Rainbows have inspired songs, movies and television shows, and even the Lucky Charms leprechaun hides his pot of gold at the end of a rainbow. But long before they were part of popular culture and made the perfect backdrop for proposals, rainbows held significance in folklore and religious traditions.
In ancient Sumeria, for example, people associated rainbows with rain. King Izdubar was one such leader who envisioned "a mass of colors like the rainbow's hues." The kind took this vision to be a "divine sanction" of war. At a later date, the same king would see a rainbow of colors above a fountain of life, which grew next to the Tree of Immortality. This is a less positive interpretation of rainbows than is typical. It's said that Ninurta, the god of farming who is oft depicted carrying a sickle, wore a crown of rainbows. Perhaps this represents the link to rain, which crops required.
The Sumerians weren't the only people who revered the rainbow in some way. In the recent movie Thor, which depicts Thor and his people in their home world of Asgard. Gods must cross the rainbow bridge, Bifrost, to get between the two planes, one mortal and one immortal. This is a representation of actual Norse beliefs, in which only the Asgard and the departed can move between the two worlds. This theory of the afterlife, like many, describes how the virtuous warriors were able to attain immortality and eventually traverse Bifrost through their virtues.
Finally, ancient Greek beliefs heralded a goddess of the rainbow. Sometimes, Iris looks like an actual rainbow. At other times, she is a human be-fitted with wings. She ruled in both sea and sky, providing clouds with the water necessary to make rain. Iris, whose parents were Electra and Thaumas, delivered messages for the gods. Unfortunately, her messages were typically devastating or negative, which is less inspiring than other folklore about rainbows. However, you can imagine her moving between worlds over the rainbow, similar to the fashion in which the Asgard uses Bifrost.
Today, rainbows continue to influence things. Take a look at Chasin Rainbows, a virtual slot game that you can play on Starspins slots. It features the familiar visage of a leprechaun, who is guarding his rainbows and pots of gold. The game uses the single-line format with progressive jackpot style. Any player who spins three rainbows will win that jackpot! However, if players are lucky, they just might make a spin that has them singing "Somewhere Over the Rainbow" at their good fortunes. Players can view the rainbow as their pathway to a different world, one filled with joy and luck, from their mortal plane.