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A one-way mirror, also called two-way mirror (or one-way glass, half-silvered mirror, and semi-transparent mirror), is a reciprocal
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:
*
mirror
A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...
that appears reflective from one side and transparent from the other though this is an illusion and would break the second law of thermodynamics. The perception of one-way transmission is achieved when one side of the mirror is brightly lit and the other side is dark. This allows viewing from the darkened side but not vice versa.
History
The first U.S. patent for a one-way mirror appeared in 1903, then named a "transparent mirror".Principle of operation
The glass is coated with, or has been encased within, a thin and almost transparent layer of metal (window film
A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent m ...
usually containing aluminium
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
). The result is a mirrored surface that reflects some light and is penetrated by the rest. Light always passes 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.
Applications
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:
* Execution chamber
An execution chamber, or death chamber, is a room or chamber in which capital punishment is carried out. Execution chambers are almost always inside the walls of a prison#Security levels, maximum-security prison, although not always at the same p ...
s
* Experimental psychology
Experimental psychology is the work done by those who apply Experiment, experimental methods to psychological study and the underlying processes. Experimental psychologists employ Research participant, human participants and Animal testing, anim ...
research
* Interrogation
Interrogation (also called questioning) is interviewing as commonly employed by law enforcement officers, military personnel, intelligence agencies, organized crime syndicates, and terrorist organizations with the goal of eliciting useful informa ...
rooms
* Market research
Market research is an organized effort to gather information about target markets and customers. It involves understanding who they are and what they need. It is an important component of business strategy and a major factor in maintaining com ...
* Reality television
Reality television is a genre of television programming that documents purportedly unscripted real-life situations, often starring ordinary people rather than professional actors. Reality television emerged as a distinct genre in the early 1990s ...
, as in the series '' Big Brother'', which makes extensive use of one-way mirrors throughout its set to allow cameramen in special black hallways to use movable cameras to film contestants without being seen.
* Security observation decks in public areas
* Train driver or conductor compartments in newer metro trains, such as Bombardier Transportation
Bombardier Transportation was a Canadian rolling stock and rail transport manufacturer, with headquarters in Toronto and Berlin. It was one of the world's largest companies in the rail vehicle and equipment manufacturing and servicing industry. ...
's '' Movia'' family of metro trains, including the Toronto Rocket
The Toronto Rocket (TR) is the fifth and latest series of rolling stock used in the Toronto subway system in Toronto, Ontario, Canada. Owned and operated by the Toronto Transit Commission (TTC), the trains were built by Bombardier Transporta ...
Smaller versions are sometimes used in:
* Low-emissivity windows on vehicles and buildings
* Mobile phone and tablet screen covers, enabling the screen to be used as a mirror when it is off
* Security camera
A closed-circuit television camera is a type of surveillance camera that transmits video signals to a specific set of monitors or video recording devices, rather than broadcasting the video over public airwaves. The term "closed-circuit televisi ...
s, where the camera is hidden in a mirrored enclosure
* Stage effects (particularly Pepper's ghost
Pepper's ghost is an Magic (illusion), illusion technique, used in theatre, Film, cinema, amusement parks, museums, television, and concerts, in which an image of an object offstage is projected so that it appears to be in front of the audience ...
)
* Teleprompter
A teleprompter, also known as an autocue, is a display device that prompts the person speaking with an electronic visual text of a speech or script.
Using a teleprompter is similar to using cue cards. The screen is in front of, and usually bel ...
s, where they allow a presenter to read from text projected onto glass directly in front of a film or television camera
* Common setups of an infinity mirror
The infinity mirror (also sometimes called an infinite mirror) is a configuration of two or more Parallel (geometry), parallel or angled mirrors, which are arranged to create a series of further and further reflections that appear to recede to inf ...
illusion
*Smart mirror ( virtual mirror) and mirror TV
*Arcade video games, such as Taito's ''Space Invaders
is a 1978 shoot 'em up video game developed and published by Taito for Arcade video game, arcades. It was released in Japan in April 1978, with the game being released by Midway Manufacturing overseas. ''Space Invaders'' was the first fixed s ...
''
The same type of mirror, when used in an optical instrument
An optical instrument is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common examples include periscopes, microscopes, telescopes, and camera ...
, is called a beam splitter
A beam splitter or beamsplitter is an optical instrument, optical device that splits a beam of light into a transmitted and a reflected beam. It is a crucial part of many optical experimental and measurement systems, such as Interferometry, int ...
and works on the same principle as a pellicle mirror
A pellicle mirror is an ultra-thin, ultra-lightweight Transparency (optics), semi-transparent mirror employed in the light path of an optical instrument, splitting the light beam into two separate beams, both of reduced light intensity. Splitting t ...
. A partially transparent mirror is also an integral part of the Fabry–Pérot interferometer
In optics, a Fabry–Pérot interferometer (FPI) or etalon is an optical cavity made from two parallel reflecting surfaces (i.e.: thin mirrors). Optical waves can pass through the optical cavity only when they are in resonance with it. It is ...
.
See also
* Optical isolator * See-through graphics *Window film
A window is an opening in a wall, door, roof, or vehicle that allows the exchange of light and may also allow the passage of sound and sometimes air. Modern windows are usually glazed or covered in some other transparent or translucent m ...
References
External links
* {{DEFAULTSORT:One-Way Mirror Mirrors Security