It’s a familiar scene from just about every police drama on television today: In one brightly lit room, a suspect is being questioned; in an adjacent room, officers watch the interrogation in near darkness. Between the rooms is a two-way mirror that allows the officers to watch the interrogation without being seen. How is that possible? Traditional mirrors are created using a process called “silvering,” in which a coating of tin and then a reflective material — such as silver or aluminum — are applied to the back of a pane of glass. Next comes a layer of copper and two coatings of paint that protect the reflective coating and ensures that all of the light is reflected back at the person looking in the mirror. This makes it impossible to look through a regular mirror. The trick of the two-way mirror is accomplished through manufacturing and lighting. To make a two-way mirror, a thin layer of metal is applied to the front 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. For the mirror to work properly, one side — the interrogation room — must be very bright, while the other side — the observation room — must be dark. The bright light in the interrogation room reflects back off the mirror’s surface, so the suspect sees when they look at it is their own reflection. Make the light levels the same in both rooms and the people in each room will be able to see each other. How can you check for a two-way mirror? Let’s say you’re in a hotel and you want to make sure the mirror in the room is truly a mirror. Simply turn off all the lights, then turn on the light on your phone and shine it at the mirror. If it’s a two-way mirror, you’ll be able to see right through it, which tells you that you need to find another hotel.
