Solution to task 501 from Chertov´s for extramural stud

501. There is liquid between the glass plate and the plano-convex lens lying on it. Find the refractive index of the liquid if the radius r3 of the third dark Newton ring when observed in reflected light with a wavelength λ = 0.6 μm is 0.82 mm. The radius of curvature of the lens is R = 0.5 m.