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.