A halo, which is also referred to as a gloriole, nimbus, or icebow, is an optical phenomenon generated by light interaction with suspended ice crystals in the atmosphere, which results in a wide variety of white or colored spots, arcs, and rings in the sky. It is as a result of sunlight traveling through ice crystals in cirrus clouds in the atmosphere. Light is refracted and reflected by these ice crystals and dispersed into different colors. The crystals act like mirrors and prisms, refracting and reflecting light between their faces, sending rays of light in certain directions.

Light goes through double refraction when travelling through a crystal of ice and the degree of bending that occurs is dependent on the diameter of the ice crystal, as illustrated in the schematic diagram above. A more detailed ray diagram should be easily drawn with the light concepts taught during our A Level Physics tuition class. A halo of 220 develops as light penetrates through a columnar crystal of ice via one side and find its way out through another side. This light refracts as it goes through the crystal of ice and one more time as it exits the crystal.

As light goes through the 60° angle of the ice prisms, which are hexagonal in shape, it’s deflected twice leading to angles of deviation between 22° and 50°. The minimum deviation angle is approximately 22°, specifically an average of 21.8°. This variation in refraction leads to the outer part of the circle being bluish while the inner part is reddish. All the clouds’ ice crystals deviate the light in a similar way, but only those from the particular ring at 22° lead to the outcome for an observer.

The sky is a bit darker in the halo’s inner side because, at angles smaller than 22°, no light is refracted. Another type of halo is the 46° halo, which is approximately twice the distance of the common 22° halo. At light deviations between 15o and 27°, this 46° halo is regularly confused with the infra-lateral and supra-lateral arcs, which intersect the parhelic circle at approximately 46° to the right and left of the sun. This 46° halo is comparable to the 22° halo but much fainter and larger. It is formed when daylight enters hexagonal ice crystals, which are randomly oriented, via a prism face and then exits via a hexagonal base.