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67-446: A looking glass is an object whose surface reflects an image. Looking Glass or Lookingglass may also refer to: Mirror A mirror , also known as a looking glass , is an object that reflects an image . Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the image in an equal yet opposite angle from which

134-482: A Doctor of Philosophy . In most countries, a degree in acoustics can represent the first step towards professional certification and the degree program may be certified by a professional body . After completing a certified degree program the engineer must satisfy a range of requirements before being certified. Once certified, the engineer is designated the title of Chartered Engineer (in most Commonwealth countries). The listed subdisciplines are loosely based on

201-481: A circular cylinder or of a parabolic cylinder . The most common structural material for mirrors is glass, due to its transparency, ease of fabrication, rigidity, hardness, and ability to take a smooth finish. The most common mirrors consist of a plate of transparent glass, with a thin reflective layer on the back (the side opposite to the incident and reflected light) backed by a coating that protects that layer against abrasion, tarnishing, and corrosion . The glass

268-528: A surround sound system. "Psychoacoustics seeks to reconcile acoustical stimuli and all the scientific, objective, and physical properties that surround them, with the physiological and psychological responses evoked by them." Speech is a major area of study for acoustical engineering, including the production, processing and perception of speech. This can include physics , physiology , psychology , audio signal processing and linguistics . Speech recognition and speech synthesis are two important aspects of

335-432: A virtual image of whatever is in the opposite angle from the viewer, meaning that objects in the image appear to exist in a direct line of sight —behind the surface of the mirror—at an equal distance from their position in front of the mirror. Objects behind the observer, or between the observer and the mirror, are reflected back to the observer without any actual change in orientation; the light waves are simply reversed in

402-461: A century, Venice retained the monopoly of the tin amalgam technique. Venetian mirrors in richly decorated frames served as luxury decorations for palaces throughout Europe, and were very expensive. For example, in the late seventeenth century, the Countess de Fiesque was reported to have traded an entire wheat farm for a mirror, considering it a bargain. However, by the end of that century the secret

469-418: A concave parabolic mirror (whose surface is a part of a paraboloid of revolution) will reflect rays that are parallel to its axis into rays that pass through its focus . Conversely, a parabolic concave mirror will reflect any ray that comes from its focus towards a direction parallel to its axis. If a concave mirror surface is a part of a prolate ellipsoid , it will reflect any ray coming from one focus toward

536-426: A different image in the same mirror. Thus, the images observed in a mirror depend upon the angle of the mirror with respect to the eye. The angle between the object and the observer is always twice the angle between the eye and the normal, or the direction perpendicular to the surface. This allows animals with binocular vision to see the reflected image with depth perception and in three dimensions. The mirror forms

603-401: A direction perpendicular to the mirror. However, when viewer is facing the object and the mirror is at an angle between them, the image appears inverted 180° along the direction of the angle. Objects viewed in a (plane) mirror will appear laterally inverted (e.g., if one raises one's right hand, the image's left hand will appear to go up in the mirror), but not vertically inverted (in the image

670-787: A good mirror are a surface with a very high degree of flatness (preferably but not necessarily with high reflectivity ), and a surface roughness smaller than the wavelength of the light. The earliest manufactured mirrors were pieces of polished stone such as obsidian , a naturally occurring volcanic glass . Examples of obsidian mirrors found at Çatalhöyük in Anatolia (modern-day Turkey) have been dated to around 6000 BCE. Mirrors of polished copper were crafted in Mesopotamia from 4000 BCE, and in ancient Egypt from around 3000 BCE. Polished stone mirrors from Central and South America date from around 2000 BCE onwards. By

737-415: A layer of paint applied over it. Mirrors for optical instruments often have the metal layer on the front face, so that the light does not have to cross the glass twice. In these mirrors, the metal may be protected by a thin transparent coating of a non-metallic ( dielectric ) material. The first metallic mirror to be enhanced with a dielectric coating of silicon dioxide was created by Hass in 1937. In 1939 at

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804-409: A left-hand glove into a right-hand glove or vice versa). When a person raises their left hand, the actual left hand raises in the mirror, but gives the illusion of a right hand raising because the imaginary person in the mirror is literally inside-out, hand and all. If the person stands side-on to a mirror, the mirror really does reverse left and right hands, that is, objects that are physically closer to

