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A one-way mirror, one-way glass, or two-way glass is a mirror that is partially reflective and partially transparent. When one side of the mirror is brightly lit and the other is dark, it allows viewing from the darkened side but not vice versa.
The glass is coated with, or has encased within, a thin and almost-transparent layer of metal (usually aluminium). The result is a mirrored surface that reflects some light and is penetrated by the rest.
A true one way mirror does not, and cannot, exist. Light always passes exactly equally in both directions. However, when one side is brightly lit and the other kept dark, the darker side becomes difficult to see from the brightly lit side because it is masked by the much brighter reflection of the lit side.
A one-way mirror is typically used as an apparently normal mirror in a brightly lit room, with a much darker room on the other side. People on the brightly lit side see their own reflection—it looks like a normal mirror. People on the dark side see through it—it looks like a transparent window. The light from the bright room reflected from the mirror back into the room itself is much greater than the light transmitted from the dark room, overwhelming the small amount of light transmitted from the dark to the bright room; conversely, the light reflected back into the dark side is overwhelmed by the light transmitted from the bright side. This allows a viewer in the dark side to observe the bright room covertly.
When such mirrors are used for one-way observation, the viewing room is kept dark by a darkened curtain or a double door vestibule. These observation rooms have been used in:
Smaller versions are sometimes used in:
There is nowadays a trend for use of glass and even foil based setups of such mirrors in showcase, arts and staging setups, somewhat comparable to Pepper's ghost, in very large dimensions. The second image blended to the scenery in these cases is then either a flat screen or a projection from one or more video beamers raising the illusion that an image is there in free air somewhat behind the mirroring surface. That technique was used at a few public locations (e.g., for creating special effects in live fashion shows). Sometimes the term holographic display stands very close even though that sort of device is a totally different category. The image is indeed as flat as the projection that it is based upon. One such commercially available entertainment product is the Musion Eyeliner.
The same type of mirror, when used in an optical instrument, is called a half-silvered mirror or beam splitter. Its purpose is quite different: to split a beam of light so that part, usually about half, passes straight through, while the other part is reflected. In a typical scientific application the two resulting beams are made to interfere after traversing different paths. An unusual single-lens reflex camera used a half-silvered mirror to create an image of the scene both in the film plane and in the viewfinder.
The popular psychological research of Obedience by Milgram contained an experiment covertly observing, through a one-way mirror, where the subjects showed different levels of obedience.