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How Do 3D Glasses Work?

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Stereoscopy—the illusion of depth created by showing a separate image to each eye—is at least as old as photography itself. In the last few years, however, stereoscopic 3D movies have come back in a big way. Theaters show you 3D movies by projecting two images on one screen and giving you specialized glasses that separate the images. But how do those glasses work?

Polar Opposites

There are several competing 3D technologies, but the most prevalent one in theaters today is based on polarized light. The technology that fueled the 1950s 3D boom, which is still in use today, is linearly polarized stereoscopy. Two images are projected through polarizers of two different orientations, typically 45 and 135 degrees relative to the horizon. The projected images are then filtered using polarizer films in the lenses of your glasses en route to your eyes. In this way, one image is excluded from your left eye while the other image is excluded from your right.

In the original formulation of this system, two projectors were used, and projectionists needed to take great care to make sure the two images were well aligned, perfectly synchronized, and equally bright. This problem has been eliminated with the rise of digital projectors. One of the other major problems, though, is inherent to the linearly polarized system: It requires the glasses to be parallel to the projector screen in order to prevent the images from leaking through their respective 'dark' lenses. This means that if you bend down to grab the popcorn, or turn to whisper to your friend, or if you sit off to the side of the theater instead of in the center, then the 3D effect will be compromised and you may get a bit of a headache.

The latter problem is corrected with circularly polarized 3D, patented in 1989. This is the method used by the RealD system, the most widely used system in theaters today. Here, one of the images is projected using light waves that trace out a left-handed spiral, and the other using light that traces out a right-handed spiral. Each lens contains a quarter wave plate, which is a passive device that transforms the two counterspiraling waves into two perpendicular linear waves. Then the familiar linear polarizers cut out one image from your left eye and the other image from your right.

So how do you know what kind of 3D glasses you're wearing? Slip into the bathroom during the movie and look in the mirror with one eye closed. The handedness of circularly polarized light is reversed when it reflects off a mirror, but the orientation of linearly polarized light is preserved. So, if the lens in front of your open eye is blacked out, then you are wearing circularly polarized glasses. If the lens in front of your closed eye is blacked out, then you are wearing linearly polarized glasses (or possibly active shutter 3D glasses—a topic for another post).

Andrew Koltonow is a graduate student in Materials Science and Engineering at Northwestern University. He's part of our College Weekend extravaganza.

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Big Questions
What's the Difference Between Stuffing and Dressing?
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For carbohydrate consumers, nothing completes a Thanksgiving meal like stuffing—shovelfuls of bread, celery, mushrooms, and other ingredients that complement all of that turkey protein.

Some people don’t say “stuffing,” though. They say “dressing.” In these calamitous times, knowing how to properly refer to the giant glob of insulin-spiking bread seems necessary. So what's the difference?

Let’s dismiss one theory off the bat: Dressing and stuffing do not correlate with how the side dish is prepared. A turkey can be stuffed with dressing, and stuffing can be served in a casserole dish. Whether it’s ever seen the inside of a bird is irrelevant, and anyone who tells you otherwise is wrong and should be met with suspicion, if not outright derision.

The terms are actually separated due to regional dialects. “Dressing” seems to be the favored descriptor for southern states like Mississippi, Tennessee, South Carolina, and Georgia, while “stuffing” is preferred by Maine, New York, and other northern areas. (Some parts of Pennsylvania call it "filling," which is a bit too on the nose, but to each their own.)

If “stuffing” stemmed from the common practice of filling a turkey with carbs, why the division? According to The Huffington Post, it may have been because Southerners considered the word “stuffing” impolite, so never embraced it.

While you should experience no material difference in asking for stuffing or dressing, when visiting relatives it might be helpful to keep to their regionally-preferred word to avoid confusion. Enjoy stuffing yourselves.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Big Questions
Why Do the Lions and Cowboys Always Play on Thanksgiving?
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Rey Del Rio/Getty Images

Because it's tradition! But how did this tradition begin?

Every year since 1934, the Detroit Lions have taken the field for a Thanksgiving game, no matter how bad their record has been. It all goes back to when the Lions were still a fairly young franchise. The team started in 1929 in Portsmouth, Ohio, as the Spartans. Portsmouth, while surely a lovely town, wasn't quite big enough to support a pro team in the young NFL. Detroit radio station owner George A. Richards bought the Spartans and moved the team to Detroit in 1934.

Although Richards's new squad was a solid team, they were playing second fiddle in Detroit to the Hank Greenberg-led Tigers, who had gone 101-53 to win the 1934 American League Pennant. In the early weeks of the 1934 season, the biggest crowd the Lions could draw for a game was a relatively paltry 15,000. Desperate for a marketing trick to get Detroit excited about its fledgling football franchise, Richards hit on the idea of playing a game on Thanksgiving. Since Richards's WJR was one of the bigger radio stations in the country, he had considerable clout with his network and convinced NBC to broadcast a Thanksgiving game on 94 stations nationwide.

The move worked brilliantly. The undefeated Chicago Bears rolled into town as defending NFL champions, and since the Lions had only one loss, the winner of the first Thanksgiving game would take the NFL's Western Division. The Lions not only sold out their 26,000-seat stadium, they also had to turn fans away at the gate. Even though the juggernaut Bears won that game, the tradition took hold, and the Lions have been playing on Thanksgiving ever since.

This year, the Lions host the Minnesota Vikings.

HOW 'BOUT THEM COWBOYS?


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The Cowboys, too, jumped on the opportunity to play on Thanksgiving as an extra little bump for their popularity. When the chance to take the field on Thanksgiving arose in 1966, it might not have been a huge benefit for the Cowboys. Sure, the Lions had filled their stadium for their Thanksgiving games, but that was no assurance that Texans would warm to holiday football so quickly.

Cowboys general manager Tex Schramm, though, was something of a marketing genius; among his other achievements was the creation of the Dallas Cowboys Cheerleaders.

Schramm saw the Thanksgiving Day game as a great way to get the team some national publicity even as it struggled under young head coach Tom Landry. Schramm signed the Cowboys up for the game even though the NFL was worried that the fans might just not show up—the league guaranteed the team a certain gate revenue in case nobody bought tickets. But the fans showed up in droves, and the team broke its attendance record as 80,259 crammed into the Cotton Bowl. The Cowboys beat the Cleveland Browns 26-14 that day, and a second Thanksgiving pigskin tradition caught hold. Since 1966, the Cowboys have missed having Thanksgiving games only twice.

Dallas will take on the Los Angeles Chargers on Thursday.

WHAT'S WITH THE NIGHT GAME?


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In 2006, because 6-plus hours of holiday football was not sufficient, the NFL added a third game to the Thanksgiving lineup. This game is not assigned to a specific franchise—this year, the Washington Redskins will welcome the New York Giants.

Re-running this 2008 article a few days before the games is our Thanksgiving tradition.

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