871-436: A lens, just as if the waves had originated from the direction of the mirror. The light can also be pictured as rays (imaginary lines radiating from the light source, that are always perpendicular to the waves). These rays are reflected at an equal yet opposite angle from which they strike the mirror (incident light). This property, called specular reflection , distinguishes a mirror from objects that diffuse light, breaking up

938-773: A mirror is said to bring seven years of bad luck . The terms "mirror" and "reflector" can be used for objects that reflect any other types of waves. An acoustic mirror reflects sound waves. Objects such as walls, ceilings, or natural rock-formations may produce echos , and this tendency often becomes a problem in acoustical engineering when designing houses, auditoriums, or recording studios. Acoustic mirrors may be used for applications such as parabolic microphones , atmospheric studies, sonar , and seafloor mapping . An atomic mirror reflects matter waves and can be used for atomic interferometry and atomic holography . The first mirrors used by humans were most likely pools of still water, or shiny stones. The requirements for making

1005-402: A person's head still appears above their body). However, a mirror does not actually "swap" left and right any more than it swaps top and bottom. A mirror swaps front and back. To be precise, it reverses the object in the direction perpendicular to the mirror surface (the normal), turning the three dimensional image inside out (the way a glove stripped off the hand can be turned inside out, turning

1072-413: A point are usually made in the shape of a paraboloid of revolution instead; they are used in telescopes (from radio waves to X-rays), in antennas to communicate with broadcast satellites , and in solar furnaces . A segmented mirror , consisting of multiple flat or curved mirrors, properly placed and oriented, may be used instead. Mirrors that are intended to concentrate sunlight onto a long pipe may be

1139-472: A protective transparent coating is added on top of the reflecting layer, to protect it against abrasion, tarnishing, and corrosion, or to absorb certain wavelengths. Thin flexible plastic mirrors are sometimes used for safety, since they cannot shatter or produce sharp flakes. Their flatness is achieved by stretching them on a rigid frame. These usually consist of a layer of evaporated aluminium between two thin layers of transparent plastic. In common mirrors,

1206-586: A small fraction of the rays are reflected. In flying relativistic mirrors conceived for X-ray lasers , the reflecting surface is a spherical shockwave (wake wave) created in a low-density plasma by a very intense laser-pulse, and moving at an extremely high velocity. A phase-conjugating mirror uses nonlinear optics to reverse the phase difference between incident beams. Such mirrors may be used, for example, for coherent beam combination. The useful applications are self-guiding of laser beams and correction of atmospheric distortions in imaging systems. When

1273-410: A sound wave reflects off of a surface, and refers to both the sound energy transmitted through and dissipated by the surface material. Reverberation is the persistence of sound caused by repeated boundary reflections after the source of the sound stops. This principle is particularly important in enclosed spaces. Diffraction is the bending of sound waves around surfaces in the path of the wave. Refraction

1340-408: A sufficiently narrow beam of light is reflected at a point of a surface, the surface's normal direction n → {\displaystyle {\vec {n}}} will be the bisector of the angle formed by the two beams at that point. That is, the direction vector u → {\displaystyle {\vec {u}}} towards the incident beams's source,

1407-480: A theatre, restaurant or railway station, enhancing the quality of music in a concert hall or recording studio, or suppressing noise to make offices and homes more productive and pleasant places to work and live. Architectural acoustic design is usually done by acoustic consultants. Bioacoustics concerns the scientific study of sound production and hearing in animals. It can include: acoustic communication and associated animal behavior and evolution of species; how sound

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1474-399: A thin coating on glass because of its naturally smooth and very hard surface. A mirror is a wave reflector. Light consists of waves, and when light waves reflect from the flat surface of a mirror, those waves retain the same degree of curvature and vergence , in an equal yet opposite direction, as the original waves. This allows the waves to form an image when they are focused through

1541-406: A variety of ways, including control by redesigning sound sources, the design of noise barriers, sound absorbers, suppressors, and buffer zones, and the use of hearing protection ( earmuffs or earplugs ). Besides noise control, acoustical engineering also covers positive uses of sound, such as the use of ultrasound in medicine , programming digital synthesizers , designing concert halls to enhance

