A one-way mirror, also called two-way mirror (or one-way glass, half-silvered mirror, and semi-transparent mirror), is a

reciprocal Reciprocal may refer to: In mathematics * Multiplicative inverse, in mathematics, the number 1/''x'', which multiplied by ''x'' gives the product 1, also known as a ''reciprocal'' * Reciprocal polynomial, a polynomial obtained from another pol ...

mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...

that appears reflective on one side and transparent at the other. 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 US 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 usually containing

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

). 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 maximum-security prison, although not always at the same prison where the death r ...

s *

Experimental psychology Experimental psychology refers to work done by those who apply experimental methods to psychological study and the underlying processes. Experimental psychologists employ human participants and animal subjects to study a great many topics, in ...

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 inf ...

rooms *

Market research Market research is an organized effort to gather information about target markets and customers: know about them, starting with who they are. It is an important component of business strategy and a major factor in maintaining competitiveness. Ma ...

Reality television Reality television is a genre of television programming that documents purportedly unscripted real-life situations, often starring unfamiliar people rather than professional actors. Reality television emerged as a distinct genre in the early 1 ...

, 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-German rolling stock and rail transport manufacturer, headquartered in Berlin, Germany. 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) train 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 Transpor ...

Smaller versions are sometimes used in: *

Low-emissivity Low emissivity (low ''e'' or low thermal emissivity) refers to a surface condition that emits low levels of radiant thermal (heat) energy. All materials absorb, reflect, and emit radiant energy according to Planck's law but here, the primary co ...

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 can produce images or recordings for surveillance or other private purposes. Cameras can be either video cameras, or digital stills cameras. Walter Bruch was the inventor of the CCTV camera. The main purpose o ...

s, where the camera is hidden in a mirrored enclosure * Stage effects (particularly

Pepper's ghost Pepper's ghost is an illusion technique used in the theatre, cinema, amusement parks, museums, television, and concerts. It is named after the English scientist John Henry Pepper (1821–1900) who began popularising the effect with a theatr ...

) *

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 be ...

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 illusion *Smart mirror ( virtual mirror) and mirror TV *Arcade video games, such as Taito's ''

Space Invaders is a 1978 shoot 'em up arcade game developed by Tomohiro Nishikado. It was manufactured and sold by Taito in Japan, and licensed to the Midway division of Bally for overseas distribution. ''Space Invaders'' was the first fixed shooter an ...

'' The same type of mirror, when used in an

optical instrument An optical instrument (or "optic" for short) 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, ...

, is called a

beam splitter A beam splitter or ''beamsplitter'' is an 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 interferometers, also finding wid ...

and works on the same principle as a pellicle mirror. 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 n ...

References

External links

* {{DEFAULTSORT:One-Way Mirror Mirrors Security

History

Principle of operation

Applications

See also

References

External links