Editing Mirror (section)

==== 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.