Saskatchewan has elected to call itself “Land of Living Skies.”  One good reason appeared in the sky on Canada Day following an afternoon thunderstorm:  a rainbow.

In space, the sky is black and the sun appears white, and that’s all there is to it. But before the light of the sun reaches us down here on the ground, it has to pass through kilometres of atmosphere–kilometres of uncountable numbers of particles, from nitrogen, oxygen and ozone molecules to much larger particles of smoke and dust to relatively huge droplets of water and crystals of ice.

As it strikes these particles, sunlight may be reflected (bounced away at an angle equal to that at which it struck the particle), refracted (bent into a different direction of travel), scattered (absorbed and then immediately emitted in a random direction) or diffracted (bent around the particle). These four things, either alone or in concert, create all the various optical effects we see in the sky.

By far the most famous and spectacular of these is the rainbow, which has drawn our gazes skyward for millennia (the Bible says God put the rainbow in the sky to seal His promise to Noah that He will never again destroy the world with a great flood).

A rainbow occurs when sunlight strikes raindrops. The color of light is determined by its wavelength. White light is actually made up of many colors–many wavelengths of light. But when white light enters a raindrop, it is slowed and bent–and each color is bent at a slightly different angle. Red light has the longest wavelength and is bent least; violet light, with the shortest wavelength, is bent most.

The separated colors then strike the back of the raindrop. Light that strikes at an angle greater than 48 degrees is reflected back out the front of the raindrop, being bent a bit more on its second journey through the drop. Violet light emerges at an angle of 40 degrees relative to the incoming sunlight, while red light exits with an angle of 42 degrees. This means that what eventually reaches your eye is just one colour of light–which one depends on the angle from which you’re viewing the raindrop, relative to the path of the sun’s light. Each raindrop shows you just one tiny flash of one specific colour, which give you some idea of the vast numbers of raindrops necessary to produce a full rainbow.

Every person looking at a rainbow actually sees a different rainbow, because for each observer, the light is coming through a different set of raindrops. Similarly, with every move you make, the light you see is being reflected from a different set of raindrops. That’s why the rainbow seems to move with you.

The rainbow’s colours appear brighter nearer the ground (sometimes, only the parts of the rainbow near the ground can be seen at all) because raindrops are bigger near the ground and are therefore able to better refract and reflect the sunlight.

Sometimes you see two rainbows. The fainter secondary rainbow appears above or outside of the primary rainbow, and its colors appear in the opposite order from that of the primary rainbow (which always has red on the outside and violet on the inside). Secondary rainbows appear when the angle of the sun’s light is such that it enters each raindrop, is refracted, then is reflected internally twice (instead of just once) before emerging.

Should you wish to attempt to find the pot of gold at the end of the rainbow, you could be in for a long search; rainbows have no end. Every rainbow is actually a perfect circle, but half of it is hidden below the horizon.

I clung to the fond hope of being able to find the end of a rainbow for years, because when I was a kid, someone told me he’d see one: a kind of blue, hazy spot attached to the ground out of which the rainbow sprang. I’m still sorry he wasn’t telling the truth.

On the other hand, isn’t the truth every bit as interesting?

Except, perhaps, for the lack of gold.

Permanent link to this article: https://edwardwillett.com/2002/07/rainbows/

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