Correct Option: C

The iridescent colours of soap bubbles are caused by interfering of (internally and externally) reflected light waves and are determined by the thickness of the film. This phenomenon is not the same as the origin of rainbow colours (caused by the refraction of internally reflected light), but rather are the same as the phenomenon causing the colours in an oil slick on a wet road. As light impinges on the film some of it reflects off of the outer surface, some of it enters the film and reemerges after reflecting off the second surface, some of it enters the film and reemerges after bouncing back and forth between the two surfaces from 1 to n times. The total reflection observed is determined by the interference of all these reflections.