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The thickness of the air film at the point of contact of lens L with glass plate P is zero. The condition for darkness is, path difference δ = 2t + λ/2 = (2n+1) λ/2 Where n = 1, 2, 3 … and λ is the wavelength of light used. One of the two reflections takes place at the surface of the denser medium and hence it introduces an additional phase change of π or an equivalent path difference λ/2 between two wavelets. If t is the thickness of the air film at a point on the film, the refracted wavelet from the lens has to travel a distance t into the film and after reflection from the top surface of the glass plate, has to travel the same distance back to reach the point again. Theory – The formation of Newton’s rings can be explained on the basis of interference between waves which are partially reflected from the top and bottom surfaces of the air film. If t is the thickness of the air film at a point on the film, the refracted wavelet from the lens has to travel a distance t into the film and after reflection from the top surface of the glass plate has to travel the same distance back to reach the point again. The formation of Newton’s rings can be explained on the basis of interference between waves which are partially reflected from the top and bottom surfaces of the air film. An important application of interference in thin films is the formation of Newton’s rings. In 1717, Sir Isaac Newton studied the pattern of the ring generated due to the interference of light.