WHY ARE SOAP BUBBLES AND OIL LAYERS COLOURED?

Coloured bands on soap bubbles and oil layers are caused by interference of light waves with themselves. Sunlight is a composite of seven colours each of which lies in a specific wavelength range. For ex. Violet light has a wavelength of 380 nanometres and red, above 600nm. Light waves falling on any thin film are partially reflected from the top and bottom surfaces of the film. A wave reflected from the bottom surface has to travel a longer distance than the one reflected from the top. This difference in distance traveled by the waves, called path difference, leads to a difference in the phases of the light waves. This phase difference depends on the thickness of the film at the point of reflection and angle of viewing. If the waves are ‘out of phase’, then they cancel each other and if they are ‘ in phase’ they add up. Thus the regions of the spectrum, which interfere constructively can be seen whereas those which interfere destructively are lost. Even if there is a slight variation in the thickness of the film, it will be coloured differently.