When the light strikes a thin film of a fluid(such as petrol)
in which light moves slower than air, there is an initial reflection off
of the surface. Some of the light is transmitted into the fluid and it reflects off the
bottom and then finally travels back out.
These waves take slightly longer
to reflect due to the travel of a very short distance through the
oil, they are now slightly out of phase with the initial reflection, creating a rainbow of interference
