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A Fabry Perot interferometer is illuminated by a diverging wave. For a given incidence angle, the phase depends on the wavelength and thus, the bright and dark fringes (located at infinity) for different wavelengths correspond to different incidence angles. The page calculates the phase dispersion and the (spectral) power of resolution according to the Rayleigh criterion as functions of the thickness and the refraction index of the Fabry Perot, the reflectance of the Fabry Perot surfaces, the wavelength and the angle of incidence.