The Sciences

Polarized rainbow, what does this mean???

Bad AstronomyBy Phil PlaitAug 18, 2011 2:20 PM


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Earlier today I posted about a ginormous blob of gas 80% of the way across the Universe that's emitting polarized light, and how that's a dead giveaway it has galaxies embedded inside it. The fact that the light was polarized helped solved a ten-year-old mystery about what's lighting it and other cosmic blobs like it. Now, that's great for something that's 100 sextillion kilometers away, but what about here on Earth? Well, it turns out polarization works down here, too. It can make rainbows disappear!

[embed width="610"][/embed]

[Set the resolution to 720p or 1080p to see it best.] Here's how this works. First, to quote my post from this morning:

Imagine two people standing on opposite sides of a tall picket fence. There are spaces between the pickets, maybe 5 cm wide and two meters tall. One person has a sheet of plywood to hand through to the person on the other side. If they hold the plywood horizontally, it can’t get through. Duh. But if they rotate the sheet so that it’s vertical, it passes between the fence pickets easily.

Polarized sunglasses are like that picket fence. They're coated with a very thin sheet of molecules that naturally align themselves in rows. The manufacturers make sure those rows are aligned vertically on the lenses. Why? Because light reflecting off pools of water or other flat surfaces are polarized horizontally. That way the molecules in the sunglasses' coating blocks most of the light from reflections, reducing glare. And that's what's going on in the video, too. Rainbows are created when sunlight reflects inside water droplets, bouncing back to you. When the light enters the droplet and also when it leaves, it bends a little bit as well (like how a spoon looks bent in a glass of water). Different colors bend by different amounts, so the sunlight colors get spread out, forming an arc in the sky. The light forming the rainbow gets polarized when it reflects off the back of the raindrop

. The amount of polarization is pretty strong, as the video shows. When I hold the glasses horizontally the light gets through, but as soon as I rotate the glasses, the rainbow disappears entirely! Almost all the polarized light is blocked, and the rainbow vanishes. But wait, there's more! In my video, the way the setting sun was being blocked by clouds meant we could only see the base arch of the rainbow. Weirdly, had that been a full rainbow, rotating the glasses would have made the base disappear, but not the top! And holding the glasses normally would've made the top disappear and not the base. Why?

It's because of the direction of the light entering the droplets to make the rainbow. The polarization happens perpendicular to the direction the light is traveling. In other words, near the base, the light you see has been bent horizontally back toward you, and the polarization is vertical. At the top of the arch, the light has been bent vertically to get back to you, so the polarization is horizontal. If it helps, think of it as the polarization being tangential to the arc of the rainbow, like in the graphic here (which I got from

, where there's a great explanation of all this, including how polarized sunglasses work). Remember, sunglasses are polarized vertically when worn normally, to block the horizontally-polarized light from reflections. So when you're wearing glasses, the horizontally polarized light from the top of the rainbow is blocked, but the base remains. Take the glasses off, spin them 90°, and boom: no base, but the top reappears. Try it after a rainstorm! One more thing: a lot of smart phone screens are polarized as well. When I take a picture with my phone, I sometimes rotate it to fit something wide in the field, and if I'm wearing those sunglasses the screen turns black! I have to take the glasses off to see what I'm photographing. This effect can be used to create 3D images and movies as well... but that's another story, for another post. So, the next time you see a rainbow, whip out your sunglasses, turn your head to the side, and yell at the top of your lungs,

Polarized rainbow!

THAT'S what it means!!!

Image credit: For those unfamiliar with the Double Rainbow mem, here's the background, and, of course, here's the original video. Watch that, and then watch Paul and Storm's fantastic satire of it. You're welcome.

Related posts: - In astronomy, a polarizing view is good - Naked rainbows - Rainbow conspiracy - Fire cloud - Do rainbow clouds foretell earthquakes?

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