Question

If a thin soap film is kept vertical, the horizontal bands resulting from the interference of light will, in time, move downwards, and simultaneously, their width will change. After some time, a rapidly increasing black spot forms in the upper part of the film and shortly afterwards the film breaks. Discuss the reasons for the movement of the bands and explain the occurrence of the black spot.

Answer 1

Each of the horizontal interference bands corresponds to some given film thickness. The water in the inner layer of the film gradually flows to the lower part; the lower part of the film becomes thicker and the upper part, thinner. The region corresponding to any given thickness of film gradually shifts. Along with it, the corresponding interference band also shifts downwards. After some time, the thickness of the film in the upper part is less than a quarter of a wavelength of the shortest light waves incident on the film. At these regions of the film there will be destructive interference of the rays reflected from the film (because of the shift of half a wavelength when the rays are reflected from an optically denser medium) and therefore this region of the film will appear black in reflected light.