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113-491: 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 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

226-480: 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

339-437: A mirror ) the angle at which the wave is incident on the surface equals the angle at which it is reflected. In acoustics , reflection causes echoes and is used in sonar . In geology, it is important in the study of seismic waves . Reflection is observed with surface waves in bodies of water. Reflection is observed with many types of electromagnetic wave , besides visible light . Reflection of VHF and higher frequencies

452-492: A three-dimensional representation of whatever the light pulses reflect off, giving an accurate representation of the surface characteristics. A LiDAR system usually consists of a laser , scanner, and GPS receiver. Airplanes and helicopters are the most commonly used platforms for acquiring LIDAR data over broad areas. One application of LiDAR is bathymetric LiDAR, which uses water-penetrating green light to also measure seafloor and riverbed elevations. ALB generally operates in

565-471: A torus . Note that these are theoretical ideals, requiring perfect alignment of perfectly smooth, perfectly flat perfect reflectors that absorb none of the light. In practice, these situations can only be approached but not achieved because the effects of any surface imperfections in the reflectors propagate and magnify, absorption gradually extinguishes the image, and any observing equipment (biological or technological) will interfere. In this process (which

678-432: 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

791-460: 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

904-405: A complex conjugating mirror, it would be black because only the photons which left the pupil would reach the pupil. Materials that reflect neutrons , for example beryllium , are used in nuclear reactors and nuclear weapons . In the physical and biological sciences, the reflection of neutrons off of atoms within a material is commonly used to determine the material's internal structure. When

1017-418: 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

1130-530: A contour target through both an active and passive system." What this means is that airborne laser bathymetry also uses light outside the visible spectrum to detect the curves in underwater landscape. LiDAR (light detection and ranging) is, according to the National Oceanic and Atmospheric Administration , "a remote sensing method that uses light in the form of a pulsed laser to measure distances". These light pulses, along with other data, generate

1243-432: 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 a thin coating on glass because of its naturally smooth and very hard surface. A mirror

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1356-426: 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

1469-400: 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

1582-469: A fan-like swath of typically 90 to 170 degrees across. The tightly packed array of narrow individual beams provides very high angular resolution and accuracy. In general, a wide swath, which is depth dependent, allows a boat to map more seafloor in less time than a single-beam echosounder by making fewer passes. The beams update many times per second (typically 0.1–50 Hz depending on water depth), allowing faster boat speed while maintaining 100% coverage of

1695-402: A flat surface forms a mirror image , which appears to be reversed from left to right because we compare the image we see to what we would see if we were rotated into the position of the image. Specular reflection at a curved surface forms an image which may be magnified or demagnified; curved mirrors have optical power . Such mirrors may have surfaces that are spherical or parabolic . If

1808-782: 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

1921-490: A great visual interpretation of coastal environments. The other method of satellite imaging, multi-spectral (MS) imaging, tends to divide the EM spectrum into a small number of bands, unlike its partner hyper-spectral sensors which can capture a much larger number of spectral bands. MS sensing is used more in the mapping of the seabed due to its fewer spectral bands with relatively larger bandwidths. The larger bandwidths allow for

2034-440: A larger spectral coverage, which is crucial in the visual detection of marine features and general spectral resolution of the images acquired. High-density airborne laser bathymetry (ALB) is a modern, highly technical, approach to the mapping the seafloor. First developed in the 1960s and 1970s, ALB is a "light detection and ranging (LiDAR) technique that uses visible, ultraviolet, and near infrared light to optically remote sense

2147-415: 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

2260-409: 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

2373-400: A longitudinal sound wave strikes a flat surface, sound is reflected in a coherent manner provided that the dimension of the reflective surface is large compared to the wavelength of the sound. Note that audible sound has a very wide frequency range (from 20 to about 17000 Hz), and thus a very wide range of wavelengths (from about 20 mm to 17 m). As a result, the overall nature of

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2486-770: 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

2599-399: A one depth at a time procedure which required very low speed for accuracy. Greater depths could be measured using weighted wires deployed and recovered by powered winches. The wires had less drag and were less affected by current, did not stretch as much, and were strong enough to support their own weight to considerable depths. The winches allowed faster deployment and recovery, necessary when

