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A plane parallel plate used in reflection can be considered as a two waves interferometer. When illuminated by a monochromatic converging/diverging wave (spherical wave), it creates ring shaped fringes located at infinity that can be projected in the back focal plane of a lens. The page calculates the radii of dark and bright fringes in the back focal plane of a lens as functions of the wavelength, the plate thickness, the plate refraction index and the number of the said rings counted from the center.