Why do rainbows sometimes appear around the sun?

It's called a solar halo. A solar halo is a colorful (or white) halo or light arc around the sun formed by cirrus clouds and cirrostratus, and the color bands are arranged in infrared purple. Solar halos are sometimes called "sun flail", including total halos and intermittent halos.

According to experts, the solar halo is formed by the refraction or reflection of ice crystals when sunlight passes through cirrostratus. When light enters the ice crystals in cirrostratus, it is refracted twice and dispersed into light of various colors in different directions. In fact, when there is cirrostratus, there are countless ice crystals floating in the sky. The ice crystals in the same circle around the sun can refract the light of the same color into our eyes, forming an internal infrared purple halo. When a cirrostratus composed of ice crystals appears in the sky, one or more colored rings of internal infrared violet centered on the sun often appear around the sun, and sometimes many colored or white spots and arcs appear. These rings, points and arcs are collectively called halos.