2712-401: 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

2825-413: 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

2938-470: 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,

3051-431: A regular or irregular grid of points connected into a surface). Historically, selection of measurements was more common in hydrographic applications while DTM construction was used for engineering surveys, geology, flow modeling, etc. Since c.  2003 –2005, DTMs have become more accepted in hydrographic practice. Satellites are also used to measure bathymetry. Satellite radar maps deep-sea topography by detecting

3164-581: 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

3277-408: 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,

3390-456: 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 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

3503-456: Is a combination of continuous remote imaging and spectroscopy producing a single set of data. Two examples of this kind of sensing are AVIRIS ( airborne visible/infrared imaging spectrometer ) and HYPERION. The application of HS sensors in regards to the imaging of the seafloor is the detection and monitoring of chlorophyll, phytoplankton, salinity, water quality, dissolved organic materials, and suspended sediments. However, this does not provide

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3616-511: 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

3729-533: Is a photon-counting lidar that uses the return time of laser light pulses from the Earth's surface to calculate altitude of the surface. ICESat-2 measurements can be combined with ship-based sonar data to fill in gaps and improve precision of maps of shallow water. Mapping of continental shelf seafloor topography using remotely sensed data has applied a variety of methods to visualise the bottom topography. Early methods included hachure maps, and were generally based on

3842-505: Is a powerful tool for mapping shallow clear waters on continental shelves, and airborne laser bathymetry, using reflected light pulses, is also very effective in those conditions, and hyperspectral and multispectral satellite sensors can provide a nearly constant stream of benthic environmental information. Remote sensing techniques have been used to develop new ways of visualizing dynamic benthic environments from general geomorphological features to biological coverage. A bathymetric chart

3955-429: Is a type of isarithmic map that depicts the submerged bathymetry and physiographic features of ocean and sea bottoms. Their primary purpose is to provide detailed depth contours of ocean topography as well as provide the size, shape and distribution of underwater features. Topographic maps display elevation above ground ( topography ) and are complementary to bathymetric charts. Bathymeric charts showcase depth using

4068-423: Is also known as phase conjugation), light bounces exactly back in the direction from which it came due to a nonlinear optical process. Not only the direction of the light is reversed, but the actual wavefronts are reversed as well. A conjugate reflector can be used to remove aberrations from a beam by reflecting it and then passing the reflection through the aberrating optics a second time. If one were to look into

4181-441: 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

4294-403: Is important for radio transmission and for radar . Even hard X-rays and gamma rays can be reflected at shallow angles with special "grazing" mirrors. Reflection of light is either specular (mirror-like) or diffuse (retaining the energy , but losing the image) depending on the nature of the interface. In specular reflection the phase of the reflected waves depends on the choice of

4407-404: Is located at the imaginary intersection of the mirrors. A square of four mirrors placed face to face give the appearance of an infinite number of images arranged in a plane. The multiple images seen between four mirrors assembling a pyramid, in which each pair of mirrors sits an angle to each other, lie over a sphere. If the base of the pyramid is rectangle shaped, the images spread over a section of

4520-400: 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

4633-600: Is not a concern) may also use a digital terrain model and artificial illumination techniques to illustrate the depths being portrayed. The global bathymetry is sometimes combined with topography data to yield a global relief model . Paleobathymetry is the study of past underwater depths. Synonyms include seafloor mapping , seabed mapping , seafloor imaging and seabed imaging . Bathymetric measurements are conducted with various methods, from depth sounding , sonar and lidar techniques, to buoys and satellite altimetry . Various methods have advantages and disadvantages and

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4746-430: Is not desired, since the light would then be directed back into the headlights of an oncoming car rather than to the driver's eyes. When light reflects off a mirror , one image appears. Two mirrors placed exactly face to face give the appearance of an infinite number of images along a straight line. The multiple images seen between two mirrors that sit at an angle to each other lie over a circle. The center of that circle

4859-498: Is returned in the direction from which it came. When flying over clouds illuminated by sunlight the region seen around the aircraft's shadow will appear brighter, and a similar effect may be seen from dew on grass. This partial retro-reflection is created by the refractive properties of the curved droplet's surface and reflective properties at the backside of the droplet. Some animals' retinas act as retroreflectors (see tapetum lucidum for more detail), as this effectively improves

