Double Rainbow

A double rainbow at the Bodie Island Lighthouse. The dark band between the rainbows is called Alexander's Dark Band.

Around 200 AD, Alexander ...
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A double rainbow at the Bodie Island Lighthouse. The dark band between the rainbows is called Alexander's Dark Band.

Around 200 AD, Alexander of Aphrodisias, a Greek Peripatetic philosopher, noticed that when there was a double rainbow, the band between them appeared darker than the rest of the sky. This is called Alexander’s Dark Band, though Alexander had no idea what caused it.

This dark band is an optical phenomenon associated with rainbows. The angle made by the line from an observer to the center of a primary rainbow and a line from the observer to the rainbow’s arc is 42°. The comparable angle of the secondary rainbow is 50°. Between these angles, no light can be scattered to the observer by primary or secondary reflections. The band is not completely dark because light can be scattered from this region by other reflections and refractions, the band is not completely dark. (From: wikipedia.org; cloudappreciationsociety.org)

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