Thin Film Interference
If we imagine a monochromatic light source striking a thin film of gasoline on water. If the ray strikes the surface nearly perpendicularly at the top surface there is some reflection and then as the light passes from the gas to the water there will be reflection. This ray will also refract when it leaves the gas going back into the air. This is shown in the diagram below.
So, two rays enter the eye from the same light source. As they are from the same light source then all the properties (e.g. wavelength) are the same. As the rays have traveled different distances there will be interference between these two waves. If the interference is constructive the the eye would see a uniformly bright film, if the interference was destructive then a uniformly dark film would be seen.
As films are hardly ever of a uniform thickness, what happens is that different paths are taken by different rays, this together with the angle at which something is viewed at causes a multicolored effect as shown below.
