light:Why do rainbow-like colours appear when light strikes oil that is floating on water?

Why do rainbow-like colours appear when light strikes oil that is floating on water? 
When oil is poured on water, it spreads out to form a very thin film on the surface of the water. However, this film is of varying thickness. In some places, it is literally a molecule thick, whilst in other places, it is much thicker.

 When light passes through the oil, some of it is reflected back off the different layers of oil, whilst some carries on, and is reflected off the surface of the water lying below. Because the light waves have now travelled different distances before being reflected, they mix together, producing a spectrum of colours. As a result, a rainbow-like colour pattern is shown on the oil surface, and this phenomenon is called thin-film interference.

 The great scientist Thomas Young in 1801 put it in a nutshell when he described thin film interference as the interaction of light waves reflected from the top surface of a thin film, with those which penetrate the film, and are reflected from the back surface of the film.