1608-511: Is a dichroic mirror that efficiently reflects the entire visible light spectrum while transmitting infrared wavelengths. A hot mirror is the opposite: it reflects infrared light while transmitting visible light. Dichroic mirrors are often used as filters to remove undesired components of the light in cameras and measuring instruments. In X-ray telescopes , the X-rays reflect off a highly precise metal surface at almost grazing angles, and only

1675-596: Is a set of strategies to reduce noise pollution by reducing noise at its source, by inhibiting sound propagation using noise barriers or similar, or by the use of ear protection ( earmuffs or earplugs ). Control at the source is the most cost-effective way of providing noise control. Noise control engineering applied to cars and trucks is known as noise, vibration, and harshness (NVH). Other techniques to reduce product noise include vibration isolation , application of acoustic absorbent and acoustic enclosures. Acoustical engineering can go beyond noise control to look at what

1742-401: Is also used to describe a set of electrokinetic effects that occur in heterogeneous liquids under influence of ultrasound. Environmental acoustics is concerned with the control of noise and vibrations caused by traffic, aircraft, industrial equipment, recreational activities and anything else that might be considered a nuisance. Acoustical engineers concerned with environmental acoustics face

1809-443: Is broken. Lettering or decorative designs may be printed on the front face of the glass, or formed on the reflective layer. The front surface may have an anti-reflection coating . Mirrors which are reflective on the front surface (the same side of the incident and reflected light) may be made of any rigid material. The supporting material does not necessarily need to be transparent, but telescope mirrors often use glass anyway. Often

1876-614: Is commonly used for inspecting oneself, such as during personal grooming ; hence the old-fashioned name "looking glass". This use, which dates from prehistory, overlaps with uses in decoration and architecture . Mirrors are also used to view other items that are not directly visible because of obstructions; examples include rear-view mirrors in vehicles, security mirrors in or around buildings, and dentist's mirrors . Mirrors are also used in optical and scientific apparatus such as telescopes , lasers , cameras , periscopes , and industrial machinery. According to superstitions breaking

1943-444: Is concerned with researching and describing the physics of music and its perception – how sounds employed as music work. This includes: the function and design of musical instruments including electronic synthesizers ; the human voice (the physics and neurophysiology of singing ); computer analysis of music and composition; the clinical use of music in music therapy, and the perception and cognition of music . Noise control

2010-403: Is no archeological evidence of glass mirrors before the third century. These early glass mirrors were made by blowing a glass bubble, and then cutting off a small circular section from 10 to 20 cm in diameter. Their surface was either concave or convex, and imperfections tended to distort the image. Lead-coated mirrors were very thin to prevent cracking by the heat of the molten metal. Due to

2077-573: Is produced by animals; the auditory mechanisms and neurophysiology of animals; the use of sound to monitor animal populations, and the effect of man-made noise on animals. This branch of acoustic engineering deals with the design of headphones, microphones , loudspeakers , sound systems, sound reproduction, and recording. There has been a rapid increase in the use of portable electronic devices which can reproduce sound and rely on electroacoustic engineering, e.g. mobile phones , portable media players , and tablet computers . The term "electroacoustics"

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2144-403: Is the best sound for a product, for instance, manipulating the sound of door closures on automobiles . Psychoacoustics tries to explain how humans respond to what they hear, whether that is an annoying noise or beautiful music. In many branches of acoustic engineering, a human listener is a final arbitrator as to whether a design is successful, for instance, whether sound localisation works in

2211-433: Is the electronic manipulation of audio signals using analog and digital signal processing . It is done for a variety of reasons, including: Audio engineers develop and use audio signal processing algorithms. Architectural acoustics (also known as building acoustics ) is the science and engineering of achieving a good sound within a building. Architectural acoustics can be about achieving good speech intelligibility in

2278-437: Is the scientific study of sound in water. It is concerned with both natural and man-made sound and its generation underwater; how it propagates, and the perception of the sound by animals. Applications include sonar to locate submerged objects such as submarines , underwater communication by animals, observation of sea temperatures for climate change monitoring, and marine biology. Acoustic engineers working on vibration study

