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A Fabry Perot is illuminated by a monochromatic diverging wave and thus creates interference rings located at infinity, each ring corresponding to a given incidence angle. The interference are observed in the back focal plane of a lens. The page calculates the orders and the radii of the bright and dark rings in the focal plane as functions of the thickness and refraction index of the Fabry Perot, of the wavelength, of the focal length and of the rings number (rings are counted from the center).