Mirror

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A mirror is a reflective surface that is smooth enough to be able to form an image. The best known example is the plane mirror that most people have at home. In it, a parallel beam of light changes its direction as a whole, whilst still remaining parallel; the images formed by a plane mirror are virtual images, of the same size as the original object. There are also concave mirrors, where a parallel beam of light becomes a convergent beam, whose rays intersect in the focus of the mirror. Finally, there are convex mirrors, where a parallel beam becomes divergent, with the apparent intersection ocurring behind the mirror.

Early mirrors consisted of a plate or sheet of polished metal, often silver when the reflected image was for viewing (such as for personal grooming) but also of other metals when only the intensity of reflected light was important.

Modern mirrors usually consist of a thin layer of aluminum (or sometimes other metals) deposited on a sheet of glass. They are usually back silvered, where the reflecting surface is viewed through the glass sheet; this makes the mirror durable, but lowers the image quality of the mirror due to extraneous reflections from the front surface of the glass. Front silvered mirrors, where the reflecting surface is placed on the front surface of the glass, have a better image quality but are easily scratched and damaged. Astronomical mirrors are of the later type, and they have to be resurfaced every now and then to keep their quality.

For scientific optical work, dielectric mirrors 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 >99.999% of the light (in a narrow range of wavelengths) which is incident on the mirror.

A beam of light reflects off of a mirror at an angle of reflection that is equal to its angle of incidence. That is, if the beam of light is shining on a mirror's surface at a 30° angle from vertical, then it reflects from the point of incidence at a 30° angle from vertical in the opposite direction.


On the Internet, a mirror is an exact copy of data stored in a different location. Popular sites use mirrors to reduce network traffic on any one server.