The Science Behind Two-Way Mirrors

Think of a two-way mirror, also known as two-way glass, and you probably think of a police interrogation room. While two-way mirrors don't have many uses in our day-to-day lives, they pop up often in Hollywood movies and detective TV shows, according to How It Works. It makes sense: it's not hard to see how two-way mirrors could be valuable for interrogation purposes.

A standard mirror reflects light in order to reflect the image of the viewer back on themselves. In contrast, as two-way mirrors are used in cop shows, the two-way mirror seems to serve as a standard mirror on only one side. While the criminal only sees a reflection of themselves, the cop can see through to watch the criminal's behavior. But though this might seem to suggest two-way mirrors work very differently than one-way mirrors, in fact, two-way mirrors are formed in much the same way, and their efficacy depends on a host of environmental factors, including, primarily, how much light is present on either side of the mirror (via Indiana Public Media).

How does a two-way mirror work?

Two-way mirrors undergo much the same creation process as one-way mirrors, with only slight modifications to the production, according to Glass. To create a one-way mirror, a metallic, reflective coating is placed behind a piece of glass, according to How It Works. When light hits the metallic coating, it bounces back into the eyes of the viewer, allowing them to see their own image.

In contrast, a two-way mirror uses a much thinner reflective layer, according to Mental Floss. This thinner layer only reflects some of the light, usually around 50% of it, while the rest of the light is absorbed into the mirror. Both sides of the mirror are actually created in the same way, meaning that, from either side, about 50% of any light is absorbed while 50% is reflected. So why can viewers only see through the mirror from one side of the pane?

The importance of lighting

The key to an effective two-way mirror isn't actually in the creation process itself but in the conditions of use. In order for a two-way mirror to work, the rooms being viewed through the mirror must be better lit than the room containing the viewers, according to Indiana Public Media. For instance, one room could be kept in shadow and one could be brightly lit by the overhead light. In the brightly lit room, the mirror will appear to be a genuine mirror, reflecting the image of the observer. From the more shadowy side, the mirror will function as a window, offering insight into the other room. If the rooms are equally lit, the mirror will serve as a window from both sides.

Two-way mirrors aren't used for many purposes, though they are sometimes installed in dance studios as well as police stations, according to Glass. Still, if you ever think you've been put face-to-face with a two-way mirror and want to test it out, try pressing a finger right against the mirror. If there is no gap between where your finger ends and the reflection of your finger begins, you may be dealing with a two-way mirror. If there is a gap, this is probably the genuine, one-way artifact (via How It Works).