What causes the rainbow pattern in oil spills?

The rainbow pattern in oil spills is caused by the phenomenon of thin-film interference of light.

When light encounters a thin film of oil on water, it splits into two beams: one is reflected off the oil's surface, and the other penetrates the oil and is reflected off the underlying water surface. These two light beams then recombine, and depending on the thickness of the oil film and the angle of incidence, they can either constructively or destructively interfere with each other. This interference results in the observed rainbow pattern.

The phenomenon of thin-film interference is a classic example of wave interference, a fundamental concept in physics. When two or more waves meet, they can either add together to create a wave of greater amplitude (constructive interference) or cancel each other out to create a wave of lesser amplitude or even no wave at all (destructive interference). In the case of an oil spill, the light waves reflected off the oil and water surfaces can either reinforce or cancel each other, depending on the thickness of the oil film and the angle at which the light hits it.

The colours seen in the oil spill are due to the fact that different colours of light have different wavelengths. When light of a particular wavelength undergoes constructive interference, that colour is amplified and becomes visible. Conversely, when light of a particular wavelength undergoes destructive interference, that colour is diminished or disappears entirely. The varying thickness of the oil film across the spill means that different colours are amplified in different areas, creating the characteristic rainbow pattern.

In addition, the angle at which you observe the oil spill can also affect the colours you see. This is because the angle of incidence of the light changes as you move around the spill, which in turn changes the conditions for constructive and destructive interference. This is why the colours in an oil spill can appear to shift and change as you move around it.