If both primary and secondary rainbows are visible, you may notice a dark space, or band, in between the two rainbows, which contrasts with the lighter area inside the primary and outside the secondary rainbow. Known as the Alexander's dark band, this effect is due to the minimum refraction angle for the primary rainbow and the maximum for the secondary. The only light in the dark region is caused by (a small amount of) scattering, and not the refraction of light in water droplets.
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