Short Wave - Why We See Rainbows

Episode Date: December 4, 2020

It's another "Back To School" episode where we take a concept you were maybe taught in school as a kid, but didn't really learn or just forgot. Short Wave producer Thomas Lu and host Maddie Sofia go on a journey to explore what a rainbow exactly is and how we see them! We all remember ROY G BIV, right?Email us your Back-To-School ideas at [e-mail] pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

Hello there, shortwave producer Thomas Liu. First time behind the mic. Howdy, howdy, Maddie, Safaya. So a couple of weeks ago, I sent you on a very serious mission for today's back-to-school episode. Right. These are episodes about something you heard about in school as a kid, but never really learned. Or maybe you might have forgotten it. Yeah. And for me, embarrassingly, on multiple levels, one thing I never really learned is what is, what is, a rainbow. Like, I kind of know, but do I, Thomas? Maddie, there's no shame in not knowing. But

just to ease your anxiety, I found some answers for you. Very kind, very kind. Oh, and by the way, I might have met someone on my mission. Let me tell you, he's very handsome, has a colorful personality, and, you know, super famous. People like Judy Garland, Willie Nelson, Israel Kamika Vivoale. the Carpenters, and my very personal hero, Kermit the Frog, all sing about them. Okay, what is going on right now? His first name is Roy. Roy. Yeah.

Oh, my God. Thomas. Full name. Get this. The full name, Roy G. Bibb. I'm so mad at you right now. You had me.

Well, so today on the show, we're going back to school to find out what is a rainbow? Why do we see them? And who exactly is Mr. Roy G. Bib? That is not an actual person. Okay, Maddie. This is Shortwave from NPR. Why are there so many songs about rainbows and what's on the other side? Okay, Tilu, we are trying to figure out what exactly is a rainbow. So who did you talk? to help me complete your mission, Maddie, I tapped into the 500 queer scientist database because rainbows. A queer scientist explaining rainbows?

I love this, Thomas. I know. I thought you might. I am Kirsten Nordstrom and I'm a professor of physics at Mount Holyoke College in South Hally, Massachusetts. And right off the bat, Maddie, let me tell you. Kirsten blew my mind. One of the most fun facts about rainbows is they are a consequence of observation.

They're not physical objects that exist in the sky. Whoa, Thomas. That's deep. So a rainbow isn't really there. Exactly. We see rainbows because the receptors in our eyes are picking up the light rays bouncing off of water droplets and our brain does the rest. In other words, rainbows are visions but only illusions.

And rainbows have nothing. Thomas, you built an extraordinary amount of. Kermit into this episode. Maddie, let me live. Anyway, he's not too far off. Rainbows are sort of like illusions that occur when the conditions are just right. The three key ingredients are having the sun behind you, having some water droplets in the sky like after a storm, and then you have to be in between those kind of at the right viewing angle, looking at the water droplets. Okay, so three things. The sun behind me, water droplets in the air, and me looking at the right angle in the right direction at the sky. You got it. That's the most ideal condition.

But how exactly does a rainbow form, Thomas? Maddie, hold your horses. I'm getting there. Sunlight is basically white light, and that doesn't mean it's lacking in color. It's actually quite the opposite. Right, because sunlight appears white because it's actually the combination of all the visible light, those colors that humans are able to see. Yep, when it comes to rainbows, Maddie, white light being the combination of visible light is important. Okay, let's start small. Take a single droplet of water.

When the light from the sun comes and hits a droplet in the sky, the droplet actually separates that light into all the colors. Oh, kind of like a prism. Something like that, yeah. So the separation of the light into different colors we see is caused by refraction. A process when light slows down and bends a tiny bit as it crosses the boundary between air and something denser like a droplet of water. The speed of light is slower in water than it is in air. But the thing is, is that for the different colors of light, the amount they slow down is different.

So then the amount they bend is different. Take the color red. It has the longest wavelength of visible light. So it bends the least versus violet with the short. broadest wavelength bins the most. Is that why we usually see red at the top of the rainbow and violet all squished at the bottom? Yes, Maddie. Yes.

Remember that handsome gentleman I was telling you about? Oh, your broigy bib joke? Yeah, I remember it. I'm still mad at you about it. Well, okay, smarty pants. For those of us who didn't know, that's actually a nice acronym to remember the order of the seven basic colors of the rainbow. Red, orange, yellow, green, blue, blue,

indigo and violet. I should say, though, there's technically an infinite number of colors in the visible light spectrum, but these are the main categories. Wow, wow, wow. I didn't learn that last point in grade school, I'll tell you that. You want to know what else you probably didn't learn in grade school? Always, Thomas, always. Up until now, we've been talking about one single droplet in the air, but in the sky there's a curtain of water droplets, each refracting and reflecting light back at you. Here's Kirsten again. So that arc of red is basically picking out all those special light rays that are reflecting off thousands, if not millions of droplets that happen to make it into your eye.

So cool. So cool. Super cool. Kirsten says this is the same for all the other colors as well, with each droplet setting you a different wavelength of color. And since you, the viewer, aren't looking at one single droplet in the sky, you're seeing the magic that's happening across a collection of. water droplets suspended at different heights in the air. Okay, wait. So does this mean if this is all about angles and where you're standing and how things are reflecting,

does this mean that no two people are seeing the exact same rainbow, like even if you're standing in the same area? Isn't that wild, Maddie? That's wild. You and your friend might be seeing a rainbow appearing in the same general direction, but your line of sight is picking up a different set of water droplets than your friends. And here's another fun fact. In principle, the rainbow is a complete circle and in principle it has no end or beginning. Wild, wild. Okay.

But so, Thomas, why don't we see that full rainbow circle goodness? Sadly, because of the earth, Maddie, the ground gets in the way. Although, if you were high enough in the air, say in a plane and in the right conditions at the right angle, you can see the full circle of a rainbow without the ground rudely interrupting. Cool. Okay. All right. I can get down with that.

Last question, serious question, Thomas, if I, as a queer person, were to run through a rainbow, does that make me more gay? Maddie, funny you asked that. Burst it has actually been asked that very question. No, she hasn't. The answer to that is, sadly, no, because you can't really run through it, because it is essentially not a fixed object in space. It is a property of you observing what's going on with light. Well, you win some, you lose some, Thomas, you know? What are you going to do, Maddie?

What are you going to do? All right, Thomas, thank you for bringing us the gayest physics episode of Shortwave ever. And honestly, that's saying something. Maddie, my gay honor is all yours. And if dear listeners have any ideas for a back-to-school episode, email us at shortwave at npr.org. This episode was produced and gaily reported by Thomas Liu, edited by Viet Le and fact-checked by Arellah Zabidi. Jases was the audio engineer. on this episode.

I'm Maddie Safaya. Thanks for listening to Shortwave from NPR.