4972-468: Is the inverse of one produced by a single mirror. A surface can be made partially retroreflective by depositing a layer of tiny refractive spheres on it or by creating small pyramid like structures. In both cases internal reflection causes the light to be reflected back to where it originated. This is used to make traffic signs and automobile license plates reflect light mostly back in the direction from which it came. In this application perfect retroreflection

5085-450: Is the process of creating an image that combines the geometric qualities with the characteristics of photographs. The result of this process is an orthoimage , a scale image which includes corrections made for feature displacement such as building tilt. These corrections are made through the use of a mathematical equation, information on sensor calibration, and the application of digital elevation models. An orthoimage can be created through

5198-441: Is used as a means of focusing waves that cannot effectively be reflected by common means. X-ray telescopes are constructed by creating a converging "tunnel" for the waves. As the waves interact at low angle with the surface of this tunnel they are reflected toward the focus point (or toward another interaction with the tunnel surface, eventually being directed to the detector at the focus). A conventional reflector would be useless as

5311-407: 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

5424-525: 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

5537-456: 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

5650-757: 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

5763-658: 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

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5876-505: 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,

5989-417: 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

6102-499: The 1960s. NOAA obtained an unclassified commercial version in the late 1970s and established protocols and standards. Data acquired with multibeam sonar have vastly increased understanding of the seafloor. The U.S. Landsat satellites of the 1970s and later the European Sentinel satellites, have provided new ways to find bathymetric information, which can be derived from satellite images. These methods include making use of

6215-502: 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

6328-754: The Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) and the Laser Airborne Depth Sounder (LADS). SHOALS was first developed to help the United States Army Corps of Engineers (USACE) in bathymetric surveying by a company called Optech in the 1990s. SHOALS is done through the transmission of a laser, of wavelength between 530 and 532 nm, from a height of approximately 200 m at speed of 60 m/s on average. High resolution orthoimagery (HRO)

6441-517: The X-rays would simply pass through the intended reflector. When light reflects off of a material with higher refractive index than the medium in which is traveling, it undergoes a 180° phase shift . In contrast, when light reflects off of a material with lower refractive index the reflected light is in phase with the incident light. This is an important principle in the field of thin-film optics . Specular reflection forms images . Reflection from

6554-463: The angle of each individual beam. The resulting sounding measurements are then processed either manually, semi-automatically or automatically (in limited circumstances) to produce a map of the area. As of 2010 a number of different outputs are generated, including a sub-set of the original measurements that satisfy some conditions (e.g., most representative likely soundings, shallowest in a region, etc.) or integrated digital terrain models (DTM) (e.g.,

6667-411: The angle of incidence equals the angle of reflection. In fact, reflection of light may occur whenever light travels from a medium of a given refractive index into a medium with a different refractive index. In the most general case, a certain fraction of the light is reflected from the interface, and the remainder is refracted . Solving Maxwell's equations for a light ray striking a boundary allows

6780-404: The animals' night vision. Since the lenses of their eyes modify reciprocally the paths of the incoming and outgoing light the effect is that the eyes act as a strong retroreflector, sometimes seen at night when walking in wildlands with a flashlight. A simple retroreflector can be made by placing three ordinary mirrors mutually perpendicular to one another (a corner reflector ). The image produced

6893-457: The auditory feel of a space. In the theory of exterior noise mitigation , reflective surface size mildly detracts from the concept of a noise barrier by reflecting some of the sound into the opposite direction. Sound reflection can affect the acoustic space . Seismic waves produced by earthquakes or other sources (such as explosions ) may be reflected by layers within the Earth . Study of

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7006-410: 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

7119-535: The cartographer's personal interpretation of limited available data. Acoustic mapping methods developed from military sonar images produced a more vivid picture of the seafloor. Further development of sonar based technology have allowed more detail and greater resolution, and ground penetrating techniques provide information on what lies below the bottom surface. Airborne and satellite data acquisition have made further advances possible in visualisation of underwater surfaces: high-resolution aerial photography and orthoimagery

7232-483: The combination of a number of photos of the same target. The target is photographed from a number of different angles to allow for the perception of the true elevation and tilting of the object. This gives the viewer an accurate perception of the target area. High resolution orthoimagery is currently being used in the 'terrestrial mapping program', the aim of which is to 'produce high resolution topography data from Oregon to Mexico'. The orthoimagery will be used to provide

