Editing Mirror (section)

=== Technology ===

==== Televisions and projectors ====

Microscopic mirrors are a core element of many of the largest [[HDTV|high-definition]] televisions and [[video projector]]s. A common technology of this type is [[Texas Instruments]]' [[digital light processing|DLP]]. A DLP chip is a postage stamp-sized microchip whose surface is an array of millions of microscopic mirrors. The picture is created as the individual mirrors move to either reflect light toward the projection surface ([[pixel]] on), or toward a light-absorbing surface (pixel off).

Other projection technologies involving mirrors include [[LCoS]]. Like a DLP chip, LCoS is a microchip of similar size, but rather than millions of individual mirrors, there is a single mirror that is actively shielded by a [[liquid crystal]] matrix with up to millions of [[pixels]]. The picture, formed as light, is either reflected toward the projection surface (pixel on), or absorbed by the activated [[LCD]] pixels (pixel off). LCoS-based televisions and projectors often use 3 chips, one for each primary color.

Large mirrors are used in rear-projection televisions. Light (for example from a DLP as discussed above) is "folded" by one or more mirrors so that the television set is compact.

==== Optical discs ====
[[Optical disc]]s are modified mirrors which encode binary data as a series of physical pits and lands on an inner layer between the metal backing and outer plastic surface. The data is read and decoded by observing distortions in a reflected laser beam caused by the physical variations in the inner layer. Optical discs typically use aluminum backing like conventional mirrors, though ones with silver and [[Gold compact disc|gold]] backings also exist.

==== Solar power ====
[[File:Parabolic trough near Harper Lake in California front and back.jpg|thumb|Parabolic troughs near [[Harper Lake]] in [[California]]]]
Mirrors are integral parts of a [[solar power]] plant. The one shown in the adjacent picture uses [[concentrated solar power]] from an array of [[parabolic trough]]s.<ref name=pale2015/>

==== Instruments ====
{{See also|Mirror support cell}}
[[File:E-ELT mirror segments under test.jpg|thumb|[[E-ELT]] mirror segments under test]]
[[Telescope]]s and other precision instruments use ''front silvered'' or [[first surface mirrors]], where the reflecting surface is placed on the front (or first) surface of the glass (this eliminates reflection from glass surface ordinary back mirrors have). Some of them use silver, but most are aluminium, which is more reflective at short wavelengths than silver.
All of these coatings are easily damaged and require special handling.
They reflect 90% to 95% of the incident light when new.
The coatings are typically applied by [[vacuum deposition]].
A protective overcoat is usually applied before the mirror is removed from the vacuum, because the coating otherwise begins to corrode as soon as it is exposed to oxygen and humidity in air. ''Front silvered'' mirrors have to be resurfaced occasionally to maintain their quality. There are optical mirrors such as [[mangin mirror]]s that are ''second surface mirrors'' (reflective coating on the rear surface) as part of their optical designs, usually to correct [[optical aberration]]s.<ref name=boba2014/>

[[File:Super-thin Mirror Under Test at ESO.jpg|thumb|left|Deformable thin-shell mirror. It is 1120 millimetres across but just 2 millimetres thick, making it much thinner than most glass windows.<ref name=eso2013/>]]

The reflectivity of the mirror coating can be measured using a [[Spectrophotometer|reflectometer]] and for a particular metal it will be different for different wavelengths of light. This is exploited in some [[optical]] work to make [[cold mirror]]s and [[hot mirror]]s. A cold mirror is made by using a transparent substrate and choosing a coating material that is more reflective to visible light and more transmissive to [[infrared]] light.

A hot mirror is the opposite, the coating preferentially reflects infrared. Mirror surfaces are sometimes given thin film overcoatings both to retard degradation of the surface and to increase their reflectivity in parts of the spectrum where they will be used. For instance, aluminium mirrors are commonly coated with silicon dioxide or magnesium fluoride. The reflectivity as a function of wavelength depends on both the thickness of the coating and on how it is applied.