2345-408: Is usually soda-lime glass, but lead glass may be used for decorative effects, and other transparent materials may be used for specific applications. A plate of transparent plastic may be used instead of glass, for lighter weight or impact resistance. Alternatively, a flexible transparent plastic film may be bonded to the front and/or back surface of the mirror, to prevent injuries in case the mirror

2412-529: The Bronze Age most cultures were using mirrors made from polished discs of bronze , copper , silver , or other metals. The people of Kerma in Nubia were skilled in the manufacturing of mirrors. Remains of their bronze kilns have been found within the temple of Kerma. In China, bronze mirrors were manufactured from around 2000 BC, some of the earliest bronze and copper examples being produced by

2479-457: The Caliphate mathematician Ibn Sahl in the tenth century. Mirrors can be classified in many ways; including by shape, support, reflective materials, manufacturing methods, and intended application. Typical mirror shapes are planar and curved mirrors. The surface of curved mirrors is often a part of a sphere . Mirrors that are meant to precisely concentrate parallel rays of light into

2546-762: The Qijia culture . Such metal mirrors remained the norm through to Greco-Roman Antiquity and throughout the Middle Ages in Europe . During the Roman Empire silver mirrors were in wide use by servants. Speculum metal is a highly reflective alloy of copper and tin that was used for mirrors until a couple of centuries ago. Such mirrors may have originated in China and India. Mirrors of speculum metal or any precious metal were hard to produce and were only owned by

2613-663: The Schott Glass company, Walter Geffcken invented the first dielectric mirrors to use multilayer coatings. The Greek in Classical Antiquity were familiar with the use of mirrors to concentrate light. Parabolic mirrors were described and studied by the mathematician Diocles in his work On Burning Mirrors . Ptolemy conducted a number of experiments with curved polished iron mirrors, and discussed plane, convex spherical, and concave spherical mirrors in his Optics . Parabolic mirrors were also described by

2680-506: The angle of incidence between n → {\displaystyle {\vec {n}}} and u → {\displaystyle {\vec {u}}} , but of opposite sign. This property can be explained by the physics of an electromagnetic plane wave that is incident to a flat surface that is electrically conductive or where the speed of light changes abruptly, as between two materials with different indices of refraction. More specifically,

2747-419: The 16th century, was to blow a cylinder of glass, cut off the ends, slice it along its length, and unroll it onto a flat hot plate. Venetian glassmakers also adopted lead glass for mirrors, because of its crystal-clarity and its easier workability. During the early European Renaissance , a fire-gilding technique developed to produce an even and highly reflective tin coating for glass mirrors. The back of

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2814-509: The 1st century CE , with the development of soda-lime glass and glass blowing . The Roman scholar Pliny the Elder claims that artisans in Sidon (modern-day Lebanon ) were producing glass mirrors coated with lead or gold leaf in the back. The metal provided good reflectivity, and the glass provided a smooth surface and protected the metal from scratches and tarnishing. However, there

2881-576: The PACS ( Physics and Astronomy Classification Scheme ) coding used by the Acoustical Society of America . Aeroacoustics is concerned with how noise is generated by the movement of air, for instance via turbulence, and how sound propagates through the fluid air. Aeroacoustics plays an important role in understanding how noise is generated by aircraft and wind turbines , as well as exploring how wind instruments work. Audio signal processing

2948-412: The bulb's walls. This phenomenon was developed into the method of evaporation coating by Pohl and Pringsheim in 1912. John D. Strong used evaporation coating to make the first aluminium -coated telescope mirrors in the 1930s. The first dielectric mirror was created in 1937 by Auwarter using evaporated rhodium . The metal coating of glass mirrors is usually protected from abrasion and corrosion by

3015-452: The challenge of measuring or predicting likely noise levels, determining an acceptable level for that noise, and determining how the noise can be controlled. Environmental acoustics work is usually done by acoustic consultants or those working in environmental health . Recent research work has put a strong emphasis on soundscapes , the positive use of sound (e.g. fountains, bird song), and the preservation of tranquility . Musical acoustics

3082-407: The glass was coated with a tin-mercury amalgam, and the mercury was then evaporated by heating the piece. This process caused less thermal shock to the glass than the older molten-lead method. The date and location of the discovery is unknown, but by the 16th century Venice was a center of mirror production using this technique. These Venetian mirrors were up to 40 inches (100 cm) square. For

