All the colours of the rainbow

A rainbow is a many-splendored thing. Over the centuries, rainbows have baffled scientists, ensorcelled storytellers, and even inspired social change.

Illustration by Launa Winship

Illustration by Launa Winship

“Do not all charms fly
At the mere touch of cold philosophy?
There was an awful rainbow once in heaven:
We know her woof, her texture; she is given
In the dull catalogue of common things.”
— John Keats, “Lamia”

Poets like Keats may prefer shimmering mystery to hard fact, but that hasn’t stopped wondering minds from chasing the meaning of the rainbow. From the physical principles that underpin its existence, to the fantastical stories that make sense of it, rainbows inspire us to recount, challenge, and explain the world. Mathematicians scratched out countless sums and angles to understand exactly where rainbows come from. Bards envisioned colourful bridges into the heavens. And activists wave the rainbow flag to support the lesbian, gay, bisexual, transgender, and intersex (LGBTI) community.   

Bending light

If you look away from the sun and extend an arm in the opposite direction, you will be pointing at the antisolar point. If there is enough sunlight, and if there happens to be an even mist or a dewy spiderweb in the bow-shaped region 40 to 42 degrees from the antisolar point, a rainbow will emerge. The colourful arc always appears at the same angle.

The exact mechanism of this constant angle puzzled scientists and philosophers for ages, resulting in many convoluted theories. Among those who finally got it right were Theodoric of Freiburg, Kamal al-Din, and most accurately Descartes, who all realized that the key steps of the process take place within individual water droplets, specifically their edges. Light passing from one medium to another, for example from air to water, changes both speed and direction — that is, refracts. Refraction only occurs when light hits an interface at an angle other than perpendicular. You may have seen this bending before: a straw in a glass of water will appear to break abruptly and change direction at the water’s surface. 

In a raindrop, sunlight enters and bends with the change in medium, reflects off the far side of the drop, and then bends again upon exiting. For a rainbow to emerge, light entering nearer the top of the drop bends in a way that causes the exiting rays to converge in a bright band at 42 degrees from where it entered the drop. This happens simultaneously in many droplets to create our rainbow.

 
Refraction, reflection, refraction. Rainbows require precise conditions. KES47/Wikimedia Commons (public domain)

Refraction, reflection, refraction. Rainbows require precise conditions. KES47/Wikimedia Commons (public domain)

 

While 42 degrees is the magic number for raindrops, every medium has its own index of refraction that determines how much the light will bend. A rainbow in the salty spray of an ocean wave will appear at a slightly lower angle than a freshwater rainbow.

A secondary rainbow forms when light enters nearer the bottom of the drop and then reflects twice internally, converging at 51 degrees. Theoretically, even more concurrent rainbows are possible at more angles with more reflections within the drop. But every reflection dims the beam, as some of the light passes through to the outside of the drop instead, so any tertiary and quaternary bows are almost always invisible. 

The most mesmerizing aspect of the rainbow — the vivid array of colours — continued to baffle until Newton revealed that sunlight is made of every colour at once, through his famous work with prisms. The refraction of light as it enters and leaves a raindrop not only concentrates the light at certain angles but also separates, or disperses, the light into its constituent colours because refraction varies with wavelength. Longer wavelengths, closer to the red end of the spectrum, have lower indices of refraction, meaning they bend less at the air-water interface and reach our eyes at higher angles. Meanwhile, shorter wavelengths, closer to the violet end of the spectrum, have higher indices of refraction, meaning they bend farther down at broader angles as they return to our eyes. So, red appears at 42 degrees while violet appears at 40 degrees. In a secondary bow, the relationship between wavelength and final angle is reversed, producing an upside-down version of the first rainbow with red at 51 degrees and violet at 55 degrees.

But a rainbow can only reflect the light it has to work with. Bows that arise during sunrise or sunset, when scattering has removed most of the blue light, are almost entirely red, and bows in moonlight, called moonbows, will often appear colourless, not because moonlight lacks the full spectrum but because the faint reflected light is far too dim to activate the colour receptors in our eyes. 

On rare occasions, a few extra, narrow arcs of colour nestle along the curve of the main rainbow. These extra bands, called supernumerary bows, come from light beams reflecting at different angles to reach the same place — your retinas — which means they travel slightly different distances. Beams that travel farther are able to complete an additional half phase, full phase, phase and a half, and so on. This discrepancy puts some beams out of sync and some beams in sync, causing them to cancel each other out or reinforce each other. Thus, brightness alternates with emptiness. Supernumerary bows are easiest to produce with a misting hose, whose drops are very similar in size.