[[File:Dielectric laser mirror from a dye laser.JPG|thumb|A dielectric coated mirror used in a [[dye laser]]. The mirror is over 99% reflective at 550 [[nanometer]]s, (yellow), but will allow most other colors to pass through.]]
[[File:Laserr mirror from a dye laser for use with rhodamine.jpg|thumb |A dielectric mirror used in [[tunable laser]]s. With a center wavelength of 600 nm and bandwidth of 100 nm, the coating is totally reflective to the orange construction paper, but only reflects the reddish hues from the blue paper.]]
For scientific [[optics|optical]] work, [[dielectric mirror]]s are often used. These are glass (or sometimes other material) substrates on which one or more layers of dielectric material are deposited, to form an optical coating. By careful choice of the type and thickness of the dielectric layers, the range of wavelengths and amount of light reflected from the mirror can be specified. The best mirrors of this type can reflect &gt;99.999% of the light (in a narrow range of wavelengths) which is incident on the mirror. Such mirrors are often used in [[laser]]s.

In astronomy, [[adaptive optics]] is a technique to measure variable image distortions and adapt a [[deformable mirror]] accordingly on a timescale of milliseconds, to compensate for the distortions.

Although most mirrors are designed to reflect visible light, surfaces reflecting other forms of electromagnetic radiation are also called "mirrors". The mirrors for other ranges of [[electromagnetic waves]] are used in
optics and [[astronomy]]. Mirrors for radio waves (sometimes known as reflectors) are important elements of [[radio telescope]]s.

Simple [[periscope]]s use mirrors.

==== Face-to-face mirrors ====
Two or more mirrors aligned exactly parallel and facing each other can give an infinite regress of reflections, called an [[infinity mirror]] effect. Some devices use this to generate multiple reflections:
* [[Fabry–Pérot interferometer]]
* [[Laser]] (which contains an [[optical cavity]])
* 3D [[kaleidoscope]] to concentrate light<ref name=more2010/>
* momentum-enhanced [[solar sail]]<ref name=meyer1987/>

==== Military applications ====
Tradition states that [[Archimedes]] used a large array of mirrors to burn [[Ancient Rome|Roman]] ships during an attack on Syracuse. This has never been proven or disproved. On the TV show ''[[MythBusters]]'', a team from [[MIT]] tried to recreate the famous "Archimedes Death Ray". They were unsuccessful at starting a fire on a ship.<ref name=myth2019/> Previous attempts to set a boat on fire using only the bronze mirrors available in Archimedes' time were unsuccessful, and the time taken to ignite the craft would have made its use impractical, resulting in the ''MythBusters'' team deeming the myth "busted". It was however found that the mirrors made it very difficult for the passengers of the targeted boat to see; such a scenario could have impeded attackers and have provided the origin of the legend. (See [[solar power tower]] for a practical use of this technique.)

Periscopes were used to great effect in war, especially during the World Wars where they were used to peer over the parapet of trenches to ensure that the soldier using the periscope could see safely without the risk of incoming direct fire from other small arms.

==== Seasonal lighting ====
[[File:Kibble Palace Mirror.JPG|left|thumb|A multi-facet mirror in the [[Kibble Palace]] conservatory, [[Glasgow]], Scotland]]

<!-- If this technique becomes popular, don't let this section grow into a vast list of examples. -->
Due to its location in a steep-sided valley, the Italian town of [[Viganella]] gets no direct sunlight for seven weeks each winter. In 2006 a €100,000 computer-controlled mirror, 8×5&nbsp;m, was installed to reflect sunlight into the town's piazza. In early 2007 the similarly situated village of [[Bondo, Switzerland]], was considering applying this solution as well.<ref name=bbcn2007/><ref name=apsw207/> In 2013, mirrors were installed to reflect sunlight into the town square in the Norwegian town of [[Rjukan]].<ref name=bbcn2013/> Mirrors can be used to produce enhanced lighting effects in greenhouses or conservatories.