When a Fabry-Perot etalon is illuminated by monochromatic light with wavelength an interference pattern, the system of

When a Fabry-Perot etalon is illuminated by monochromatic light with wavelength λ an interference pattern, the system of concentric rings, appears in the focal plane of a lens (Fig. 5.18). The thickness of the talon is equal to d.
Determine how
(a) The position of rings;
(b) The angular width of fringes depends on the order of interference.