7345-761: The deep reflections of waves generated by earthquakes has allowed seismologists to determine the layered structure of the Earth . Shallower reflections are used in reflection seismology to study the Earth's crust generally, and in particular to prospect for petroleum and natural gas deposits. Seafloor mapping Bathymetric charts (not to be confused with hydrographic charts ), are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or terrain as contour lines (called depth contours or isobaths ) and selected depths ( soundings ), and typically also provide surface navigational information. Bathymetric maps (a more general term where navigational safety

7458-405: The depths measured were of several kilometers. Wire drag surveys continued to be used until the 1990s due to reliability and accuracy. This procedure involved towing a cable by two boats, supported by floats and weighted to keep a constant depth The wire would snag on obstacles shallower than the cable depth. This was very useful for finding navigational hazards which could be missed by soundings, but

7571-470: The derivation of the Fresnel equations , which can be used to predict how much of the light is reflected, and how much is refracted in a given situation. This is analogous to the way impedance mismatch in an electric circuit causes reflection of signals. Total internal reflection of light from a denser medium occurs if the angle of incidence is greater than the critical angle . Total internal reflection

7684-431: The different depths to which different frequencies of light penetrate the water. When water is clear and the seafloor is sufficiently reflective, depth can be estimated by measuring the amount of reflectance observed by a satellite and then modeling how far the light should penetrate in the known conditions. The Advanced Topographic Laser Altimeter System (ATLAS) on NASA's Ice, Cloud, and land Elevation Satellite 2 (ICESat-2)

7797-445: The form of a pulse of non-visible light being emitted from a low-flying aircraft and a receiver recording two reflections from the water. The first of which originates from the surface of the water, and the second from the seabed. This method has been used in a number of studies to map segments of the seafloor of various coastal areas. There are various LIDAR bathymetry systems that are commercially accessible. Two of these systems are

7910-432: The forward radiation cancels the incident light, and backward radiation is just the reflected light. Light–matter interaction in terms of photons is a topic of quantum electrodynamics , and is described in detail by Richard Feynman in his popular book QED: The Strange Theory of Light and Matter . When light strikes the surface of a (non-metallic) material it bounces off in all directions due to multiple reflections by

8023-403: The glass is the combination of the forward radiation of the electrons and the incident light. The reflected light is the combination of the backward radiation of all of the electrons. In metals, electrons with no binding energy are called free electrons. When these electrons oscillate with the incident light, the phase difference between their radiation field and the incident field is π (180°), so

8136-405: 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

8249-422: 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

8362-439: 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

8475-537: The individual atoms (or oscillation of electrons, in metals), causing each particle to radiate a small secondary wave in all directions, like a dipole antenna . All these waves add up to give specular reflection and refraction, according to the Huygens–Fresnel principle . In the case of dielectrics such as glass, the electric field of the light acts on the electrons in the material, and the moving electrons generate fields and become new radiators. The refracted light in

8588-414: The light is reflected with equal luminance (in photometry) or radiance (in radiometry) in all directions, as defined by Lambert's cosine law . The light sent to our eyes by most of the objects we see is due to diffuse reflection from their surface, so that this is our primary mechanism of physical observation. Some surfaces exhibit retroreflection . The structure of these surfaces is such that light

8701-402: 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 the wave and scattering it in many directions (such as flat-white paint). Thus, a mirror can be any surface in which

8814-411: The locality and tidal regime. Occupations or careers related to bathymetry include the study of oceans and rocks and minerals on the ocean floor, and the study of underwater earthquakes or volcanoes. The taking and analysis of bathymetric measurements is one of the core areas of modern hydrography , and a fundamental component in ensuring the safe transport of goods worldwide. Another form of mapping

8927-474: The measurement of ocean depth through depth sounding . Early techniques used pre-measured heavy rope or cable lowered over a ship's side. This technique measures the depth only a singular point at a time, and is therefore inefficient. It is also subject to movements of the ship and currents moving the line out of true and therefore is not accurate. The data used to make bathymetric maps today typically comes from an echosounder ( sonar ) mounted beneath or over

9040-434: The microscopic irregularities inside the material (e.g. the grain boundaries of a polycrystalline material, or the cell or fiber boundaries of an organic material) and by its surface, if it is rough. Thus, an 'image' is not formed. This is called diffuse reflection . The exact form of the reflection depends on the structure of the material. One common model for diffuse reflection is Lambertian reflectance , in which

