forming a Fabry-Perot etalon at normal incidence. (a) Choose a spacing d so that the etalon's half-width Δν1/2 is one third of the line separation, a safe margin for telling the two lines apart. (b) With that spacing, check that the two lines lie well within a single free spectral range, so that neighbouring orders are not confused. (c) State the resulting free spectral range, linewidth, interference order and resolving power at 546.1 nm.