Discuss the below:
Fabry-Perot Etalon: Radius of a bright ring
Q: A source of visible light is placed before an air-spaced Fabry-Perot etalon od diameter 20mm and mirror separation d = 1mm. The light transmitted by the etalon passes through a convex lens of focal length f and is incident on a screen positioned in the focal plane of the lens.
For an extended monochromatic source of wavelength λ = 500nm, a set of ring-shaped fringes is produced on the screen, with a maximum of density at the centre of the pattern. Show that, in the small angle approximation, the radius r of the smallest bright ring is given by
r = f*sqrt(λ/d)
where sqrt means square-root.