[Intro]

Hank Green: A couple of weeks ago I told you about how rainbows are formed, but I did not tell you ANYTHING about double rainbows. Many of your voiced your curiosity about this phenomenon in the comments, and luckily, I know just the rainbow expert to explain. 

LeVar Burton: (laughing) Thanks Hank. Hi, I'm LeVar Burton, and you may know me from Star Trek: The Next Generation, or as Hank alluded to, thirty years of Reading Rainbow. 

And, if you're a fan of the late, great, Community, not to worry. Through the magic of green screen, Hank and I have yet to meet, so he can't disappoint me. Though I have to say, Hank, your video about how to field strip a fusion reactor or realign the power transfer tunnel in a warp coil generator is lacking in the most rudimentary understanding of matter-antimatter principles. 

Now, let's get back to rainbows. As Hank has explained before, rainbows simply require light, water, and you. So long as you're in the right place, at the right time. 

From there, it's only a matter of reflection and refraction. When sunlight enters droplets of water, it's reflected off the back of the droplets, and as it moves from the air through the water and back into the air again, each color of the visible spectrum refracts, or bends, at a slightly different angle. This scattering of light creates the array of colors we know as the rainbow. 

But sometimes, this process of internal reflection really takes off. In these cases, the light bounces around inside the droplets, reflecting not once, but twice. This creates a second rainbow that we see above the first. Exactly nine degrees above, to be precise. 

Primary rainbows stretch from 40 to 42 degrees from your line of sight, while its double appears from 50 to 53 degrees. 

Now, that double, or secondary rainbow, is always fainter than the primary because only some of the light, reflected a second time, reaches where you're standing. And the second round of reflection reverses the order of the colors of the spectrum like a mirror. So the second rainbow appears to be inverted compared to the first. You'll see red on the bottom and violet on the top.

There's another aspect of the double rainbow phenomenon that you may not be aware of. The area between the the two bows appears to be extra dark. That's known as Alexander's band, named for Alexander of Aphrodisias who first noticed this about 1,800 years ago. It's dark because the droplets in that range are refracting sunlight at angles that don't reach your eyes.

But, rainbows don't have to stop at just two. Light rays can bounce three and even four times before exiting raindrops, forming tertiary or quaternary rainbows, but they're very rare and extremely difficult to see unless you have a visor.

Then, there are supernumerary rainbows, which appear as extra repeated bands of color inside of the primary rainbow. These are caused by the interference of overlapping light waves as the crests of one wave coincide with the troughs of another. 

Even more amazing are twinned rainbows, which are duplicated, uninverted pairs of rainbows which seem to split off of the same base. Now scientists don't fully understand them yet, but the current thinking is that they are formed when there are two different sizes of raindrops inside a storm, some larger and not quite spherical, while others are smaller and more round.

So, the next time you see a double rainbow- "Double rainbow, all the way across the sky! What does it mean?"- that although amazing and beautiful, it's not quite out of this world. It's a simple interaction of the electromagnetic spectrum, air, water, and you.

So, now that I've talked about one of Hank's favorite subjects, science, he's going to talk about one of mine, reading. Over to you, Hank.

Hank: Thanks, LeVar. The reason LeVar Burton just came on SciShow is because we both love reading, and we love rainbows, and we love libraries and we love the Internet. 

LeVar wants to make the new Reading Rainbow library of books and video field trips available to everyone by putting the app experience online. Right now it's available on tablets, which is awesome, but not every reader has access to one, and schools need a specialized version just for them. LeVar and his team at Reading Rainbow are launching a Kickstarter campaign to fund the project. 

Unlimited access to information is something we really value here at SciShow, so we are thrilled that Reading Rainbow is launching this project which will be giving thousands of classrooms in need the app for free. If you want to help, and I know I do, you can go to the Reading Rainbow Kickstarter by clicking right here, or there's a link in the description. 

Thanks for watching, and for getting smarter with us, and with LeVar Burton, and thanks to LeVar for teaching us about double rainbows.

[Outro]