Alexander's Band in the double rainbow

Tuesday, January 28, 2020

Photo by Kate Harriger

By Cynthia Sheridan

Kate Harriger took this photo Sunday afternoon through the living room window, looking north from Shoreline. The family described it as “absolutely glorious” with incredibly vibrant colors on the primary bow.

According to Wikipedia, secondary rainbows are caused by a double reflection of sunlight inside the water droplets; the colors of the secondary rainbow appear reversed compared to those of the primary bow.

The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky.

Each rainbow reflects white light inside its colored bands, but that is "down" for the primary and "up" for the secondary. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.


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