in optics, a reflecting surface that forms an image of an object when light rays coming from that object fall upon it (see reflection). Usually mirrors are made of plate glass, one side of which is coated with metal or some special preparation to serve as a reflecting surface. The junction of this reflecting surface and the plate glass is called the mirror line. Highly polished metal and other materials serve also as mirrors; fused quartz is used for applications that require high precision because of its very low thermal expansion. Three common types of mirror are the plane mirror, which has a flat, or plane, surface; the convex mirror; and the concave mirror.
In a plane mirror the rays of light falling on it are reflected with little change in their original character and their relationship to one another in space. The apparent position of the image is the same distance behind the mirror as the actual object is in front of the mirror; the image is the same size as the object and is called a virtual image (i.e., the rays of light from the object do not actually go to the image, but extensions of the reflected light rays appear to intersect behind the mirror).
Convex and concave mirrors are known collectively as spherical mirrors, since their curved reflecting surfaces are usually part of the surface of a sphere. The concave type is one in which the midpoint or vertex of the reflecting surface is farther away from the object than are the edges. The center of the imaginary sphere of which it is a part is called the center of curvature and each point of the mirror surface is, therefore, equidistant from this point. A line extending through the center of curvature and the vertex of the mirror is the principal axis, and rays parallel to it are all reflected in such a way that they meet at a point on it lying halfway between the center of curvature and the vertex. This point is called the principal focus.
The size, nature, and position of an image formed by a concave spherical mirror depend on the position of the object in relation to the principal focus and the center of curvature. If the object is at a point farther from the mirror than the center of curvature, the image is real (i.e., it is formed directly by the reflected rays), inverted, and smaller than the object. If the object is at the center of curvature, the image is the same size as the object and is real and inverted. If the object is between the center of curvature and the principal focus, the image is larger, real, and inverted. If the object is inside the principal focus, the image is virtual, erect (right side up), and larger than the object. The position of the object can be found from the equation relating the focal length f of the mirror (the distance from the mirror to the principal focus), the distance do of the object from the mirror, and the distance di of the image from the mirror: 1/f=1/do+1/di. In the case of the virtual image, this equation yields a negative image distance, indicating that the image is behind the mirror. In the case of both the real and the virtual image, the size of the image is to the size of the object as the distance of the image from the mirror is to the distance of the object from the mirror.
In a convex spherical mirror the vertex of the mirror is nearer to the object than the edges—the mirror bulges toward the object. The image formed by it is always smaller than the object and always erect. It is never real because the reflected rays diverge outward from the face of the mirror and are not brought to a focus, and the image, therefore, is determined by their prolongation behind the mirror as in the case of the plane mirror.
The mirror of the ancient Greeks and Romans was a disk of metal with a highly polished face, sometimes with a design on the back, and usually with a handle. Glass mirrors date from the Middle Ages. They were made in large quantities in Venice from the 16th cent., the back being covered with a thin coating of tin mixed with mercury; after 1840 a thin coating of silver was generally substituted. The introduction of plate glass for mirrors (17th cent.) stimulated the use of large stationary mirrors as part of household furniture. Small bits of silvered glass were much used in the East to adorn articles of dress and of decoration. The metal trench hand mirror of World War I revived the manufacture of mirrors of this type. More recently, aluminum was introduced as the reflecting material because it is almost as efficient as silver but is more resistant to oxidation. Mirrors play an important part in the modern astronomical telescope.
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