The use of plane or curved reflecting surfaces for the purpose of reverting, directing, or forming images. An optical surface which specularly reflects the largest fraction of the incident light is called a reflecting surface. Such surfaces are commonly fabricated by polishing of glass, metal, or plastic substrates, and then coating the surface of the substrate with a thin layer of metal, which may be covered in addition by a single or multiple layers of thin dielectric films. The law of reflection states that the incident and reflected rays will lie in the plane containing the local normal to the reflecting surface and that the angle of the reflected ray from the normal will be equal to the angle of the incident ray from the normal. See Geometrical optics
The formation of images in the plane mirrors is easily understood by applying the law of reflection. The illustration shows the formation of the image of a point formed by a plane mirror. Each of the reflected rays appears to come from a point image located a distance behind the mirror equal to the distance of the object point in front of the mirror. The face of the observer can be considered as a set of points, each of which is imaged by the plane mirror. Since the observer is viewing the facial image from the object side of the mirror, the face will appear to be reversed left for right in the virtual image formed by the mirror. The illustration also indicates the redirection of light by a plane mirror, in that a viewer who cannot observe the object point directly can observe the virtual image of the point formed by the mirror. A simple optical device which is based on this principle is the simple mirror periscope, which uses two mirrors to permit viewing of scenes around an obstacle.
A curved mirror, either spherical or conic in form, will produce a real or virtual image in much the same manner as a lens, but generally with reduced aberrations. There will be no chromatic aberrations since the law of reflection is independent of the color or wavelength of the incident light. See Aberration (optics), Optical image
Both concave and convex spherical mirrors are commonly encountered. Convex mirrors are commonly used as wide-angle rearview mirrors in automobiles or on trucks. A common application of concave mirrors is the magnifying shaving mirror frequently found in bathrooms.
A spherical mirror will form an image which is not perfect, except for particular conjugate distances. The use of a mirror which has the shape of a rotated conic section, such as a parabola, ellipsoid, or hyperboloid, will form a perfect image for a particular set of object-image conjugate distances and will have reduced aberrations for some range of conjugate relations. The most familiar applications for conic mirrors are in reflecting telescopes. See Optical prism, Optical surfaces, Reflection of electromagnetic radiation, Telescope