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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 Is the Meaning Behind "420"?
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Whether or not you’re a marijuana enthusiast, you’re probably aware that today is an unofficial holiday for those who are. April 20—4/20—is a day when pot smokers around the world come together to, well, smoke pot. Others use the day to push for legalization, holding marches and rallies.

But why the code 420? There are a lot of theories as to why that particular number was chosen, but most of them are wrong. You may have heard that 420 is police code for possession, or maybe it’s the penal code for marijuana use. Both are false. There is a California Senate Bill 420 that refers to the use of medical marijuana, but the bill was named for the code, not the other way around.

As far as anyone can tell, the phrase started with a bunch of high school students. Back in 1971, a group of kids at San Rafael High School in San Rafael, California, got in the habit of meeting at 4:20 to smoke after school. When they’d see each other in the hallways during the day, their shorthand was “420 Louis,” meaning, “Let’s meet at the Louis Pasteur statue at 4:20 to smoke.”

Somehow, the phrase caught on—and when the Grateful Dead eventually picked it up, "420" spread through the greater community like wildfire. What began as a silly code passed between classes is now a worldwide event for smokers and legalization activists everywhere—not a bad accomplishment for a bunch of high school stoners.

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Why Is a Pineapple Called a Pineapple?
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by James Hunt

Ask an English-speaking person whether they've heard of a pineapple, and you'll probably receive little more than a puzzled look. Surely, every schoolchild has heard of this distinctive tropical fruit—if not in its capacity as produce, then as a dessert ring, or smoothie ingredient, or essential component of a Hawaiian pizza.

But ask an English-speaking person if they've ever heard of the ananas fruit and you'll probably get similarly puzzled looks, but for the opposite reason. The average English speaker has no clue what an ananas is—even though it's the name given to the pineapple in almost every other major global language.

In Arabic, German, French, Dutch, Greek, Hebrew, Hindi, Swedish, Turkish—even in Latin and Esperanto—the pineapple is known as an ananas, give or take local variations in the alphabet and accents. In the languages where it isn't, it's often because the word has been imported from English, such as in the case of the Japanese パイナップル (painappuru) and the Welsh pinafel.

So how is it that English managed to pick the wrong side in this fight so spectacularly? Would not a pineapple, by any other name, taste as weird and tingly?

To figure out where things went wrong for English as a language, we have to go back and look at how Europeans first encountered the fruit in question, which is native to South America. It was first catalogued by Columbus's expedition to Guadeloupe in 1493, and they called it piña de Indes, meaning "pine of the Indians"—not because the plant resembled a pine tree (it doesn't) but because they thought the fruit looked like a pine cone (umm, ... it still doesn't. But you can sort of see it.)

Columbus was on a Spanish mission and, dutifully, the Spanish still use the shortened form piñas to describe the fruit. But almost every other European language (including Portuguese, Columbus's native tongue) decided to stick with the name given to the fruit by the indigenous Tupí people of South America: ananas, which means "excellent fruit."

According to etymological sources, the English word pineapple was first applied to the fruit in 1664, but that didn't end the great pineapple versus ananas debate. Even as late as the 19th century, there are examples of both forms in concurrent use within the English language; for example, in the title of Thomas Baldwin's Short Practical Directions For The Culture Of The Ananas; Or Pine Apple Plant, which was published in 1813.

So given that we knew what both words meant, why didn't English speakers just let go of this illogical and unhelpful linguistic distinction? The ultimate reason may be: We just think our own language is better than everyone else's.

You see, pineapple was already an English word before it was applied to the fruit. First used in 1398, it was originally used to describe what we now call pine cones. Hilariously, the term pine cones wasn't recorded until 1694, suggesting that the application of pineapple to the ananas fruit probably meant that people had to find an alternative to avoid confusion. And while ananas hung around on the periphery of the language for a time, when given a choice between using a local word and a foreign, imported one, the English went with the former so often that the latter essentially died out.

Of course, it's not too late to change our minds. If you want to ask for ananas the next time you order a pizza, give it a try (though we can't say what you'd up with as a result).

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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