The reason the rainbow is seen is because the layer of clear nail polish is so thin that it reflects light in different wavelengths. Slight difference in thickness cause light waves of different length to interfere with each other– sometimes cancelling and sometimes reinforcing. Other examples of where we see this in nature is with car oil spills on the wet roads, as well as when looking at many colorful bird feathers such as peacocks. The peacock feather is actually fairly dark brown and not colorful. But there are thin variations in the thickness of the colorful side, and this causes light to reflect in different wavelengths, producing the colors we see. If we were to smash a feather totally flat, the color would be greatly reduced.
