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Non Sequitur Fridays

How a Rainbow Works

This post is part of our Non Sequitur Fridays series, which will feature a different Wistia team member's take on a non-Wistia-related topic each week. It's like our "employee of the month" but less "of the month"-y. Jeff Vincent is director of customer happiness at Wistia. His last Non Sequitur was about Andrew Jackson.

Rainbows are one of the most beautiful and transfixing phenomena in nature.

Because of their rarity, rainbows have always been associated with magical events, like leprechauns with their hidden pot of gold. Seeing one has always been considered connected with good luck.

Well, that's total crap.

Rainbows are sunlight being refracted through a medium before it strikes your eyes. That's it. Sorry, there's no gold, and a little later on you'll even see why it's not possible to find the end of a rainbow. You aren't more lucky for having seen a rainbow (although you are totally entitled to feel more awesome).

Destruction of blissful ignorance aside, we gotta give nature some credit! There is a very intricate collaboration that has to happen between the sun, the rain, and your eye in order for a rainbow to appear. It's nature, and it's pretty neat. Let's get into how it all works.

What is happening to create a rainbow?

If you grew up in the '70s, or are awesome, you no doubt recognize Pink Floyd's Dark Side of the Moon cover. Every rock nerd I know (and love) owns the t-shirt. It's also an awesome recreation of an experiment that Sir Isaac Newton ran in 1672.

Newton passed light through a prism, and using refraction (essentially the bending of the light) split the single beam of light into its color components. Because the colors are at slightly different wavelengths, they refract at slightly different angles when entering a new medium (which for Newton was a prism, and for a rainbow is water). These slightly different angles are also what creates the vision of bands in the rainbow.

Now, let's apply this situation to the water droplet. Light from the sun enters the droplet, refracts, reflects off the back wall, and refracts again when exiting the water droplet and hurtling towards your eyes. The light is now a single color, instead of the white light it started with.

Red light, because of its longer wavelength, refracts at 42 degrees, while blue light refracts at 40 degrees. Between those two measurements is where the entirety of the rainbow lives. Cool!

What are the conditions for seeing a rainbow?

First condition: There has to be a lot of sun, and rain. Okay, that's obvious. But think about it: the sun cannot be blocked by clouds, but there are water droplets in the air. It's a pretty a rare combination in weather (although something you can easily recreate in your backyard using a hose).

Second condition: You must be positioned between the sun and the rain. The sun must be "behind" you in the sky (in other words, your shadow goes out in front of you), and the water droplets must be in front of you (in other words, your shadow is closer to the rain than you are). This is important because the light must reflect off the back wall of a rain drop, and head back to your eye.

Third condition: You cannot be wearing sunglasses - at least the polarized kind. All rainbow light is polarized light, so you won't be able to see it if you're sporting those slick Gucci shades.

How do I see a rainbow?

To track a rainbow, you must first find your antisolar point. Imagine a line being drawn from the center of the sun, down through your head, and to the head of your shadow. This line is actually the center of your rainbow. But more on that later. For now just remember "anti" "solar", as in "opposite of sun".

If the three conditions we talked about are met, the instructions that professor Walter Lewin gave in his wonderful book, For the Love of Physics, are the best I've heard:

First, turn your back to the sun. Then locate the shadow of your head, and look about 42 degrees in any direction away from the imaginary line [ed: which is what Prof. Lewin calls the *antisolar point*, because he is awesome]. If there's enough sunlight, and if there are enough raindrops, the collaboration will work and you'll see a colorful bow.

So at this point, I've covered the basic science behind a rainbow (i.e., a rainbow is refracted light hitting your eyes). But for all you curious folks, learning something new tends to create more questions than answers. Here are some of the coolest questions I could think of regarding rainbows.

Why is a rainbow... bowed?

If you used one of those protractors from middle school, selected a single point (your antisolar point), and then sketched a constant point 42 degrees from the point, you'd get a circle. A rainbow is actually a full circle - it only looks like an arch because the darn ground gets in the way! Next time you're on a plane, look down at some rainy clouds and you might just see a perfectly circular rainbow!

Man, what's with all this talk about angles? Geometry is the worst!

Try this. Roll up a piece of paper like a cone, and leave a tiny hole in the smaller end.

Facing away from a large light source, aim your "looking glass" directly at the head of your shadow. From your line of sight, the edges of the looking glass look like a circle, right?

In the event all the conditions for seeing a rainbow were met and your telescope was wide enough (i.e. almost 90 degrees total), the rainbow would exactly match the edge of your improvised looking glass.

Where's the end of the rainbow?

A rainbow is not a stationary thing - if you move, so does your shadow, and therefore so does your antisolar line. Not only is it not possible to reach the end of the rainbow, it's not even possible to see a rainbow at an angle!

This is actually my favorite part about rainbows. The light beams you see are the ones that perfectly hit your eye. The light beams that hit your eye are different than those that hit someone else's, even if they are standing right next to you. Everyone sees their own individual rainbow. Whoa, mind blown.

Why is the order of the colors always the same?

The order of the colors of a rainbow is always the same because the angle the light refracts at is always the same. From top to bottom, it's always red, then green, then blue (or, because Newton moonlighted as a Benjamin Moore salesperson, red, orange, yellow, green, blue, indigo, and violet). This is true unless you are looking at a secondary rainbow, a.k.a. a double rainbow, which we'll cover in a minute.

Let's go through an example.

Imagine three water droplets, stacked on top of each other. Each water droplet refracts the sun's light, breaking it into the color spectrum. Then, it reflects back towards your eyes. But because our eyes are this teeny tiny lens, only light moving at a specific angle will make it back to your eye. So each droplet contributes a small part of the total spectrum you see as a rainbow - the top droplet contributes the red light, the middle droplet the green light, and the bottom droplet contributes the blue light.

What's a double rainbow?

The double rainbow, made most famous by a young man named Bear Vasquez, is really always there, it can just be hard to see. Some of the light inside the water droplet reflects not just once, but twice. This fainter second rainbow then appears outside the first (at an angle ~50 degrees), and, because of the second reflection, the colors are flipped! That's right - in a double rainbow, the blue light is on top, and the red at the bottom.

Now that's beauty all the way across the sky.

When is the best time to see a rainbow?

The lower the sun is in the sky, the higher the arc of the rainbow. This is because of the angle at which the light is refracted (crazy how this whole rainbow thing is driven by that single principle).

The early morning and the late afternoon, then, are the best times for rainbows. So if there's a sunshower just before dinner, run outside, there is probably an awesome rainbow to see.

What's the best way to capture an awesome rainbow?

If you've recently upgraded your camera arsenal, and want to shoot your next video with a rainbow in the background, you'll need some specific equipment.

To capture the full arc of the rainbow, you'd need a lens of at least 84 degrees (42 x 2). This would probably require a 19mm wide-angle lens or something similar for a typical DSLR.

What's your favorite rainbow-related character?

Definitely Lady Rainicorn.

There is so much more to rainbows, let alone the subject of physics. If you have suddenly found yourself more interested in recreational physics, I suggest checking out these awesome resources:

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