March 02, 2018
This colorful splotch was observed in the parking area of a market in Karviná, Czech Republic. It's about 20 inches (0.5 m) in diameter and results from diffraction of sunlight in drops of motor oil (or perhaps gasoline), likely dripping from an automobile engine. Since the film on the drop's surface is very thin, light is diffracted (not refracted). Thus, outgoing spherical waves of light from the filmy surface overlap and interfere with each other, producing the metallic colors and the corona-like rings. Variations in the viewer's line of sight and in the thickness of the film affect coloration. Photo taken on December 28, 2017.