Mirror
From Wikipedia, the free encyclopedia
A mirror is an object with a surface that has good specular reflection; that is, it is smooth enough to form an image. The most familiar type of mirror is the plane mirror, which has a flat surface. Curved mirrors are also used, to produce magnified or demagnified images or focus light or simply distort the reflected image.
Mirrors are most commonly used for personal grooming, 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 wavelengths of electromagnetic radiation are also used, especially in optical instruments.
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[edit] Effects
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 (see mirror image). There are also concave mirrors, where a parallel beam of light becomes a convergent beam, whose rays intersect in the focus of the mirror. Lastly, there are convex mirrors, where a parallel beam becomes divergent, with the rays appearing to diverge from a common 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.
A beam of light reflects off a mirror at an angle of reflection that is 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 30° angle from vertical, then it reflects from the point of incidence at a 30° angle from vertical 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).
- See also: Mirror image
[edit] Composition
Early mirrors were often little more than a sheet of polished metal, often silver or copper, for example the Aranmula kannadi. Most modern mirrors consist of a thin layer of aluminium deposited on a sheet of glass. This layer is called the Tain. They are back silvered, where the reflecting surface is viewed through the glass sheet; this makes the mirror durable, but lowers the image quality of the mirror due to extraneous reflections from the front surface of the glass (ordinary glass typically reflects around 4% of the light). This type of mirror reflects about 80% of the incident light. The "back side" of the mirror is often painted or coated in some way to completely seal the metal from corrosion.
Mirrors for precision optical applications are more likely to have the reflective coating on the front surface of the mirror, to eliminate reflection from the glass. Metal films on the front surface are generally covered with a thin, transparent coating to protect them from corrosion. This is often made of silica. In some cases this coating may also enhance reflectivity.
Mirrors designed for special applications, such as in lasers and other advanced optical devices, use a reflective optical coating composed of many layers of different dielectric materials. Such coatings can be designed to have extremely high reflectivity and are reasonably durable. Since they absorb very little of the incident light they can be used with high power lasers without absorbing the energy and being damaged.
[edit] Applications
[edit] Safety and easier viewing
Rear-view mirrors are widely used in and on vehicles (such as automobiles, or bicycles), to allow drivers to see other vehicles coming up behind them. Some motorcycle helmets have a built-in so-called MROS (Multiple Reflective Optic System): a set of reflective surfaces inside the helmet which together function as a rear-view mirror.[1] There exist rear view sunglasses, of which the left end of the left glass and the right end of the right glass work as mirrors.
Convex mirrors are used to provide a wider field of view than a flat mirror. They are sometimes placed at road junctions, and corners of places such as parking lots to allow people to see around corners to avoid crashing into other vehicles or shopping carts. They are also sometimes used as part of security systems, so that a single video camera can show more than one angle at a time.
Corner reflectors use three flat mirrors to reflect light back towards its source. They are used for emergency location, and even laser ranging to the Moon.
With the sun as light source, a mirror can be used to signal, by variations in the orientation of the mirror. The signal can be used over long distances, possibly up to 60 kilometres on a clear day. This technique was used by Native American tribes and numerous militaries to transmit information between distant outposts.
Mirrors can also be used for rescue, especially to attract the attention of search and rescue helicopters. Specialised signalling mirrors are available and are often included in military survival kits.
[edit] Two-way mirrors
A two-way mirror, also sometimes referred to as a one-way mirror or One-way Glass, reflects some percentage of the light and lets some other percentage pass. It is a sheet of glass coated with a layer of metal only a few dozen atoms thick, allowing some of the light through the surface (from both sides). It is used between a dark room and a brightly lit room. People on the brightly lit side see their own reflection - it looks like a normal mirror. People on the dark side see through it - it looks like a transparent window. It may be used to observe criminal suspects or customers (to watch out for theft). The same type of mirror, when used in an optical instrument, is called a half-silvered mirror or beam splitter. Its purpose is to split a beam of light so that half passes straight through, while the other half is reflected — this is useful for interferometry. The reality television programme Big Brother makes extensive use of two-way mirrors throughout its set (known as the "house") to allow cameramen in special black hallways to use movable cameras to videotape contestants without them coming in contact with the workers. This is due to the rule that contestants are not allowed contact with the outside world.
