Those dirty curbside puddles – you know the ones I mean – those with the motor oil slick across the surface just waiting to pollute the waterways. Walk past them the right way and they are lovely with a rainbow sheen as colourful as the most expensive opal.
How does something as black and sticky as oil become a fabulous modern art picture on water?
The answer is two-fold. Firstly oil and water don’t like to mix – just as the oil always floats to the top of the vinegar in salad dressings, motor oil on a pool forms a thin layer on the surface. When this layer is really thin (and we’re talking a couple of hundred nanometers) it’s about the same thickness as the wavelength of light.
As sunlight (a mix of all coloured lights from red and yellow to green and blue) hits the oil, some of the light is reflected back. Some passes through the oil but is reflected back from the surface of the water beneath. (Some passes right through the water as well but it’s not important).
Light waves are a bit like waves on the oceans, with crests and troughs.
As the light reflecting back from the surface of the water passes back out through the oil layer and into the air, it starts to interract with the light reflected back from the surface of the oil. Crests and troughs interract so that in some areas the wave is additive and in other areas, it cancels out (a trough meets a crest). The different colours in sunlight have different wavelengths, with red the longest and indigo the shortest in the visible spectrum. Depending on the angle of the sunlight, the angle you are looking at the puddle and the thickness of the oil film, different wavelengths of light (colours) cancel out or are magnified. Hence the rainbow effect as you walk past.
This interaction is called thin film interference and is also the reason soap bubbles have a multi-coloured finish.