An Etymological Dictionary of Astronomy and Astrophysics
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A dark space or band between the primary and secondary rainbows when both are visible. This effect is due to the minimum refraction angle for the → primary rainbow and the maximum for the → secondary rainbow. The only light in the dark region is caused by (a small amount of) scattering, and not the refraction of light in water droplets.

Named for Alexander of Aphrodisias, Greek Peripatetic philosopher and commentator, who first described the effect in 200 AD.