Contrary to popular belief, passive one-way mirrors that operate directionally between equally lit rooms do not exist. The laws of physics do not allow for real, passive one-way mirrors or windows (ones that do not need external energy); if such a device were possible, one could break the second law of thermodynamics and make energy flow from a cold object to a hot one, by placing such a mirror between them. One-way windows can be made to work with polarized light, however, without violating the second law.[1][2] Optical isolators are one-way devices that are commonly used with lasers.
[edit] Technology
[edit] Televisions and projectors
Microscopic mirrors are a core element of many of the largest high-definition televisions and video projectors. A common technology of this type is Texas Instruments' DLP. A DLP chip is a postage stamp-sized microchip whose surface is comprised of an array of millions of microscopic mirrors. The picture is created as the individual mirrors move to either reflect light toward the projection surface (pixel on), or toward a light absorbing surface (pixel off).
Other projection technologies involving mirrors include LCoS. Like a DLP chip, LCoS is a microchip of similar size, but rather than millions of individual mirrors, there is a single mirror that is actively shielded by a liquid crystal matrix with up to millions of pixels. The picture is formed as light is either reflected toward the projection surface (pixel on), or absorbed by the activated LCD pixels (pixel off). LCoS-based televisions and projectors often use 3 chips, one for each primary color.
[edit] Instruments
Telescopes and other precision instruments use front silvered or first surface mirrors, where the reflecting surface is placed on the front (or first) surface of the glass (this eliminates reflection from glass surface ordinary back mirrors have). Some of them use silver, but most are aluminum, which is more reflective at short wavelengths than silver. All of these coatings are easily damaged and require special handling. They reflect 90% to 95% of the incident light when new. The coatings are typically applied by vacuum deposition. A protective overcoat is usually applied before the mirror is removed from the vacuum, because the coating otherwise begins to corrode as soon as it is exposed to oxygen and humidity in the air. Front silvered mirrors have to be resurfaced occasionally to keep their quality.
The reflectivity of the mirror coating can be measured using a reflectometer and for a particular metal it will be different for different wavelengths of light. This is exploited in some optical work to make cold mirrors and hot mirrors. A cold mirror is made by using a transparent substrate and choosing a coating material that is more reflective to visible light and more transmissive to infrared light. A hot mirror is the opposite, the coating preferentially reflects infrared. Mirror surfaces are sometimes given thin film overcoatings both to retard degradation of the surface and to increase their reflectivity in parts of the spectrum where they will be used. For instance, aluminum mirrors are commonly coated with silicon dioxide or magnesium fluoride. The reflectivity as a function of wavelength depends on both the thickness of the coating and on how it is applied.
For scientific optical work, dielectric mirrors are often used. These are glass (or sometimes other material) substrates on which one or more layers of dielectric material are deposited, to form an optical coating. By careful choice of the type and thickness of the dielectric layers, the range of wavelengths and amount of light reflected from the mirror can be specified. The best mirrors of this type can reflect >99.999% of the light (in a narrow range of wavelengths) which is incident on the mirror. Such mirrors are often used in lasers.
In astronomy, adaptive optics is a technique to measure variable image distortions and adapt a deformable mirror accordingly on a timescale of milliseconds, to compensate for the distortions.
Although the most of mirrors are designed to reflect visible light, surfaces reflecting other forms of electromagnetic radiation are also called "mirrors". The mirrors for other ranges of electromagnetic waves are used in optics and astronomy. Mirrors for radio waves are important elements of radio telescopes.
