How many rainbows have you seen at a given time? One or two at most if you were lucky to have found yourself in favorable atmospheric conditions of rain and shine. Here is photographic evidence of a rainbow that has not been reported earlier. The author of this paper has recorded photographs of a quinary (fifth-order) rainbow.

Primary and secondary rainbows are a common observation owing to the strong internal reflections within water droplets. However, higher order rainbows such as tertiary, quaternary, quinary and so on are harder to see for their weaker strength after facing multiple scattering.

Rainbows are called primary or secondary due to the number of internal reflections a light ray encounters inside a water droplet before exiting; hence, the quinary rainbow manifests itself after five internal reflections in the droplet.

The author’s photographic record is a clear proof of a natural quinary rainbow present in the so-called Alexander’s dark band that exists between the primary and secondary bow. In the photographs, green and blue color bands of this fifth order rainbow are clearly visible next to the secondary rainbow whereas the red band is not obvious due to the presence of bright secondary rainbow.

The typical water drop size that contributes to the quinary rainbow was estimated to be ~1 mm in diameter and 1/2 mm diameter droplet size was attributed to the primary rainbow, from the images recorded by the author. These results agree with theoretical models, confirming the position of respective color bands of the quinary rainbow at their expected locations.

“The quinary rainbow may not be that rare, at least in this region”, as the author remarks in his paper; the ideal geographic location and atmospheric conditions were conducive to help him record the quinary rainbow, not to mention some luck.

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