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The useful glare-blocking properties of polarized sunglasses are well-known to just about anyone who goes outside. What isn't so well-known is how they reduce glare in the first place. That answer is deceptively complicated!

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If you’ve been watching SciShow for a while, and you’re super dedicated – and I appreciate that if you are – the title of this video might look a little familiar. That’s because a few years ago, we made a video on polarized lenses and how they reduce glare.

But thanks to some comments from viewers, we recently realized that we mixed some things up in that video, and we wanted to set the record straight. So here’s how polarized lenses actually reduce glare, and where were wrong! Light travels as something called a transverse wave, where the direction it waves is perpendicular to the direction it moves.

It’s like how water waves — which are also transverse until they reach shore — move across the water: the wave moves forward across the surface, but the water itself just bobs up and down. Similarly, light waves can oscillate up-down, or left-right, or any diagonal in between as they move forward through space. And the direction light oscillates is called its polarization.

Now, just as we English-speakers say that the direction between north and east is a combination of the two -- in other words, “northeast” -- physicists think about all polarizations as a combination of just two key ones. There’s vertical polarization, which oscillates up and down as the light moves forward, and horizontal polarization, which oscillates left-right as it moves forward. And just like northeast is a combination of north and east, any diagonal oscillations are just a combination of horizontal and vertical.

Light from something like the Sun starts unpolarized, meaning it’s oscillating in random directions. Some light is fully horizontally polarized, some is fully vertical, most is all sorts of combinations of the two. Okay, so far, so good.

That’s it for the vocab words, and we had most of this in the last video. But here’s where we get to the explanation of how polarized sunglasses work and where we went awry. We said that unpolarized light reflecting off a horizontal surface, like a lake, all gets polarized in the same direction.

And that isn’t quite true; unless unpolarized light hits a perfectly flat surface at exactly the right angle, some horizontally polarized light gets reflected and some vertically polarized light gets reflected. The closer you are to that perfect angle, the closer you get to reflecting only light that’s polarized in a certain direction. The bigger problem was that we said light reflected off a horizontal surface is vertically polarized — that it oscillates up and down.

When actually, it’s the exact opposite of that. Light reflected off something horizontal tends to get horizontally polarized. It oscillates side-to-side, parallel to the surface it reflected off.

So the glare off a lake or road is mostly horizontally polarized light. And this is where polarized lenses come in. They tend to be coated with long, thin molecules lined up in horizontal rows across the lenses.

These long molecules can only absorb light that oscillates along the molecule, so the rows of horizontal molecules absorb horizontally polarized light before it gets through to your eyes. But they let vertically polarized light right through. So, polarized sunglasses will almost entirely block glare from horizontal surfaces since it’s mostly made up of horizontally polarized light.

They also block the horizontally polarized part of sunlight, and since sunlight is unpolarized, that means some light can get through, but not all of it. If you turn the lenses on their side, suddenly the horizontally polarized light isn’t in the same direction as those molecules any more, and you can watch the glare come back. And, in the last video, we got that backwards as well.

Since we were saying that glare was vertically polarized, we said that horizontal molecules absorb vertically polarized light, which is not the case. What is true is that polarized lenses tend to work best fairly early and fairly late in the day, when the Sun is close to that critical angle where just about all its reflected light is horizontally polarized. Either way, their ability to block glare without making everything super dark makes polarized sunglasses pretty useful.

But it’s definitely the horizontally polarized light that’s getting blocked. Thanks for watching this episode of SciShow! We fact-check all our videos, and we work really hard to do that, but we appreciate you all letting us know that this one slipped through the cracks.

If you’re interested in learning more about the physics of light, you might want to check out our video about sonoluminescence, where sound creates light. ♪.