A Mangin mirror is a combination lens and concave mirror and is widely used in optical instruments and even sometimes in cameras.[2] [3][4]
[edit] Military applications
It has been said that Archimedes used a large array of mirrors to burn Roman ships during an attack on Syracuse. This has never been proven or disproved; however, many have put it to the test. Recently, on a popular Discovery Channel show, MythBusters, a team from MIT tried to recreate the famous "Archimedes Death Ray". They were successful at starting a fire on a ship at 75 feet away, however, previous attempts to light the boat on fire using only the bronze mirrors available in Archimedes' time were unsuccessful, and the time taken to ignite the craft would have made its use impractical, resulting in the MythBusters team deeming the myth implausible.
Modern thermonuclear weapons use x-ray mirrors to focus radiation from the primary onto the secondary to create conditions favoring nuclear fusion (see Nuclear weapons design for more details).
[edit] Seasonal lighting
Due to its location in a steep-sided valley, the Italian town of Viganella gets no direct sunlight for 7 weeks each winter. In 2006 a €100,000 computer-controlled mirror, 8 m × 5 m, was installed to reflect sunlight into the town's piazza. In early 2007 the similarly situated village of Bondo, Switzerland, was considering applying this solution as well.[3][4]
[edit] Leisure
[edit] Decoration
Mirrors, typically large and unframed, are frequently used in interior decoration to create an illusion of space, and amplify the apparent size of a room.
Mirrors are used also in some schools of feng shui, an ancient Chinese practice of placement and arrangement of space to achieve harmony with the environment.
The softness of old mirrors is sometimes replicated by contemporary artisans for use in interior design. These reproduction antiqued mirrors are works of art and can bring color and texture to an otherwise hard, cold reflective surface. It is an artistic process that has been attempted by many and perfected by few.
A decorative reflecting sphere of thin metal-coated glass, working as a reducing wide-angle mirror, is sold as a Christmas ornament called a bauble.
[edit] Entertainment
The hall of mirrors, commonly found in amusement parks, is an attraction in which a number of distorted mirrors are used to produce unusual reflections of the visitor. Mirror mazes, also found in amusement parks, contain large numbers of mirrors and sheets of glass. The idea is to navigate the disorientating array without bumping into the walls.
Mirrors are often used in magic to create an illusion. One effect is called Pepper's ghost.
Illuminated rotating disco balls covered with small mirrors are used to cast moving spots of light around a dance floor.
Mirrors are employed in kaleidoscopes, personal entertainment devices invented in Scotland by sir David Brewster.
[edit] Mirror in Art
Filippo Brunelleschi discovered the linear perspective with the help of the mirror, Leonardo called the mirror the "master of painters". He recommended. "When you wish to see whether your whole picture accords with what you have portrayed from nature take a mirror and reflect the actual object in it. Compare what is reflected with your painting and carefully consider whether both likenesses of the subject correspond, particularly in regard to the mirror. The mirror is the central device in some of the greatest of European paintings: Jan Van Eyck's Marriage of Arnolfini, Diego Velazquez's Las Meninas and Edouard Manet’s A Bar at the Folies-Bergère. Without a mirror, the great self portraits by Dürer, Rembrandt, Van Gogh and Frida Kahlo could not have been painted. M.C. Escher used special shape of mirrors in order to have a much more complete view of the surroundings than by direct observation (Hand with Reflecting Sphere). István Orosz’s anamorphic works are images distorted such way that they only become clearly visible when reflected in a suitably-shaped and positioned mirror. Some other contemporary artists use mirrors as the material of art, like in mirror-sculptures and paintings on mirror surfaces.
[edit] Anamorphosis
One kind of anamorphosis has an interposed cylindrical or conical mirror making it possible to apprehend an image which is the reflection of a deformed image conceived for this purpose. The deformed image is painted on a plane surface surrounding the mirror. By looking uniquely into the mirror, the image appears normal.
[edit] Mirrors and superstition
It is a common superstition that someone who breaks a mirror will receive seven years of bad luck.
There is a fictional story in which an unattractive person looks in a mirror which then cracks.
[edit] Mirrors and animals
Experiments have shown that only large brained social animals are able to recognise that a mirror shows a reflection of themselves. [5]
Animals that have shown they are able to use a mirror to study themselves:-
- Asian elephants
- Bonobos
- Common chimpanzees
- Dolphins
- Humans
- Orangutans