It is believed by some Hindus that the halo shown around the heads of holy people is a depiction of their "activated" crown chakra.
Halos are visual phenomena that appear near or around the Sun or Moon, and sometimes near other strong light sources such as street lights. There are many types of optical halos, but they are mostly caused by ice crystals in certain types of clouds. The shape of the crystals changes the type of halo seen. Light is reflected by the ice and may split into colors, similar to the rainbow.
Media related to Halo phenomena at Wikimedia Commons