A colourful oil stain

Here's an oil stain in the road at Castleton in Derbyshire.  What happens is that oil forms a thin film, like the skin on a bubble.  Remember that not only can light pass through glass, it can also reflect as well - "a man who looks on glass, on it may stay his eye, or if he pleaseth through it pass and then the heavens espy"!  When light hits the top surface of the oil, both things happen.  Some light reflects towards your eye and some goes in to the oil.  The light that goes into the oil hits the far side of the oil and reflects back towards your eye.  So there are now two rays of the same light heading for your eye.  Where the waves line up correctly peak-with-peak you see a colour.  This is called constructive interference.  The colours all have different wavelengths so they line up in different places giving you the rainbow effect in the oil.