 
Supernumerary bows appear as extra bands alongside the main rainbow. Mika-Pekka Markkanen/Wikimedia Commons (CC BY-SA 4.0)

Supernumerary bows appear as extra bands alongside the main rainbow. Mika-Pekka Markkanen/Wikimedia Commons (CC BY-SA 4.0)

 

Contrary to an often repeated belief, rainbows are not usually complete circles whose other halves are hidden by the horizon. In fact, a rainbow cannot exist without a beholder. Any rainbow that appears at 42 degrees from the antisolar point will disappear into the ground unless you are well above the surface of the earth. You have much better chances of spotting a glory — a spectrum forming a ring 5 to 10 degrees from the antisolar point — from an airplane passing over a layer of clouds, or from the rim of a canyon filled with fog. Glories can also appear in concentric multiples, like bullseyes.

What does it mean?

Well, that depends on your culture. Several myths around the world interpret rainbows as bridges. The rainbow serves as a path to the afterlife in Native American, Japanese, Hawaiian, Polynesian, Austrian, Maori, and French traditions. A famous rainbow bridge, Bifrost of Norse mythology, stretches from Earth to Asgard where the vigilant, far-seeing Heimdall guards the realm of the gods against invading giants and demons. 

 

Double rainbows, what do they mean? This man's fascination with the rare phenomenon went viral on the Internet in 2010. 

 

That Iris herself, the Greek god who personifies the rainbow, functioned more as a messenger than a weather-maker, speaks to the ability of the rainbow to link disparate worlds. If you have a pet or, especially, if you are part of a dog rescue community, you might say “See you at the bridge,” as a farewell to an animal who has died, in reference to a peaceful meadow where pets who have passed on await their owners before they cross over a rainbow bridge into heaven together.

In other mythologies, the rainbow spanning the sky is a monstrous snake. The Australian rainbow serpent controls rains and monsoons, sucking up and expelling water and retreating to deep waterholes during drought. Similarly, the Ewe peoples of West Africa imagine the rainbow as a serpent that stretches across the sky to drink from the other side of the world. The Shoshoni in North America also tell of a rainbow snake who scratches his back against the firmament, dislodging pieces of ice and precipitating the weather. And the Semang of Malaysia consider the rainbow a python that infects the earth it touches.

Others interpret the rainbow as a sign of good fortune. In the Bible, the rainbow represents God’s covenant with mankind after the great flood, a promise to never again destroy the world in a deluge. According to a medieval Germanic belief, no rainbow can appear for 40 years before the end of the world, so the sight of any rainbow again reassures humanity that the apocalypse is not impending. Some Mojave of Arizona believe that a violent storm will end when every colour of the rainbow appears. 

And of course, the end of the rainbow has been supposed to hide all kinds of treasures, like magic pearls, precious beads, magical golden dishes, and gold coins.

Yet some see the rainbow as an ill omen. The Nias people of Indonesia fear the rainbow, a giant net spread by a malevolent spirit to catch souls. In Slavonic tales, a fey being sits at the foot of the rainbow, combing her hair, and her glance brings death. Many cultures hold that pointing at a rainbow provokes curses; according to Hungarian folk belief, your finger will wither, and a Chinese text says your hand will ulcerate. The Sumu of Honduras and Nicaragua hide their children indoors at the appearance of a rainbow. Some even perform rituals to make a rainbow go away. Children in Staffordshire, England, have been known to cross a pair of sticks and place a stone on top of them to this end.

 
Oshumare — a rainbow god from Togo and the Volta region of Ghana — spends half the year as a hunter and half the year as a mermaid. xenmate/Flickr (CC BY 2.0)

Oshumare — a rainbow god from Togo and the Volta region of Ghana — spends half the year as a hunter and half the year as a mermaid. xenmate/Flickr (CC BY 2.0)

 

Rainbows also wield power over gender. In the folklore of several Eastern European countries, anyone who passes under the arch will change sexes. Relatedly, many rainbow deities are androgynous, split into male and female aspects — red versus violet or primary rainbow versus secondary rainbow — or attracted to the same sex. The androgynous West African god and rainbow serpent Oshumare spends half the year as a hunter and half the year as a mermaid. Rainbow gods like the gender-fluid Aido Hwedo of Vodou belief have also influenced contemporary culture, showing up in the poetry of lesbian writer Audre Lorde.

Perhaps not surprisingly, rainbow flags have become a symbol of pride for the LGBTI community, first appearing as such in 1978 when designed by San Francisco artist Gilbert Baker. Each of the original eight colours represented an idea: pink for sexuality, red for life, orange for healing, yellow for sun, green for nature, blue for art, indigo for harmony, and violet for spirit. Before becoming synonymous with fabulous pride movements, the rainbow flag has stood for many social movements. The flag has represented peace, in an Italian peace march in 1961 and later internationally, as well as Buddhism, Andean indigenous pride, and in the 16th century during the German Peasants’ War, social change.

These iterations of the rainbow through cultures and eras share some common threads of acceptance and community. But ultimately, the rainbow doesn’t carry an inherent meaning, but instead reflects, and refracts, unique moments and communities.

Edited by Bryonie Scott and Tessa Evans, and sponsored by Lizzy Bart