A mirror is a device whose surface 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.

The most common use of mirrors is in the home for personal grooming, but mirrors are also used in scientific apparatus such as telescopes and lasers, and in 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.

Effects

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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. Finally, 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. (The ideal of focusing to a point is a commonly-used approximation, however.) 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, however.

A beam of light reflects off a mirror at an angle of reflection that is equal to its angle of incidence. 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.

Image in a mirror

For an object with approximate reflection symmetry, a reflection in some mirror plane corresponds to a combination of:

• a translation if the mirror is parallel to the symmetry plane of the object, and otherwise a rotation about the line of intersection of the two planes by an angle which is twice the angle between the two planes
• a reflection in the approximate symmetry plane of the object (due to the assumption this is a minor change) We can apply this to the image in a mirror of, say, a standing person, because people have approximate bilateral symmetry. The image is the most realistic if it is still vertical, i.e., if the rotation is about a vertical axis. This is the case if the mirror is vertical. In this case the image of the person is in normal standing orientation and vertically in a normal position, at a horizontally different position and with an orientation rotated about a vertical axis, the latter except if the mirror is parallel to the approximate symmetry plane of the person.

  In particular, if one looks at one's image in a vertical mirror in left-right orientation, the image corresponds to a rotation by 180° about the vertical axis in the mirror, combined with a reflection in one's approximate symmetry plane.

  The question is sometimes asked, "Why does the mirror reverse left to right and not top to bottom?", and it is worth thinking about. The counter-intuitive answer is that it actually does. Imagine you had a complete copy of your body that you could manipulate into different positions, and imagine that it is directly in front of you and facing the same direction as you, so that you are looking at its back. If you twist the copy around the vertical axis, as if it were turning to face you, and then compare that with your reflection in the mirror. The reflection will be different from the model because everything that should be on the left will be on the right.

  But imagine instead that you twist the model about a horizonal axis, as if it were doing a handstand. The model would be upside down, facing you. If you compare this to yourself, and the reflection, then left and right are all correct. Your wedding ring, eye patch, and false leg are all on the correct side, be that east west north or south, but something is glaringly wrong about the reflection compared with the model, the reflection's feet are down at the bottom, where its head should be! Or, you could just keep the model in front of you so that you are looking at its back, and compare that with the reflection. Now left, right, up and down are all correct, but the reflection has its back where its front should be.

  The model represents the way the 'real you' ought to look. If some other person looks at you, what would they see?. So you compare the reflection against what you think you 'ought' to see. If you think you ought to see what you look like when you're doing a handstand, then the reflection is upside down, and if you think you ought to see your own back, then the reflection is flipped front and back. But most people want to see themselves from the front, whilst standing up, they think their reflection ought to look like they would look if they turned around, and they think that left has been flipped with right.

  In some sense what has 'really' been flipped is front and back. If you were to describe the body with co-ordinates, east/west north/south up/down, and the mirror has been facing south whilst you are looking north, then the difference is in the north/south direction; front and back. So I don't think my reflection looks a bit wrong because left has flipped with right, I think it looks hideously deformed because it has a face where the back of its head should be.

  The mathematical or geometrical version of the question is: "why does a chiral object (such as a right hand or glove) appear as an object of opposite chirality (left hand or glove) in the mirror?" The answer is that chirality of the three-dimensional space is dictated by the choice of the directions of the three axes. When the direction of one axis is reversed, as is the case in a mirror image, the chirality (or "handedness") of space changes to the opposite one. If two mirrors are set side by side (with, say, a 90° angle between them), the axes in the doubly reflected image are inverted twice and the "handedness" of the image is not changed. In such a double mirror, a right hand looks like a right hand. This set-up lets you see how you really look, but most people find it very difficult at first to use a mirror like this for shaving.

  Composition

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  Early mirrors were usually 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. 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. This type of mirror reflects about 80% of the incident light. The "back side" of the mirror is often painted black 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 improve the image quality. Metal films on the front surface are generally covered with a thin, transparent coating (often silica) to protect them from corrosion. In some cases, this coating may be designed to enhance the reflectivity as well.

  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 being damaged by the intense beam.

  Applications

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  Viewing

  Viewing one's own body

  A mirror is used for inspecting parts of one's body which are difficult or impossible to see directly, such as the face, neck or the whole body. This may be to check physical appearance (including clothing, make-up, hair, etc.) or to control applying make-up, shaving, cutting hair, fixing one's tie, etc.

  The optimal size and position for a mirror to view one's whole body is half of one's height with the top edge between the top of one's head and eyes.

  It is a common superstition that someone who breaks a mirror will receive seven years of bad luck.

  Safety and easier viewing

  Rear-view mirrors are applied in and on vehicles (such as cars, 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 ^*.

  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.

  Rounded (convex) mirrors are sometimes placed at road junctions, and corners of places such as parking lots or stores, allowing people to see around corners to avoid crashing into other vehicles or shopping carts.

  Mirrors are also sometimes used as part of security systems, so that a single video camera can show more than one angle at a time.

  High quality flat mirrors are essential for making corner reflectors, which are used for emergency location, and even laser ranging to the Moon.

  A mirror is sometimes used for voyeurism, e.g. upskirt. Other uses of mirrors in hedonistic acts include the classic 'mirror on the ceiling' for use during sex (see The Eagles' Hotel California), and the use of mirrors for 'cutting' and snorting cocaine. Use of a large number of mirrors in a confined space can act to satisfy people's desire for satisfaction of their ego, as in the hall of mirrors in the Palace of Versailles.

