In the rain sky, the fog drops, drizzle droplets, or raindrops cause sunlight or moonlight undergo two refraction and one reflection process to form a group of concentric arc optical phenomena, called rainbow, also called 42° rainbow. There are two kinds of rainbows: the "main rainbow" is the color halo of the inner ultraviolet rainbow, and the "secondary rainbow" has an extra reflection process during the formation process presenting the color halo of inner infrared purple. 51° rainbow, also known as "second rainbow".