Editing Optical coating (section)

===Antireflection coatings===
{{main|Anti-reflective coating}}
[[Image:Anti-reflective coating comparison.jpg|thumb|right|150px|Comparison of uncoated glasses (top) and glasses with an anti-reflective coating (bottom).]]
Antireflection coatings are used to reduce reflection from surfaces. Whenever a [[ray (optics)|ray]] of light moves from one [[Medium (optics)|medium]] to another (such as when light enters a sheet of [[glass]] after travelling through [[air]]), some portion of the light is reflected from the surface (known as the ''interface'') between the two media.

A number of different effects are used to reduce reflection. The simplest is to use a thin layer of material at the interface, with an index of refraction between those of the two media. The reflection is minimized when 
:<math>n_1 = \sqrt{n_0 n_S}</math>,
where <math>n_1</math> is the index of the thin layer, and <math>n_0</math> and <math>n_S</math> are the indices of the two media. The optimum refractive indices for multiple coating layers at angles of incidence other than 0° is given by Moreno et al. (2005).<ref>{{cite web|url=http://planck.reduaz.mx/~imoreno/Publicaciones/OptLett2005.pdf|title=''Thin-film spatial filters''|format=PDF|access-date=2007-05-30|archive-url=https://web.archive.org/web/20090219185627/http://planck.reduaz.mx/~imoreno/Publicaciones/OptLett2005.pdf|archive-date=2009-02-19|url-status=dead}}</ref>

Such coatings can reduce the reflection for ordinary glass from about 4% per surface to around 2%. These were the first type of antireflection coating known, having been discovered by [[John William Strutt, 3rd Baron Rayleigh|Lord Rayleigh]] in 1886. He found that old, slightly tarnished pieces of glass transmitted more light than new, clean pieces due to this effect.

Practical antireflection coatings rely on an intermediate layer not only for its direct reduction of reflection coefficient, but also use the [[Interference (wave propagation)|interference]] effect of a thin layer. If the layer's thickness is controlled precisely such that it is exactly one-quarter of the wavelength of the light in the layer (a ''quarter-wave coating''), the reflections from the front and back sides of the thin layer will destructively interfere and cancel each other.

[[Image:Optical-coating-2.svg|right|thumb|Interference in a quarter-wave antireflection coating]]
In practice, the performance of a simple one-layer interference coating is limited by the fact that the reflections only exactly cancel for one wavelength of light at one angle, and by difficulties finding suitable materials. For ordinary glass (''n''≈1.5), the optimum coating index is ''n''≈1.23. Few useful substances have the required refractive index.  [[Magnesium fluoride]] (MgF<sub>2</sub>) is often used, since it is hard-wearing and can be easily applied to substrates using [[physical vapor deposition|physical vapour deposition]], even though its index is higher than desirable (n=1.38). With such coatings, reflection as low as 1% can be achieved on common glass, and better results can be obtained on higher index media.

Further reduction is possible by using multiple coating layers, designed such that reflections from the surfaces undergo maximum destructive interference. By using two or more layers, broadband antireflection coatings which cover the visible range (400-700&nbsp;nm) with maximum reflectivities of less than 0.5% are commonly achievable. Reflection in narrower wavelength bands can be as low as 0.1%. Alternatively, a series of layers with small differences in refractive index can be used to create a broadband antireflective coating by means of a [[gradient-index optics|refractive index gradient]].