Mirror
"looking glass" redirects here. For other uses, see Looking Glass (disambiguation).
This article is about wave reflectors (mainly, specular reflection of visible light). For other uses, see Mirror (disambiguation).
A mirror, reflecting a vase
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection. This is different from other light-reflecting objects that do not preserve much of the original wave signal other than color and diffuse reflected light. 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 simply distort the reflected image.
Mirrors are commonly used for personal grooming or admiring oneself (in which case the archaic term looking-glass is sometimes still used), decoration, and architecture. Mirrors are also used in scientific apparatus such as telescopes and lasers, cameras, and industrial machinery. Most mirrors are designed for visible light; however, mirrors designed for other types of waves or other wavelengths of electromagnetic radiation are also used, especially in non-optical instruments.
History
The first mirrors used by people were most likely pools of dark, still water, or water collected in a primitive vessel of some sort. The earliest manufactured mirrors were pieces of polished stone such as obsidian, a naturally occurring volcanic glass. Examples of obsidian mirrors found in Anatolia (modern-day Turkey) have been dated to around 6000 BC. Polished stone mirrors from Central and South America date from around 2000 BC onwards.[1] Mirrors of polished copper were crafted in Mesopotamia from 4000 BC,[1] and in ancient Egypt from around 3000 BC.[2] In China, bronze mirrors were manufactured from around 2000 BC,[3] some of the earliest bronze and copper examples being produced by the Qijia culture. Mirrors made of other metal mixtures (alloys) such as copper and tin speculum metal may have also been produced in China and India.[4] Mirrors of speculum metal or any precious metal were hard to produce and were only owned by the wealthy.
Metal-coated glass mirrors are said to have been invented in Sidon (modern-day Lebanon) in the first century AD,[6] and glass mirrors backed with gold leaf are mentioned by the Roman author Pliny in his Natural History, written in about 77 AD.[7] The Romans also developed a technique for creating crude mirrors by coating blown glass with molten lead.
Parabolic mirrors were described and studied in classical antiquity by the mathematician Diocles in his work On Burning Mirrors.[9] Ptolemy conducted a number of experiments with curved polished iron mirrors,[10] and discussed plane, convex spherical, and concave spherical mirrors in his Optics.[11] Parabolic mirrors were also described by the physicist Ibn Sahl in the 10th century,[12] and Ibn al-Haytham discussed concave and convex mirrors in both cylindrical and spherical geometries,[13] carried out a number of experiments with mirrors, and solved the problem of finding the point on a convex mirror at which a ray coming from one point is reflected to another point.[14] By the 11th century, clear glass mirrors were being produced in Moorish Spain.
In China, people began making mirrors with the use of silver-mercury amalgams as early as 500 AD.[16] Some time during the early Renaissance, European manufacturers perfected a superior method of coating glass with a tin-mercury amalgam. The exact date and location of the discovery is unknown, but in the 16th century, Venice, a city famed for its glass-making expertise, became a center of mirror production using this new technique. Glass mirrors from this period were extremely expensive luxuries.[17] The Saint-Gobain factory, founded by royal initiative in France, was an important manufacturer, and Bohemian and German glass, often rather cheaper, was also important.
Manufacturing
Mirrors are manufactured by applying a reflective coating to a suitable substrate. The most common such substrate is glass, due to its transparency, ease of fabrication, rigidity, hardness, and ability to take a smooth finish. The reflective coating is typically applied to the back surface of the glass, so that the reflecting side of the coating is protected from corrosion and accidental damage by the glass on one side and the coating itself and optional paint for further protection on the other.
In classical antiquity, mirrors were made of solid metal (bronze, later silver) and were too expensive for widespread use; they were also prone to corrosion. Due to the low reflectivity of polished metal, these mirrors also gave a darker image than modern ones, making them unsuitable for indoor use with the artificial lighting of the time (candles or lanterns).
The method of making mirrors out of plate glass was invented by 16th-century Venetian glassmakers on the island of Murano, who covered the back of the glass with mercury, obtaining near-perfect and undistorted reflection. For over one hundred years, Venetian mirrors installed in richly decorated frames served as luxury decorations for palaces throughout Europe, but the secret of the mercury process eventually arrived in London and Paris during the 17th century, due to industrial espionage. French workshops succeeded in large scale industrialization of the process, eventually making mirrors affordable to the masses, although mercury's toxicity remained a problem.
In modern times, the mirror substrate is shaped, polished and cleaned, and is then coated. Glass mirrors are most often coated with non-toxic silver[19] or aluminium, implemented by a series of coatings.
Tin(II) chloride
Silver
Chemical activator
Copper
Paint
Types of mirror
There are many types of glass mirrors, each representing a different manufacturing process and reflection type.
An aluminium glass mirror is made of a float glass manufactured using vacuum coating, i.e. aluminium is spread over the glass in the vacuum chamber and then coated with two or more layers of waterproof protective paint. An aluminium glass mirror provides an actual and accurate reflection.
A safety glass mirror is made by sticking a special protective film on the back surface of a silver glass mirror, which prevents injuries in case the mirror is broken. This kind of mirror is used for furniture, doors, glass walls, commercial shelves, or public areas.
A silkscreen printed glass mirror is produced using inorganic color ink that prints patterns through a special screen onto glass. Various colors, patterns, and glass shapes are available. Such a glass mirror is durable and more moisture resistant than ordinary printed glass and can serve for over 20 years. This type of glass is widely used for decorative purposes (e.g., on mirrors, table tops, doors, windows, kitchen chop boards, etc.).
A silver glass mirror is an ordinary mirror, coated on its back surface with silver, that produces images by reflection. This kind of glass mirror is produced by coating a silver, copper film and two or more layers of waterproof paint on the back surface of float glass, which perfectly resists acid and moisture. A silver glass mirror provides clear and actual images, is quite durable, and is widely used for furniture, bathroom and other decorative purposes.[citation needed]
Decorative glass mirrors are usually handcrafted. A variety of shades, shapes and glass thickness are often available.
Shape of a mirror's surface
A beam of light reflects off a mirror at an angle of reflection equal to its angle of incidence (if the size of a mirror is much larger than the wavelength of light). That is, if the beam of light is shining on a mirror's surface at a θ° angle vertically, then it reflects from the point of incidence at a θ° angle from vertically in the opposite direction. This law mathematically follows from the interference of a plane wave on a flat boundary (of much larger size than the wavelength).
In a plane mirror, a parallel beam of light changes its direction as a whole, while still remaining parallel; the images formed by a plane mirror are virtual images, of the same size as the original object
In a concave mirror, parallel beams of light becomes a convergent beam, whose rays intersect in the focus of the mirror.
In a convex mirror, parallel beams become divergent, with the rays appearing to diverge from a common point of intersection "behind" the mirror.
Spherical concave and convex mirrors do not focus parallel rays to a single point due to spherical aberration. However, the ideal of focusing to a point is a commonly-used approximation. Parabolic reflectors resolve this, allowing incoming parallel rays (for example, light from a distant star) to be focused to a small spot; almost an ideal point. Parabolic reflectors are not suitable for imaging nearby objects because the light rays are not parallel.
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