Belonging to the group of photometeors, the definition of a rainbow is: a group of concentric arcs with colors ranging from violet to red, produced on a "screen" of water drops (rain, drizzle or fog) in the atmosphere by light from the sun or moon.
The phenomenon is mainly due to refraction and reflection of light. When rainbows are produced by the sun, their colors are usually brilliant. When produced by the moon, their colors are much weaker or sometimes absent.

The primary rainbow is a colored bow which appears on a "screen" of water drops when light from the luminary falls upon them. The colored bow is opposite the luminary by which it is produced and its centre is on the prolongation of the line joining the luminary and the observer. Thus, the rainbow may form a complete ring when seen from a high tower or from an aircraft. It is rare that all the so-called "colors of the rainbow" (red, orange, yellow, green, blue, indigo and violet) are observed. The size of the droplets determines which colors are present and the width of the band occupied by each of them. In all cases the violet is on the inside and the red on the outside. The sky is darker outside the bow than inside.

In addition to the primary bow, there may be a secondary bow, much less bright than the primary, and with a breadth almost twice that of the primary. The red is on the inside and the violet on the outside.

Rainbows may be bordered by narrow colored bow (green, violet, orange) due to interference. These bows are called supernumerary bows. They occur inside the primary rainbow or on very rare occasions on the outside the secondary rainbow.

The picture, with view towards the east, was taken moments after a severe thunderstorm. The sun was still quite high, hence the primary bow appeared rather flat from the point of observer. Also a piece of the weaker secondary rainbow appeared on the left.