A two-way mirror, also called a one-way mirror by some, is often used to provide one-way observation during a police interrogation or camouflage for a surveillance camera. Television teleprompters also use the same technology to reflect the script towards the performer. This gives the illusion that the performer is looking directly at the audience while delivering lines. Some unscrupulous store owners or hotel managers have also been accused of using this technology to illegally spy on their customers or invade a guest's privacy. The use of these mirrors can be very controversial at times.
A pane of glass used for mirrors of any kind has two separate surfaces suitable for the tinting process known as silvering. The first surface is the outermost layer of the front side. The second layer is directly behind the thickness of the glass, or the "front of the back" as it were. In traditional mirrors, a layer of reflective metal such as silver, tin, or nickel is applied to the second layer, often with a second coating of paint to completely darken the back of the mirror pane. All of the light is reflected forward through the glass pane to the observer in front of the mirror. There is no way to see any image when viewed from behind.
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The circumstances change during the creation of a two-way mirror, however. A very thin layer of metal or reflective acrylic is applied to the first surface of the glass pane. An observer looking directly at the mirror from the front would see nothing but a reflected image, no different than a traditional mirror. The reflective surface is so thin, however, that some light penetrates the pane instead of being reflected back to the observer. A person looking through the back of the mirror can see an image as if he or she were looking through a tinted window, which is essentially what he or she is doing.
If that two-way mirror is installed between two rooms, it will look like a large mirror in one room and a large pane of tinted glass in the other. In order to get the maximum benefit, the mirrored room must be significantly brighter than the observation room. The additional light makes it difficult to see through the reflective side and also provides more illumination for the observers on the other side. It is this pronounced difference in lighting levels which make the two-way effect possible. If the lighting circumstances were reversed, a person on the mirrored side could see directly into the other room.
There are several ways to tell if a two-way mirror is present in a room. One way is called the fingernail test. The observer should touch the surface of the suspected mirror with a fingernail and observe the reflection. In a traditional mirror, the two fingernails should not meet directly. Because the second layer of the mirror is silvered, there will be a gap between the object and its reflected image. If there is no such gap, then the first surface is silvered.
A commercial two-way mirror is actually marketed as tinted window glass, so it is generally installed as a window, not framed and hung like a traditional mirror. If the suspected mirror appears permanently installed on a wall instead of housed in a removable frame, it may be two-way. Turning off all the lights in the room and shining a bright flashlight through the suspected mirror should reveal the presence of a hollow space or second room behind it. If the lighting cannot be changed, a person may be able to look through the mirror by forming a shade with his or her hands and looking through the glass for any signs of light or defined shapes.
Some say that this type of mirror makes a different sound when tapped with a finger than a traditional one. Since there is no framing or support behind the mirror's glass, it may sound brighter or sharper when tapped. The use of a two-way surveillance mirror is legal under many circumstances, but not when there is a reasonable expectation of privacy, such as in hotel rooms, public restrooms, or dressing rooms.