An air-spaced Fabry-Perot interferometer has mirror spacing 0.15 mm and mirror reflectivity R = 0.99. It is used to measure the spectrum of the sodium doublet, which consists of two closely spaced emission lines at 588.995 and 589.592 nm.
(a) Determine the mode number of the FP resonance.
(b) By how much must the plate spacing be changed in order to scan a single mode from one of the lines to the other?
(c) By how much must the plate spacing be changed so that the same emission line is seen again in a different order? If the plate spacing was increased, is the new mode number higher or lower?
(d) What is the wavelength resolution of the resulting spectrum?