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Mirrors

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Definition: devices which reflect light

German: Spiegel

Category: general optics

How to cite the article; suggest additional literature

A mirror is an optical device which can reflect light. Usually, however, only those devices are meant where the angle of reflection equals the angle of incidence (see Figure 1). This means that diffraction gratings, for example, are not considered as mirrors, although they can also reflect light.

Mirror surfaces do not need to be flat; there are mirrors with a curved reflecting surface (see below).

Properties of a Mirror

Various basic properties characterize a mirror:

mirror

Figure 1: Reflection of light on a mirror.

Additional properties can be relevant in various applications:

Types of Mirrors

Metal-coated Mirrors

Ordinary mirrors as used in households are often silver mirrors on glass. These basically consist of a glass plate with a silver coating on one side. The coating is thick enough to suppress any significant transmission from any side. Nevertheless, the reflectivity is substantially below 100% since there are absorption losses of a few percent (for visible light) in the silver layer. Household mirrors typically have the coating on the inner side, so that one has a robust glass surface outside, which can be cleaned easily. For other applications, the light is often incident directly on the coating and does not reach the glass substrate.

For use in laser technology and general optics, more advanced types of metal-coated mirrors have been developed. These often have additional layers on top of the metallic coating in order to improve the reflectivity and/or to protect the metallic coating against oxidation. Different metals can be used, e.g. gold, silver, copper and nickel/chrome alloys.

The article on metal-coated mirrors gives more details.

Dielectric Mirrors

The most important type of mirror in laser technology and general optics is the dielectric mirror. This kind of mirror contains multiple thin dielectric layers. One exploits the combined effect of reflections at the interfaces between the different layers. A frequently used design is that of a Bragg mirror (quarter-wave mirror), which is the simplest design and leads to the highest reflectivity at a particular wavelength (the Bragg wavelength).

In contrast to some metal-coated mirrors, dielectric mirrors are usually made as front surface mirrors, which means that the reflecting surface is at the front surface, so that the light does not propagate through some transparent substrate before being reflected. That way, not only possible propagation losses in the transparent medium are avoided, but most importantly additional reflections at the front surface, which could be particularly relevant for non-normal incidence.

See the article on dielectric mirrors for more details.

Laser mirrors as used to form laser resonators, for example, are also usually dielectric mirrors, having a particularly high optical quality and often a high optical damage threshold. Also, there are supermirrors with a reflectivity extremely close to 100%, and dispersive mirrors with a systematically varied thin-film thickness.

Curved Mirrors

Most curved mirrors have a spherical surface, characterized by some radius of curvature R. A mirror with a concave (inwards-curved) surface acts a focusing mirror, while a convex surface leads to defocusing behavior. Apart from the change of beam direction, such a mirror acts like a lens. For normal incidence, the focal length (disregarding its sign) is simply R / 2, i.e., half the curvature radius. For non-normal incidence with an angle θ against the normal direction, the focal length is (R / 2) · cos θ in the tangential plane and (R / 2) / cos θ in the sagittal plane.

There are also parabolic mirrors, having a surface with a parabolic shape. For tight focusing, one often uses off-axis parabolic mirrors, which allow one to have the focus well outside the incoming beam.

Dichroic Mirrors

Dichroic mirrors are mirrors which have substantially different reflection properties for two different wavelengths. They are usually dielectric mirrors with a suitable thin-film design.

See also: metal-coated mirrors, dielectric mirrors, Bragg mirrors, quarter-wave mirrors, crystalline mirrors, laser mirrors, output couplers, supermirrors, dichroic mirrors, diffraction gratings, laser-induced damage, focal length, beam splitters
and other articles in the category general optics

In the RP Photonics Buyer's Guide, 155 suppliers for mirrors are listed.

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