Tag: sunglasses

Polarized rainbow, what does this mean???

By Phil Plait | August 18, 2011 10:20 am

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!

[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!

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CATEGORIZED UNDER: Cool stuff, Pretty pictures, Science

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