How Do Two-Way Mirrors Work?


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 Does Asparagus Make Your Pee Smell Funny?
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The asparagus has a long and storied history. It was mentioned in the myths and the scholarly writings of ancient Greece, and its cultivation was the subject of a detailed lesson in Cato the Elder's treatise, On Agriculture. But it wasn't until the turn of the 18th century that discussion of the link between asparagus and odorous urine emerged. In 1731, John Arbuthnot, physician to Queen Anne, noted in a book about food that asparagus "affects the urine with a foetid smell ... and therefore have been suspected by some physicians as not friendly to the kidneys." Benjamin Franklin also noticed that eating asparagus "shall give our urine a disagreeable odor."

Since then, there has been debate over what is responsible for the stinky pee phenomenon. Polish chemist and doctor Marceli Nencki identified a compound called methanethiol as the cause in 1891, after a study that involved four men eating about three and a half pounds of asparagus apiece. In 1975, Robert H. White, a chemist at the University of California at San Diego, used gas chromatography to pin down several compounds known as S-methyl thioesters as the culprits. Other researchers have blamed various "sulfur-containing compounds" and, simply, "metabolites."

More recently, a study demonstrated that asparagusic acid taken orally by subjects known to produce stinky asparagus pee produced odorous urine, which contained the same volatile compounds found in their asparagus-induced odorous urine. Other subjects, who normally didn't experience asparagus-induced odorous urine, likewise were spared stinky pee after taking asparagusic acid.

The researchers concluded that asparagusic acid and its derivatives are the precursors of urinary odor (compared, in different scientific papers, to the smell of "rotten cabbage," "boiling cabbage" and "vegetable soup"). The various compounds that contribute to the distinct smell—and were sometimes blamed as the sole cause in the past—are metabolized from asparagusic acid.

Exactly how these compounds are produced as we digest asparagus remains unclear, so let's turn to an equally compelling, but more answerable question:

WHY DOESN'T ASPARAGUS MAKE YOUR PEE SMELL FUNNY?

Remember when I said that some people don't produce stinky asparagus pee? Several studies have shown that only some of us experience stinky pee (ranging from 20 to 40 percent of the subjects taking part in the study, depending on which paper you read), while the majority have never had the pleasure.

For a while, the world was divided into those whose pee stank after eating asparagus and those whose didn't. Then in 1980, a study complicated matters: Subjects whose pee stank sniffed the urine of subjects whose pee didn't. Guess what? The pee stank. It turns out we're not only divided by the ability to produce odorous asparagus pee, but the ability to smell it.

An anosmia—an inability to perceive a smell—keeps certain people from smelling the compounds that make up even the most offensive asparagus pee, and like the stinky pee non-producers, they're in the majority.

Producing and perceiving asparagus pee don't go hand-in-hand, either. The 1980 study found that some people who don't produce stinky pee could detect the rotten cabbage smell in another person's urine. On the flip side, some stink producers aren't able to pick up the scent in their own urine or the urine of others.

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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What Legal Authority Does Judge Judy Have?
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While Judith Sheindlin was a real, live judge—New York City Mayor Ed Koch appointed her to family court in 1982 and then made her Manhattan's supervising family court judge in 1986—she's not acting as one on her show. Neither are any of the other daytime TV judges (whether they passed the bar and served as actual judges or not).

TV court shows don't take place in real courtrooms and they don't feature real trials, though they are usually real cases—the producers often contact parties who have pending litigation in small claims court and offer them the opportunity to appear on TV instead. What you're seeing on these TV court shows is really just arbitration playing dress-up in small claims court's clothes.

Arbitration is a legal method for resolving disputes outside the court. The disputing parties present their cases to a neutral, third-party arbitrator or arbitrators who hear the case, examine the evidence, and make a (usually binding) decision. Like a court-based case, arbitration is adversarial, but generally less formal in its rules and procedures.

The power that Judge Judy and the rest of the TV arbitrators have over the disputing parties is granted by a contract, specific to their case, that they sign before appearing on the show. These contracts make the arbitrators' decision final and binding, prevent the disputing parties from negotiating the terms of the arbitration, and allow the "judges" wide discretion on procedural and evidentiary rules during the arbitration.

TV judges make their decision on the case and either decide for the plaintiff, in which case the show's producers award them a judgment fee, or with the defendant, in which case the producers award both parties with an appearance fee. This system seems to skew things in favor of the defendants and gives them an incentive to take their case from court to TV. If they have a weak case, appearing on the show absolves them of any financial liability; if they have a strong case, they stand to earn an appearance fee along with their victory.

If one party or the other doesn't like the arbitrator's decision, it can really only be successfully appealed if it addresses a matter outside the scope of the contract. In 2000, Judge Judy had one of her decisions overturned for that reason by the Family Court of Kings County. In the case B.M. v. D.L., the parties appeared in front of Sheindlin to solve a personal property dispute. Sheindlin ruled on that dispute, but also made a decision on the parties' child custody and visitation rights. One of the parties appealed in court, and the family court overturned the custody and visitation part of the decision because they weren't covered by the agreement to arbitrate.

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

This post originally appeared in 2012.

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