There’s just something about rainbows. They’ve been immortalized in endless songs, myths, stories, movies and even cellphone commercials based on the painfully hilarious mushroom-induced exaltations of an overly-enthusiastic youtube star.
Rainbows just capture the imagination. For the vikings of old, they were the Bifrost Bridge between Asgard, the home of the gods and our world of Midgard. In ancient Rome, they were the path of a messenger between Earth and the heavens and of course we all know they’re where leprechauns store their pots of gold.
But what are they really? Why can you never find the rainbow’s end? I think Kermit the Frog had it right when he labelled them ‘only illusions’.
Rainbows are the product of an observer standing in the right place at just the right time. What is that right place? It’s about 42 degrees from the direction opposite the sun. You can never get to the end of the rainbow because it will keep moving with you as you walk towards it. Stray off the bearing and the image will vanish into the mist.
What you’re seeing is sunlight being refracted, dispersed and reflected back at you through millions of water droplets suspended in the air. You usually only get enough water hanging around after a storm has passed, hence the name ‘rainbow’. Of course, if you’re standing next to a waterfall, fountain or someone’s sprinkler, you can often get the same effect if you’re in that magical optical sweet spot.
I’ll try and keep the physics simple, but here’s how it works. The white sunlight enters the water droplet and is dispersed into the full spectrum of colours. Then, it’s reflected off the back of the raindrop, just like the inside of a camera. On the way back out of the drop, each wavelength is refracted (their direction of travel is changed) as it passes from the water back out into the air. How much each wave is refracted depends on the wavelength (colour) of the light. Red light (short wavelengths) are refracted less than blue (long wavelengths). The result is what was once a beam of ‘white’ light is now spread out into an arc of continuous colour. Double rainbows appear when the light is reflected off the back of the droplet lens twice. This second fan of light comes out at a slightly different angle and the spectrum of colour is inverted.
To us, the viewer, we see that colour in bands of red, orange, yellow, green, blue, indigo and violet because of the way photopigments in our eyes receive the light that is then interpreted by our brains. Take a black and white photo of a rainbow and you won’t see any bands, just a continuous gradation in intensity. Animals whose brains can interpret wavelengths we can’t, like ultraviolet or infrared, would see a completely different rainbow than we do.
I think that’s what I find so fascinating about rainbows. Their beauty is truly in the eye of the beholder.