Alexander's band or Alexander's dark band is an
optical phenomenon
Optical phenomena are any observable events that result from the interaction of light and matter.
All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...
associated with
rainbow
A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular ...
s which was named after
Alexander of Aphrodisias
Alexander of Aphrodisias (; AD) was a Peripatetic school, Peripatetic philosopher and the most celebrated of the Ancient Greek Commentaries on Aristotle, commentators on the writings of Aristotle. He was a native of Aphrodisias in Caria and liv ...
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
In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...
of
water
Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...
prevents light from being deviated at smaller angles. The
minimum deviation
In a Prism (optics), prism, the angle of deviation () decreases with increase in the angle of incidence () up to a particular angle. This angle of incidence where the angle of deviation in a prism is minimum is called the minimum deviation positio ...
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.
References
External links
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Atmospheric optical phenomena
de:Alexanders dunkles Band
fr:Arc-en-ciel#La bande sombre d'Alexandre
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