Perhaps unsurprisingly, the rainbow has often been used in literature, religion, and art to symbolise the bridge between the land and the sky, the earth and heavens. But rainbow symbolism is also wide-ranging in different cultures, so it’s worth exploring the different meanings of the rainbow throughout the centuries, and in different religions and works of literature. So, let’s take a closer look at some prominent rainbow symbolism.
Perhaps the most famous religious symbolism associated with the rainbow is found in Christianity and Judaism: after the Biblical Flood in the Book of Genesis, the rainbow appeared in the sky as God’s covenant with Noah that he would not flood the world again (Genesis 9:11-13):
And I will establish my covenant with you, neither shall all flesh be cut off any more by the waters of a flood; neither shall there any more be a flood to destroy the earth.
And God said, This is the token of the covenant which I make between me and you and every living creature that is with you, for perpetual generations:
I do set my bow in the cloud, and it shall be for a token of a covenant between me and the earth.
In ancient Greek mythology, Iris was the goddess of the rainbow, bringing the gods’ commands down from Mount Olympus to the land of the mortals: once again, the arc or arch of the rainbow suggests a bridge between earthly and heavenly, land and sky. Iris was the divine messenger, and the rainbow was the brightly-coloured embodiment of her path from Olympus to the land of the living. The section of the eye surrounding the pupil is called the ‘iris’ for this reason: because its pigmentation suggests the colours of the rainbow.
Meanwhile, in Norse legend, a rainbow was literally a bridge between the world and the heavens, named Byfrost. In the East, the rainbow was a staircase of seven colours, used by Buddha when he returns from heaven. Also in East Asia, the rainbow was associated with the serpent (presumably because of its shape), and specifically with the mythic serpent who was the offspring of the Underworld.
Indeed, although rainbows often symbolise something positive and benign, this isn’t always the case, and the rainbow-as-serpents 
symbolism can be found in numerous cultures. In Vietnam, for example, there is a long-standing association between rainbows and snakes, with the rainbow being called the ‘perilous sky-serpent’ or, more specifically, two serpents interconnected.
In the Americas, the Incans didn’t dare look at the rainbow, regarding it as a malevolent omen. As The Penguin Dictionary of Symbols (Penguin dictionaries) points out, ancient Peruvians would often cover their mouths with their hands when a rainbow appeared in the sky. Once again, Incans believed the rainbow to be a sky-serpent, with all of the dangerous symbolism encoded in such an association.
Where does the myth about there being a pot of gold at the end of the rainbow come from? That appears to be Celtic in origin: ancient Celtic gold coins were called ‘rainbow saucers’. The pot of gold is supposed to be treasure belonging to Irish leprechauns: little fairies clothed in green and known for making shoes. The myth states that a leprechaun could be persuaded into giving up his treasure to someone who captured him, but the leprechaun would always trick his captor into looking the other way, whereupon both leprechaun and gold would promptly vanish. The idea of finding the end of the rainbow, by extension, denotes pursuing a fruitless and unrealistic aim.
In poetry, the rainbow has proved a powerful symbol of everything from God’s divinity to science’s ability to help us understand the natural world. In ‘The Rainbow’, the eighteenth-century poet James Thomson celebrates Newton’s discoveries (the word ‘awful’ here means ‘inspiring awe’):
Here, awful Newton, the dissolving clouds
Form, fronting on the sun, thy showery prism;
And to the sage-instructed eye unfold
The various twine of light, by thee disclosed
From the white mingling maze.
Thomson was writing during the great age of the Enlightenment or Age of Reason, but the Romantics, writing around half a century later, took a slightly different view, and thought that scientific enquiry risked destroying the awe and wonder of nature. In his poem Lamia, John Keats famously lamented the fact that Isaac Newton’s work with prisms had managed to ‘unweave a rainbow’.
Talking of the Romantics, in his short poem ‘My Heart Leaps Up’, William Wordsworth observes a rainbow in the sky and is filled with joy at the sight of it:
My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
Or let me die!
The Child is father of the Man;
And I could wish my days to be
Bound each to each by natural piety.
For him, the rainbow symbolises a joy that was there when Wordsworth was very young, and is still there now he has attained adulthood. He trusts it will still be with him until the end of his days. If he loses this thrilling sense of wonder, what would be the point of living?
Following the Romantics, many artists, especially landscape artists like J. M. W. Turner and John Constable, and the later impressionists like Monet, have sought to capture the colours of the rainbow, as well as its illusory quality. Since a rainbow is, quite literally, a trick of the light, with the sun’s white light being refracted and reflected through lots of tiny raindrops, it proved especially popular with nineteenth-century romantic and impressionist painters. In such paintings, the rainbow symbolises exactly what it is: a trick of light, colour created through the interplay of observer, rain, and sunlight, producing a miraculous and beautiful sight.