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 this phenomenon in Aphrodisias, Commentary on Book IV of Aristotle's Meteorology (also known as: Commentary on Book IV of Aristotle's De Meteorologica or On Aristotle's Meteorology 4), commentary 41.
The dark band 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. The refractive index of water prevents light from being deviated at smaller angles. 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.