  One-way mirror

  A one-way mirror 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. Persons on the brightly lit side see their own reflection - it looks like a normal mirror. Persons 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.

  Contrary to popular belief, one-way mirrors that function well between equally lit rooms do not exist. The laws of physics do not allow for real, passive one-way mirrors (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. (There is no prohibition against one-way windows, however.Rayleigh, On the magnetic rotation of light and the second law of thermodynamics, Nature (London), Vol. 64, p. 577 (Oct. 10, 1901). Optical isolators are one-way devices, that are commonly used with lasers.)

  Technology

  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, which gives better image quality. 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 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 kinds of waves are also called "mirrors". The mirrors for other range of electromagnetic waves are used in optics and astronomy. Mirrors for radio waves are important elements of radio telescopes. The acoustic mirrors were in worldwide use during period between World War 1 and World War 2 for reflection (and detection) of sound of aircrafts. The research of atomic mirrors aims the creation of images using atoms instead of light.

  Signaling

  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.

  In war

  It has been said that Archimedes, an ancient Greek mathematician, physicist, engineer, astronomer, and philosopher, used a large array of mirrors to burn Roman ships during an attack on Syracuse. This has never been proved 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.

  Acoustic mirrors were in worldwide use in the 1930s for detection of powered aircraft.

  Leisure

  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.

  A decorative reflecting sphere of thin metal-coated glass, working as a reducing wide-angle mirror, is sold as a Christmas tree decoration called a bauble.

  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.

  Mirrors are often used in magic to create an illusion.

  Illuminated rotating disco balls covered with small mirrors are used to cast moving spots of light around a dance floor.

  Anamorphosis

  One kind of anamorphosis has an interposed cylindrical or conical mirror making it possible to apprehend an image which is the reflexion 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.

  Mirrors in literature

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  In Greek mythology, the hero Perseus killed Medusa by using a mirrored shield so as not to gaze upon her monstrous appearance.

  In English literature, a famous example is Lewis Carroll's Through the Looking Glass, in which protagonist Alice uses a mirror as a portal to a strange alternate world.

  Another example, from the German tales of the brothers Grimm, is Snow White, in which the Wicked Queen consults a magic mirror to determine the identity of the most beautiful woman in the world.

  Tennyson's Lady of Shalott has a mirror which shatters, releasing a curse on her.

  Mirrors, along with labyrinths, figure prominently in the work of Argentine writer Jorge Luis Borges, who often used them as symbols of infinity, impersonation, and illusion. In Dreamtigers, he writes of fearing that his reflection would move independently or change shape before his eyes. In Tlön, Uqbar, Orbis Tertius, a fictional heresiarch declares that "mirrors and copulation are abominable, since they both multiply the numbers of men."

  Similarly, Edgar Allan Poe expresses an aversion to mirrors in The Philosophy of Furniture:

  ^*egarded apart from its reflection, the mirror presents a continuous, flat, colourless, unrelieved surface, – a thing always and obviously unpleasant. Considered as a reflector, it is potent in producing a monstrous and odious uniformity: and the evil is here aggravated, not in merely direct proportion with the augmentation of its sources, but in a ratio constantly increasing. In fact, a room with four or five mirrors arranged at random, is, for all purposes of artistic show, a room of no shape at all. If we add to this evil, the attendant glitter upon glitter, we have a perfect farrago of discordant and displeasing effects.

  The King in the Window works off some of the concepts laid out by Lewis Carroll to develop its own tale of a war between the benevolent habitants of windows and the soul-stealing Master of Mirrors.

  In the Harry Potter series of novels, the Mirror of Erised is a magic mirror that reflects its viewer's deepest desires. Also featured in the series is a type of device (a dark detector) that functions as a mirror, depicting not only the gazer but also a number of shadowy figures in the background; they are enemies and their proximity to the viewer represents their imminent threat.

  In the comic series Johnny the Homicidal Maniac, the protagonist makes a habit of focusing on a mirror and willing himself to be able to go "through" to another, better world, and believes that the reason he is always thwarted is due to a lack of patience.

  In Kurt Vonnegut's Breakfast of Champions, Kilgore Trout calls mirrors "leaks," and tells people he believes they lead to other worlds. He often tells people that where he is from "taking a leak" means stealing a mirror.

  In numerous cartoons, mirrors are often employed to trick both heroes and villains, as well as reflecting laser-like beams back at their source.

  In Witches Abroad by Terry Pratchett, a character who increases her magical powers by standing between two mirrors becomes lost in the reflections, unable to tell if she is real.

  The Mirror Master in DC Comics Flash uses various scientific and mystical mirrors to commit crimes.

  Bibliography

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  • On reflection, Jonathan Miller. National Gallery Publications Limited. 1998

  See also

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  • Aranmula kannadi
  • Digital Micromirror Device
  • Periscope
  • Cold Mirror and Hot Mirror
  • Mirror Master
  • Venus effect
  • Anamorphosis and Anamorphism
  • TLV Mirror - An ancient type of Chinese mirror from the Han Dynasty.
  • Bronze Mirrors
  • Mirror Image
  • Home decor

  Notes

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  External links

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  • History of mirrors and glass

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