A mirror is a surface that reflects light in a way that preserves much of its original direction. This is different from other surfaces that scatter light in many directions. Mirrors reflect visible light well, but they can also reflect other types of electromagnetic radiation, such as X-rays and ultraviolet radiation. The reflection of X-rays by mirrors was first observed in 1873 by Ernst Abbe and Otto Schott.
The most familiar type of mirror is the plane mirror, which has a flat surface. Curved mirrors are also used, to produce magnified or diminished images or focus light or other waves. Mirrors are commonly used for personal grooming (shaving, make-up), decoration, and architecture (mirrored glass). They are also used in scientific apparatus such as telescopes and lasers, for industrial inspection (e.g., of welds), and for medical applications such as endoscopes.
Light waves change direction when they reflect off a mirror—they bounce off the surface at an angle equal to their angle of incidence (the angle at which the waves hit the surface). The angle between the reflected wave and the incident wave is known as the angle of reflection. The law of reflection states that this angle is always equal to the angle of incidence; i.e., θi = θr, where θi is theangle of incidence and θr is the angleof reflection (see Figure 1).
FIGURE 1 Anglesofincidenceandreflectionforplanemirror.(a)Forincominglightparalleltosurface(b)Forincominglightperpendiculartosurface
The distance between an object and its image in a mirror can be calculated using geometry (see Figure 2). If we know the distance d between the object and the mirror (measured along a line perpendicular to the surface of the mirror), and we know h—the height ofthe object above or belowthe midpoint Mof themirror’ssurface—then wecancalculatethe distance Hbetweenits imageandMusingth equation: H=d^2/4h . This relationshipisknownasthe MirrorFormula .
FIGURE 2 Geometryusedtocalculatedistancebetweentheobjectanditsimageinthemirror.(a)Objectabovethemidpointofthesurface(b)Objectbelowthemidpointofthesurface