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How Do Two-Way Mirrors Work?

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A two-way mirror in the film The Cabin In The Woods. Photo Courtesy Lionsgate Entertainment.

It’s a familiar scene from every police procedural: In one brightly lit room, a perp is being questioned. In an adjacent room, officers watch the proceedings in near darkness, downing cup after cup of coffee. Between the rooms is a two-way mirror, which allows the officers to watch the suspect’s questioning without being seen. But how is that even possible?

Traditional vs. Two-Way

Traditional mirrors are created using a process called silvering, in which a coating of a reflective material (such as silver, tin or nickel) is applied to the back of a pane of glass. After a layer of copper is added to prevent oxidation of the metal, a layer of paint is applied. It serves two purposes: to protect the reflective coating, and to ensure that all light is reflected forward to the person standing in front of the mirror—which means that it’s impossible to look through a regular mirror.

The trick of the two-way mirror is accomplished through manufacturing and lighting. To make this type of mirror (which are also sometimes referred to as one-way mirrors), a thin layer of metal—usually aluminum—is applied to the front of a pane of glass. The layer is so thin that only half of the light that hits it is reflected back; the rest goes through the pane.

Let There Be Light

In order for the mirror to work properly, one side—the interrogation room, for example—must be very bright, while the other side—the police observation room—must be dark. The bright light in the interrogation room reflects back off the mirror’s surface; all a criminal sees when he looks at it is his own reflection. The observation room, meanwhile, is kept dark so that very little light is transmitted into the interrogation room. The large amount of light coming from the criminal’s side is what allows the detectives to observe his activity as if they were looking through a regular tinted window. Make the light levels the same in both rooms, however—either by turning the lights in the observation room on or switching the lights in the interrogation room off—and the people in each room will be able to see into the other.

There are many uses for two-way mirrors besides interrogation rooms, including teleprompters, scientific and marketing research, security cameras, and to create various stage effects.

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Big Questions
Why Do Baseball Managers Wear Uniforms?
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Basketball and hockey coaches wear business suits on the sidelines. Football coaches wear team-branded shirts and jackets and often ill-fitting pleated khakis. Why are baseball managers the only guys who wear the same outfit as their players?

According to John Thorn, the official historian of Major League Baseball since 2011, it goes back to the earliest days of the game. Back then, the person known as the manager was the business manager: the guy who kept the books in order and the road trips on schedule. Meanwhile, the guy we call the manager today, the one who arranges the roster and decides when to pull a pitcher, was known as the captain. In addition to managing the team on the field, he was usually also on the team as a player. For many years, the “manager” wore a player’s uniform simply because he was a player. There were also a few captains who didn’t play for the team and stuck to making decisions in the dugout, and they usually wore suits.

With the passing of time, it became less common for the captain to play, and on most teams they took on strictly managerial roles. Instead of suits proliferating throughout America’s dugouts, though, non-playing captains largely hung on to the tradition of wearing a player's uniform. By the early to mid 20th century, wearing the uniform was the norm for managers, with a few notable exceptions. The Philadelphia Athletics’s Connie Mack and the Brooklyn Dodgers’s Burt Shotton continued to wear suits and ties to games long after it fell out of favor (though Shotton sometimes liked to layer a team jacket on top of his street clothes). Once those two retired, it’s been uniforms as far as the eye can see.

The adherence to the uniform among managers in the second half of the 20th century leads some people to think that MLB mandates it, but a look through the official major league rules [PDF] doesn’t turn up much on a manager’s dress. Rule 1.11(a) (1) says that “All players on a team shall wear uniforms identical in color, trim and style, and all players’ uniforms shall include minimal six-inch numbers on their backs" and rule 2.00 states that a coach is a "team member in uniform appointed by the manager to perform such duties as the manager may designate, such as but not limited to acting as base coach."

While Rule 2.00 gives a rundown of the manager’s role and some rules that apply to them, it doesn’t specify that they’re uniformed. Further down, Rule 3.15 says that "No person shall be allowed on the playing field during a game except players and coaches in uniform, managers, news photographers authorized by the home team, umpires, officers of the law in uniform and watchmen or other employees of the home club." Again, nothing about the managers being uniformed.

All that said, Rule 2.00 defines the bench or dugout as “the seating facilities reserved for players, substitutes and other team members in uniform when they are not actively engaged on the playing field," and makes no exceptions for managers or anyone else. While the managers’ duds are never addressed anywhere else, this definition does seem to necessitate, in a roundabout way, that managers wear a uniform—at least if they want to have access to the dugout. And, really, where else would they sit?

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
How Long Could a Person Survive With an Unlimited Supply of Water, But No Food at All?
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How long could a person survive if he had unlimited supply of water, but no food at all?

Richard Lee Fulgham:

I happen to know the answer because I have studied starvation, its course, and its utility in committing a painless suicide. (No, I’m not suicidal.)

A healthy human being can live approximately 45 to 65 days without food of any kind, so long as he or she keeps hydrated.

You could survive without any severe symptoms [for] about 30 to 35 days, but after that you would probably experience skin rashes, diarrhea, and of course substantial weight loss.

The body—as you must know—begins eating itself, beginning with adipose tissue (i.e. fat) and next the muscle tissue.

Google Mahatma Gandhi, who starved himself almost to death during 14 voluntary hunger strikes to bring attention to India’s independence movement.

Strangely, there is much evidence that starvation is a painless way to die. In fact, you experience a wonderful euphoria when the body realizes it is about to die. Whether this is a divine gift or merely secretions of the brain is not known.

Of course, the picture is not so pretty for all reports. Some victims of starvation have experienced extreme irritability, unbearably itchy skin rashes, unceasing diarrhea, painful swallowing, and edema.

In most cases, death comes when the organs begin to shut down after six to nine weeks. Usually the heart simply stops.

(Here is a detailed medical report of the longest known fast: 382 days.)

This post originally appeared on Quora. Click here to view.

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