Alexander's band or Alexander's dark band is an optical phenomenon associated with rainbows which was named after Alexander of Aphrodisias who first described it. It occurs due to the deviation angles of the primary and secondary rainbows. Both bows exist due to an optical effect called the angle of minimum deviation. Light which is deviated at smaller angles than this can never reach the observer.
The minimum deviation angle for the primary bow is 137.5°. Light can be deviated up to 180°, causing it to be reflected right back to the observer. Light which is deviated at intermediate angles brightens the inside of the rainbow.
The minimum deviation angle for the secondary bow is about 230°. The fact that this angle is greater than 180° makes the secondary bow an inside-out version of the primary. Its colors are reversed, and light which is deviated at greater angles brightens the sky outside the bow.
Between the two bows lies an area of unlit sky referred to as Alexander's band. Light which is reflected by raindrops in this region of the sky cannot reach the observer, though it may contribute to a rainbow seen by another observer elsewhere.
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