mirror

Light rays reflected from a regular (plane) mirror. The angle of incidence is the angle between the ray and a perpendicular line drawn to the surface and the angle of reflection is the angle between the reflected ray and a perpendicular to the surface. The image of an object in a plane mirror is described as virtual or imaginary because it appears to be the position from which the rays are formed.

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).

Formula

Materials Materials often used for mirrors in scientific and engineering work are aluminium, copper, molybdenum, and silicon carbide. For accurate reflective mirrors, such as those used in large astronomical telescopes, chemical-vapour-deposited silicon carbide is used with the electrolytic in-process dressing (ELID), a grinding technique first introduced in 1989, which can give a surface accuracy of a few angstroms.

Liquid mirrors Mirrors using, for example, mercury are formed by rotating the liquid so that gravity and centrifugal forces shape it into a perfect parabola. Liquid mirrors 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; in 1995 the largest liquid-mirror telescope (3 m/9.8 ft across) was completed for NASA's Orbital Debris Observatory in New Mexico, USA.