<P> A very similar experiment consists in using a cylindrical glass vessel filled with water or a solid transparent cylinder and illuminated either parallel to the circular base (i.e. light rays remaining at a fixed height while they transit the cylinder) or under an angle to the base . Under these latter conditions the rainbow angles change relative to the natural phenomenon since the effective index of refraction of water changes (Bravais' index of refraction for inclined rays applies). </P> <P> Other experiments use small liquid drops, see text above . </P> <P> Rainbows occur frequently in mythology, and have been used in the arts . One of the earliest literary occurrences of a rainbow is in the Book of Genesis chapter 9, as part of the flood story of Noah, where it is a sign of God's covenant to never destroy all life on earth with a global flood again . In Norse mythology, the rainbow bridge Bifröst connects the world of men (Midgard) and the realm of the gods (Asgard). Cuchavira was the god of the rainbow for the Muisca in present - day Colombia and when the regular rains on the Bogotá savanna were over, the people thanked him offering gold, snails and small emeralds . The Irish leprechaun's secret hiding place for his pot of gold is usually said to be at the end of the rainbow . This place is appropriately impossible to reach, because the rainbow is an optical effect which cannot be approached . </P> <P> Rainbows sometimes appear in heraldry too, even if its characteristic of multiple colours doesn't really fit into the usual heraldic style . </P>

What does it mean when the rainbow comes out
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