Question

colour on water surface having oil on it is due to?

Answer 1

It is due to the combination of refraction, reflection and a bit of diffraction. Oil is less dense than water, and so will float on top of the water. When sunlight, which is a continuum of wavelengths and hence colours enters the surface of the liquids. As it enters the top of the oil layer, part of the light is reflected, and part of it is refracted, which splits the light into its different colours. The light continues on through the oil until it meets the interface of the oil and water (i.e. the top of the water layer), where secondary reflection occurs, with other light refracting into the water. There can be some reflection of the bottom of the water too and the reflected ray experiences similar behaviour as it passes back through the oil layer.

This has the effect of creating different colours upon the surface of the oil, with the colours being separated by areas without colour. The areas without colour occur due to destructive interference due to diffraction effects. A full detailed explanation can be found here http://physics.bu.edu/~duffy/py106/Diffraction.html