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   Alexander's Dark Band 

Photo ©Les Cowley       

Dark band.

The sky between the primary and secondary bows is noticeably darker than elsewhere.

Alexander of Aphrodisias first described the effect in 200 AD and it now carries his name.

Light rays undergoing a single reflection in raindrops form the primary rainbow or brighten the sky inside it.

Rays reflected twice are deviated to form the secondary bow or brighten the sky outside.

Raindrops along lines of sight between the two bows cannot send light to your eye and so the sky is darker there.