A semi-reflective mirror is a type of Miroir whose characteristic is to reflect only part of the Lumière that it receives, and to let pass the other part. In other words, it separates a incidental ray in two luminous flows, one considered, the other refracted (the part diffused, of smaller quantity, being Négligeable).

Because the mirror is not the only technical possibility to obtain this optical characteristic (an optical system composed of prisms can achieve the same goal), one will call the semi-reflective face indifferently separating blade or separating or dividing of beam (or of ray),

Parameters

The distribution of the Luminous intensity between these two flows depends:

• of the optical System chosen,
• of the Color of the light, i.e. of its Wavelength,
• of the Angle which the luminous ray with the surface of the mirror makes,
• in the case of a mirror covered with a thin Layer, thickness of the selected metal deposit which can filter certain wavelengths of the incidental ray.

Various means of obtaining this separating blade

Three types of these mirrors exist:

• Two prism S triangular stuck with Balsam of Canada and forming a Cube makes it possible to think partially the light of the level of their junction. Some of these prisms like the Prism of Wollaston, made out of birefringent materials , give two rays of different polarizations (they are then Polariseur S).
• Another method consists in depositing a fine layer of a compound Métal lic or Diélectrique on the surface of a blade of Verre. One used a long time tin, mercury, money by chemical reaction. In the current technique of vacuum Deposit, it is the Aluminum which is applied. This technique of control depth of the deposit takes place with a precision such (near the luminous wavelength, that is to say only a few tens of Atome S) which it makes it possible to choose the transmitted or considered colors (anti-reflecting glass, analyzes by color).
• a third method consists in using a dichroic Prisme.

Uses

In Physical

Interferometry

The semi-reflective mirrors are very important in Interférométrie. This technique indeed requires to make interfere two rays resulting from the even source. This is why these mirrors play a central role there. One finds them in particular in the Interféromètre of Michelson and the Interféromètre of Fabry-Pérot.

Others

Other experiments of physics require this type of mirror. For example, if one wants to study two characteristics of a beam, one can separate it into two thanks to one from these mirrors, then to study these two rays separately.

In the everyday life

This type of mirror is used in the everyday life under the designation of mirror without silvering or two-way mirror (by historical reference to the Tain). It is about a semi-reflective mirror separating two contiguous parts, one enlightened, the other sinks:

• the people located in the enlightened part see their reflection on this surface, as on a normal mirror, and cannot thus see the people in the other part;
• the people located in the dark part see through this surface as through a transparent Vitre , and can thus see the people in the other part.

It thus allows the people who are in the dark part “to see without being considering”.

Uses

• the mirrors without silvering are used in the police stations of police force:

• in rooms arranged especially to allow the Pilot S to identify the Suspect S without fearing reprisals,
• in the rooms of interrogation, to allow other people to observe the exchange without distracting the questioned person.
• One as finds of it in the Commerce S in order to supervise as the customers Vol ent nothing, or to study their practices of Consommation and to draw the conclusions from them to improve the Marketing. In the same order of idea, they are also used in the meetings of consumers, so that the silent partners of the study can assist to with it discreetly.
• the mirrors without silvering find also an application in the Prompteur S, in order to be able to film through a surface reflecting a text located on other side of surface.
• One places also a mirror without silvering in the interior rear view mirrors of the Véhicule S, in order to limit the dazzling by the Phare S of the prosecuting vehicles, the night.

A Caméra can be placed behind such a mirror, in order to film individuals without their knowledge.

Works

• In an architectural project entitled Cinema , the American contemporary artist daN Graham imagines a Cinema whose screen and walls, which are mirrors without silvering, form a Triangle. When the spectators, who are inside, view film, the room is plunged in the black, and the passers by, which are outside, can see:

• not only the film projected on the screen (the image being however reversed),
• but also spectators looking at film, without the latter not being able to see them: they can thus peacefully observe this relation film/spectator.
• the Mirror without silvering is a Romance published in 1947 by Pierre Humbourg.
• the Mirror without silvering is a novel published in 1964 by Guarini Ferida.
• the Mirror without silvering is a novel published in 1991 by Michelle Meyer.
• a mirror without silvering is a novel published in 1995 by Jean Chèvre.
• Mirror without silvering, Tiroir without hand , whose title plays on a Contrepétrie, is a novel of Franck Zerbib (member of the group Wriggles) which is still being prepared in 2007.
• Behind the mirror without silvering is a pornographic Film carried out in 1982 by Jean-Claude Roy, in which in particular Cathy Menard plays.

Limits with this device

In the device describes above, it is enough to light the dark part so that the mirror without silvering becomes again a transparent pane. In spite of that, there exist optical systems which let pass the light only in one direction. In fact the insulators of Faraday are usually used with the lasers diode of laboratory. Windows , however. Optical isolator S are one-way devices, that Laser are commonly used with S.) -->

Alternative

Certain applicable adhesive films on a pane allow the constitution of a mirror without silvering (by respecting the difference in illumination of the two zones).

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