How Does a Two-Way Mirror Work?

Composition

When creating a mirror of any kind, the glass being used must have two separate surfaces that can be tinted (a process known as silvering). In common everyday mirrors, the second surface is covered with a reflective metal like silver, tin or nickel and then painted over to fully darken the back of the pane of glass. This allows whoever is using the mirror to only see through that first layer because that is where all the light is being reflected through. The difference when putting together a two-way mirror comes when the first surface of the glass pane is covered with a thin layer of metal or reflective acrylic. From the front, only an ordinary, reflected image would be seen, but the reflective surface now has light breaking through. A person observing through the back of the mirror is then able to see images coming from the front, like a tinted window.

Conditions

A two-way mirror does not act as a mirror on one side and a window on the other. Each side is really letting half the light through and reflecting the other half. For a two-way mirror to work properly there are certain lighting requirements. The mirror should be placed between one room that is bright with light and another that is fairly dim. The person in the brightly lit room will experience reflectivity that brightens the room considerably, while the little light that is coming from the other room only weakens. This makes the image in the darkened room insufficient enough for the eye to see. If the person in the illuminated room turns off the lights and shines light through the mirror, he should be able to see the second room or signs of defined shapes. A two-way mirror has to be well preserved since any scratches on the surface will allow light to invade and ruin the effect.

Reasons

Whether people realize it or not, two-way mirrors are used in many everyday situations and places they frequent. A misconception is that they are solely used for police interrogations, but that's just one of the many uses. Two-way mirrors can be found in grocery stores, banks and post offices. They are used in observing focus groups, childhood education or daycare facilities, to reflect scripts off of teleprompters in broadcast television, to protect concealed cameras and to provide security in public venues.