Mirrors

A mirror is a device whose surface has good specular reflection; that is, it is smooth enough to form an image. The most familiar type of mirror is the plane mirror, which has a flat surface. Curved mirrors are also used, to produce magnified or demagnified images. more...

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The most common use of mirrors is in the home for personal grooming, but mirrors are also used in scientific apparatus such as telescopes and lasers, and in industrial machinery. Most mirrors are designed for visible light, however, mirrors designed for other wavelengths of electromagnetic radiation are also used, especially in optical instruments.

Effects

In a plane mirror, a parallel beam of light changes its direction as a whole, while still remaining parallel; the images formed by a plane mirror are virtual images, of the same size as the original object (see mirror image). 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 rays appearing to diverge from a common intersection "behind" the mirror. Spherical concave and convex mirrors do not focus parallel rays to a single point due to spherical aberration. (The ideal of focusing to a point is a commonly-used approximation, however.) Parabolic reflectors resolve this, allowing incoming parallel rays (for example, light from a distant star) to be focused to a small spot; almost an ideal point. Parabolic reflectors are not suitable for imaging nearby objects, however.

A beam of light reflects off 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.

Image in a mirror

Composition

Early mirrors were usually a sheet of polished metal, often silver or copper, for example the Aranmula kannadi.

Most modern mirrors consist of a thin layer of aluminium deposited on a sheet of glass. They are 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. This type of mirror reflects about 80% of the incident light. The "back side" of the mirror is often painted black to completely seal the metal from corrosion.

Mirrors for precision optical applications are more likely to have the reflective coating on the front surface of the mirror, to improve the image quality. Metal films on the front surface are generally covered with a thin, transparent coating (often silica) to protect them from corrosion. In some cases, this coating may be designed to enhance the reflectivity as well.

Read more at Wikipedia.org