3149-423: The greater availability of affordable mirrors. Mirrors are often produced by the wet deposition of silver, or sometimes nickel or chromium (the latter used most often in automotive mirrors) via electroplating directly onto the glass substrate. Glass mirrors for optical instruments are usually produced by vacuum deposition methods. These techniques can be traced to observations in the 1920s and 1930s that metal

3216-408: The illusion that those objects are behind the mirror, or (sometimes) in front of it . When the surface is not flat, a mirror may behave like a reflecting lens . A plane mirror yields a real-looking undistorted image, while a curved mirror may distort, magnify, or reduce the image in various ways, while keeping the lines, contrast , sharpness , colors, and other image properties intact. A mirror

3283-440: The incident rays are parallel among themselves but not parallel to the mirror's axis, or are divergent from a point that is not the focus – as when trying to form an image of an object that is near the mirror or spans a wide angle as seen from it. However, this aberration can be sufficiently small if the object image is sufficiently far from the mirror and spans a sufficiently small angle around its axis. Mirrors reflect an image to

3350-523: The light shines upon it. This allows the viewer to see themselves or objects behind them, or even objects that are at an angle from them but out of their field of view, such as around a corner. Natural mirrors have existed since prehistoric times, such as the surface of water, but people have been manufacturing mirrors out of a variety of materials for thousands of years, like stone, metals, and glass. In modern mirrors, metals like silver or aluminium are often used due to their high reflectivity , applied as

3417-563: The machine processing of speech. Ensuring speech is transmitted intelligibly , efficiently and with high quality; in rooms, through public address systems and through telephone systems are other important areas of study. Ultrasonics deals with sound waves in solids, liquids and gases at frequencies too high to be heard by the average person. Specialist areas include medical ultrasonics (including medical ultrasonography ), sonochemistry , nondestructive testing , material characterisation and underwater acoustics ( sonar ). Underwater acoustics

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3484-527: The mirror always appear closer in the virtual image, and objects farther from the surface always appear symmetrically farther away regardless of angle. Acoustical engineering One goal of acoustical engineering can be the reduction of unwanted noise, which is referred to as noise control . Unwanted noise can have significant impacts on animal and human health and well-being, reduce attainment by students in schools, and cause hearing loss. Noise control principles are implemented into technology and design in

3551-407: The motions and interactions of mechanical systems with their environments, including measurement, analysis and control. This might include: ground vibrations from railways and construction; vibration isolation to reduce noise getting into recording studios; studying the effects of vibration on humans ( vibration white finger ); vibration control to protect a bridge from earthquakes , or modelling

3618-444: The normal vector n → {\displaystyle {\vec {n}}} , and direction vector v → {\displaystyle {\vec {v}}} of the reflected beam will be coplanar , and the angle between n → {\displaystyle {\vec {n}}} and v → {\displaystyle {\vec {v}}} will be equal to

3685-447: The observer. However, unlike a projected image on a screen, an image does not actually exist on the surface of the mirror. For example, when two people look at each other in a mirror, both see different images on the same surface. When the light waves converge through the lens of the eye they interfere with each other to form the image on the surface of the retina , and since both viewers see waves coming from different directions, each sees

3752-457: The other focus. A convex parabolic mirror, on the other hand, will reflect rays that are parallel to its axis into rays that seem to emanate from the focus of the surface, behind the mirror. Conversely, it will reflect incoming rays that converge toward that point into rays that are parallel to the axis. A convex mirror that is part of a prolate ellipsoid will reflect rays that converge towards one focus into divergent rays that seem to emanate from

3819-531: The other focus. Spherical mirrors do not reflect parallel rays to rays that converge to or diverge from a single point, or vice versa, due to spherical aberration . However, a spherical mirror whose diameter is sufficiently small compared to the sphere's radius will behave very similarly to a parabolic mirror whose axis goes through the mirror's center and the center of that sphere; so that spherical mirrors can substitute for parabolic ones in many applications. A similar aberration occurs with parabolic mirrors when

