oilspot

Oilspot refers to a visual phenomenon observed in certain liquids, most commonly water, where patches of iridescent color appear on the surface. These colors are not due to a pigment within the liquid itself but rather to the optical effect of thin-film interference. When a thin layer of a substance with a different refractive index, such as oil or a similar non-polar liquid, spreads across the surface of a more polar liquid like water, light waves reflect off both the top and bottom surfaces of the thin film.

The thickness of this film, which varies across its surface, causes the reflected light waves to interfere with each other. Depending on the wavelength of light and the thickness of the film, certain wavelengths (colors) will be constructively interfered with, appearing brighter to an observer, while others will be destructively interfered with, becoming less visible. This differential reflection of light across the varying thickness of the oil film creates the characteristic rainbow-like patterns seen in oilspots. The colors observed can range from blues and greens to reds and yellows, and their intensity and distribution depend on the type of oil, the thickness of the film, and the angle of observation. Oilspots are a common indicator of oil pollution on water bodies, such as oceans, lakes, and rivers.