Question: When light hits oil it creates a rainbow effect how does it work??

Andrew McKinley answered on 17 Nov 2014:

This is such a brilliant question which you asked in the chat last week, and I’m sorry I couldn’t answer it there. You’ll soon see why!

When you look through a glass window, you will see a ghostly reflection of yourself looking back. In fact, about 4% of the light hitting the window comes back towards you. You will be familiar with this of course! If you look through double glazing, you will see the same effect, but you will see two reflections; one from the inner pane, and one from the outer pane. The further apart the panes, the less good the fit of one reflection over the other.

How does this relate to the rainbows on oil though? Well, the closer the panes of glass are together, the better the “fit” of one reflection over the other. Once the panes of glass get so close together (i.e. as close to each other as the wavelength of light – around 400 to 700 nanometres), weird stuff starts to happen! Depending on the separation, the images will ‘interfere’ with each other, and this has the effect of cancelling out some of the wavelengths of light, while enhancing others.

Of course, it is impossible to get two glass surfaces so close together, but a film of oil over water can very easily get this close; you get one reflection from the top surface, and a second from the bottom surface. This leads to this ‘interference’, and because the separation between the two surfaces will vary, you will ‘select’ different wavelengths of light for enhancement – different wavelengths = different colours, so you see the rainbow!

I hope this helps to answer your question!