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
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.
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. |
mirror
| Literary term from the late Renaissance, meaning an ideal example for all to follow, or an accurate depiction of a particular subject. It is seen in such titles as A Mirror for Magistrates (1559) and Mirrour of the Blessed Lyf of Iesu Christ (about 1400), and is used by Shakespeare, for example, in his description of Henry V as ‘the mirror of all Christian kings’. |
| In classical Japanese literature, kagami (‘mirror’) is used in the title of many historical works to suggest a way of seeing into the past. Examples are Ōkagami/The Great Mirror (about 1119), chronicling the court and the Fujiwara family during the early Heian period; its approximate sequel Ima kagami/The Mirror of the Present (about 1180); and Mizu kagami/The Water Mirror (second half of the 12th century), dealing with emperors before 850 AD, including the mythical prehistoric ones. |
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