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Excerpt from Field Guide to Spectroscopy
A mirror is an optical component that has a highly reflective surface. The surface can be on the top of some substrate, like glass. This is a front surface mirror. On a rear surface mirror, the reflecting surface can also be on the back of a transparent substrate. For visible-light applications, the reflecting surface can be aluminum or silver. Infrared mirrors use a gold reflecting surface. Like lenses, many front surface mirrors have a thin coating to protect the reflecting surface.
A flat mirror, or plane mirror, is used to change the direction of a beam of light but not its divergence. A spherical mirror can take a collimated beam of light and focus the light at a point, called the focal point, F. The distance between the mirror and the focal point is called the focal length f of the mirror. For a spherical mirror, the focal length is half of the radius of curvature, R, of the mirror surface.
A source of light located at the focal point produces a collimated beam of light after the rays reflect off the spherical mirror surface. An off-axis spherical mirror can be used so that the beam path of the collimated light is not blocked by the source.
An aspherical mirror can be either parabolic or hyperbolic. Like a spherical mirror, a parabolic mirror forms a collimated beam of light if a source is located at its focus. A hyperbolic mirror focuses light from one focal point to the other focal point.