This week, thanks to a question from Temple, New Hampshire resident Lorraine von Sneidern, the Curiosity Desk not only went chasing answers, we also went chasing rainbows.
"Why are the rainbows that I see outside after a rainstorm always a perfect arc?" asked von Sneidern.
Rainbows are, of course, one of nature’s most charming tricks of light. And light is what Massachusetts Institute of Technology Associate Professor Mathias Kolle spends his days pondering and studying in his lab.
As Kolle explained, in order for a rainbow to appear in the sky, you need three ingredients, so to speak: Sunlight, raindrops and you. And all three have to be in a particular position for the magic to happen.
“The sun has to be behind you,” he said. “Then water in the atmosphere in front of you. And that's usually when it rains... then what you also want to do is you want to look at the right spot.”
WATCH: Why are rainbows shaped in a perfect arc?
These positions are crucial because of the two phenomena responsible for a rainbow’s appearance — a couple of old friends you might remember from middle school science class: refraction and reflection.
Refraction is responsible for a rainbow’s colors. When sunlight enters a raindrop, it bends. White light, like sunlight, is comprised of a spectrum of different colored light. Each color exists at a slightly different wavelength and bends at a slightly different angle when it enters the droplet.
“So all of a sudden we get this splitting of colors,” said Kolle. “The reds and the oranges and the greens and the blues go in slightly different directions.”
After the light enters the raindrop and is split into its component colors, a small portion of it reflects off the back of the raindrop and heads in a new direction. With each color now on a slightly different path, the light is refracted one more time as it leaves the droplet.
Then it all heads back toward the earth, each color at a different angle: The red light at approximately 40 degrees, the violet at around 42 and all the other colors in between.
And that’s just one raindrop. All of this is happening on a grand scale, with billions of rays of light hitting billions of droplets of rain.
“It happens in every raindrop,” said Kolle. “As long as it sees a ray of sun, it will do that refraction, reflection, refraction thing. But if you're not standing in the right position, you will not see this by most of the raindrops.”
But if you are in the right position, at that magic angle between 40 and 42 degrees, there will be raindrops galore throwing innumerable, distinct beams of red, orange, yellow, blue, indigo and violet light right at you.
And the raindrops that are at that perfect angle — relative to the sun and your position — are what create the distinct arching shape of a rainbow.
You can think of it like a cone, explained Kolle, with you at the point and the rainbow appearing along the circumference of the base.
So why doesn't the entire circumference of the cone’s base light up like a rainbow? Well, the ground usually gets in the way. In fact, if you could get up high enough so that there was also plenty of rain below you, that arching bow would continue uninterrupted until the two ends met back up to create as Koll put it, “not a rainbow anymore, but a rain circle.”
And if you think the idea of rain circles is cool, consider this: The countless individual raindrops that are responsible for the rainbow you see are unique to your position, and they are changing moment to moment. So forget about leprechauns and pots of gold. The real magic is in the science. And the fact that any time you see one, you are literally smack dab at the center of your own, personal, rainbow.
If there is something you've been itching to know more about, email The Curiosity Desk or send in your question below. Edgar might just dig up the answer in a future episode. For more from The Curiosity Desk, follow Edgar B. Herwick III on Twitter and subscribe to the GBH News YouTube Channel.
