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  4. What causes change in the colours of the soap or oil films for the given beam of light?

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Q. What causes change in the colours of the soap or oil films for the given beam of light?

Punjab PMETPunjab PMET 2006Ray Optics and Optical Instruments

Solution:

When a thin film of oil spreads over the surface of water, and observed in broad day light then brilliant colours are observed. These colours arise due to interference of sunlight reflected from lower and upper surfaces of the film.
If thickness of film at point is such that optical path difference $ \delta =0,\,\,\lambda ,\,\,2\lambda ,\,\,3\lambda ,... $ then the film appears dark and if
$ \delta =\frac{\lambda }{2},\,\,\frac{3\lambda }{2},\,\,\frac{5\lambda }{2},... $ film will appear bright.
Hence, colour of soap film changes because of thickness of film.