ETYM Old Eng. mirour, French miroir, Old Fren. also mireor, from (assumed) Late Lat. miratorium, from mirare to look at, Latin mirari to wonder. Related to Marvel, Miracle, Mirador.
1. A faithful depiction or reflection.
2. A polished surface that forms images by reflecting light.
Any polished surface that reflects light; often made from “silvered” glass (in practice, a mercury-alloy coating of glass). A plane (flat) mirror produces a same-size, erect “virtual” image located behind the mirror at the same distance from it as the object is in front of it. A spherical concave mirror produces a reduced, inverted real image in front or an enlarged, erect virtual image behind it (as in a shaving mirror), depending on how close the object is to the mirror. A spherical convex mirror produces a reduced, erect virtual image behind it (as in a car’s rear-view mirror).
In a plane mirror the light rays appear to come from behind the mirror but do not actually do so. The inverted real image from a spherical concave mirror is an image in which the rays of light pass through it. The focal length f of a spherical mirror is half the radius of curvature; it is related to the image distance v and object distance u by the equation 1/v + 1/u = 1/f.
Liquid mirrors using, for example mercury, are formed by rotating the liquid so that gravity and centrifugal forces shape it into a perfect parabola. They have a number of advantages over solid mirrors: they do not sag and so can theoretically be made much larger; they are cheaper, and need no polishing. In 1994 a small number of liquid-mirror telescopes had been built for research purposes.