3886-807: The poor quality, high cost, and small size of glass mirrors, solid-metal mirrors (primarily of steel) remained in common use until the late nineteenth century. Silver-coated metal mirrors were developed in China as early as 500 CE. The bare metal was coated with an amalgam , then heated until the mercury boiled away. The evolution of glass mirrors in the Middle Ages followed improvements in glassmaking technology. Glassmakers in France made flat glass plates by blowing glass bubbles, spinning them rapidly to flatten them, and cutting rectangles out of them. A better method, developed in Germany and perfected in Venice by

3953-407: The propagation of structure-borne sound through buildings. Although the way in which sound interacts with its surroundings is often extremely complex, there are a few ideal sound wave behaviours that are fundamental to understanding acoustical design. Complex sound wave behaviors include absorption , reverberation , diffraction , and refraction . Absorption is the loss of energy that occurs when

4020-407: The reflective layer is usually some metal like silver, tin, nickel , or chromium , deposited by a wet process; or aluminium, deposited by sputtering or evaporation in vacuum. The reflective layer may also be made of one or more layers of transparent materials with suitable indices of refraction . The structural material may be a metal, in which case the reflecting layer may be just the surface of

4087-588: The same. Metal concave dishes are often used to reflect infrared light (such as in space heaters ) or microwaves (as in satellite TV antennas). Liquid metal telescopes use a surface of liquid metal such as mercury. Mirrors that reflect only part of the light, while transmitting some of the rest, can be made with very thin metal layers or suitable combinations of dielectric layers. They are typically used as beamsplitters . A dichroic mirror , in particular, has surface that reflects certain wavelengths of light, while letting other wavelengths pass through. A cold mirror

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4154-978: The sound of orchestras and specifying railway station sound systems so that announcements are intelligible . Acoustic engineers usually possess a bachelor's degree or higher qualification in acoustics , physics or another engineering discipline. Practicing as an acoustic engineer usually requires a bachelor's degree with significant scientific and mathematical content. Acoustic engineers might work in acoustic consultancy, specializing in particular fields, such as architectural acoustics , environmental noise or vibration control . In other industries, acoustic engineers might: design automobile sound systems; investigate human response to sounds, such as urban soundscapes and domestic appliances; develop audio signal processing software for mixing desks, and design loudspeakers and microphones for mobile phones. Acousticians are also involved in researching and understanding sound scientifically. Some positions, such as faculty require

4221-433: The wave and scattering it in many directions (such as flat-white paint). Thus, a mirror can be any surface in which the texture or roughness of the surface is smaller (smoother) than the wavelength of the waves. When looking at a mirror, one will see a mirror image or reflected image of objects in the environment, formed by light emitted or scattered by them and reflected by the mirror towards one's eyes. This effect gives

4288-661: The wealthy. Common metal mirrors tarnished and required frequent polishing. Bronze mirrors had low reflectivity and poor color rendering , and stone mirrors were much worse in this regard. These defects explain the New Testament reference in 1 Corinthians 13 to seeing "as in a mirror, darkly." The Greek philosopher Socrates urged young people to look at themselves in mirrors so that, if they were beautiful, they would become worthy of their beauty, and if they were ugly, they would know how to hide their disgrace through learning. Glass began to be used for mirrors in

4355-432: Was an important manufacturer, and Bohemian and German glass, often rather cheaper, was also important. The invention of the silvered-glass mirror is credited to German chemist Justus von Liebig in 1835. His wet deposition process involved the deposition of a thin layer of metallic silver onto glass through the chemical reduction of silver nitrate . This silvering process was adapted for mass manufacturing and led to

4422-425: Was being ejected from electrodes in gas discharge lamps and condensed on the glass walls forming a mirror-like coating. The phenomenon, called sputtering , was developed into an industrial metal-coating method with the development of semiconductor technology in the 1970s. A similar phenomenon had been observed with incandescent light bulbs : the metal in the hot filament would slowly sublimate and condense on

4489-464: Was leaked through industrial espionage. French workshops succeeded in large-scale industrialization of the process, eventually making mirrors affordable to the masses, in spite of the toxicity of mercury's vapor. The invention of the ribbon machine in the late Industrial Revolution allowed modern glass panes to be produced in bulk. The Saint-Gobain factory, founded by royal initiative in France,

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