9153-516: The mirror always appear closer in the virtual image, and objects farther from the surface always appear symmetrically farther away regardless of angle. Reflection (physics) Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light , sound and water waves . The law of reflection says that for specular reflection (for example at

9266-573: The natural system more than any physical driver. Marine topographies include coastal and oceanic landforms ranging from coastal estuaries and shorelines to continental shelves and coral reefs . Further out in the open ocean, they include underwater and deep sea features such as ocean rises and seamounts . The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes , oceanic trenches , submarine canyons , oceanic plateaus and abyssal plains . Originally, bathymetry involved

9379-444: 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

9492-447: 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

9605-454: The ocean. These shapes are obvious along coastlines, but they occur also in significant ways underwater. The effectiveness of marine habitats is partially defined by these shapes, including the way they interact with and shape ocean currents , and the way sunlight diminishes when these landforms occupy increasing depths. Tidal networks depend on the balance between sedimentary processes and hydrodynamics however, anthropogenic influences can impact

9718-468: The origin of coordinates, but the relative phase between s and p (TE and TM) polarizations is fixed by the properties of the media and of the interface between them. A mirror provides the most common model for specular light reflection, and typically consists of a glass sheet with a metallic coating where the significant reflection occurs. Reflection is enhanced in metals by suppression of wave propagation beyond their skin depths . Reflection also occurs at

9831-456: 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

9944-529: 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

10057-401: The photographic data for these regions. The earliest known depth measurements were made about 1800 BCE by Egyptians by probing with a pole. Later a weighted line was used, with depths marked off at intervals. This process was known as sounding. Both these methods were limited by being spot depths, taken at a point, and could easily miss significant variations in the immediate vicinity. Accuracy

10170-804: 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

10283-432: The reflecting surface is very smooth, the reflection of light that occurs is called specular or regular reflection. The laws of reflection are as follows: These three laws can all be derived from the Fresnel equations . In classical electrodynamics , light is considered as an electromagnetic wave, which is described by Maxwell's equations . Light waves incident on a material induce small oscillations of polarisation in

10396-402: The reflection varies according to the texture and structure of the surface. For example, porous materials will absorb some energy, and rough materials (where rough is relative to the wavelength) tend to reflect in many directions—to scatter the energy, rather than to reflect it coherently. This leads into the field of architectural acoustics , because the nature of these reflections is critical to

10509-405: 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

10622-417: The research of the world's oceans. The development of multibeam systems made it possible to obtain depth information across the width of the sonar swath, to higher resolutions, and with precise position and attitude data for the transducers, made it possible to get multiple high resolution soundings from a single pass. The US Naval Oceanographic Office developed a classified version of multibeam technology in

10735-616: The same role for ocean waterways. Coastal bathymetry data is available from NOAA's National Geophysical Data Center (NGDC), which is now merged into National Centers for Environmental Information . Bathymetric data is usually referenced to tidal vertical datums . For deep-water bathymetry, this is typically Mean Sea Level (MSL), but most data used for nautical charting is referenced to Mean Lower Low Water (MLLW) in American surveys, and Lowest Astronomical Tide (LAT) in other countries. Many other datums are used in practice, depending on

10848-588: 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

10961-442: The sea floor started by using sound waves , contoured into isobaths and early bathymetric charts of shelf topography. These provided the first insight into seafloor morphology, though mistakes were made due to horizontal positional accuracy and imprecise depths. Sidescan sonar was developed in the 1950s to 1970s and could be used to create an image of the bottom, but the technology lacked the capacity for direct depth measurement across

11074-462: The seafloor is through the use of satellites. The satellites are equipped with hyper-spectral and multi-spectral sensors which are used to provide constant streams of images of coastal areas providing a more feasible method of visualising the bottom of the seabed. The data-sets produced by hyper-spectral (HS) sensors tend to range between 100 and 200 spectral bands of approximately 5–10 nm bandwidths. Hyper-spectral sensing, or imaging spectroscopy,

11187-449: The seafloor. Attitude sensors allow for the correction of the boat's roll and pitch on the ocean surface, and a gyrocompass provides accurate heading information to correct for vessel yaw . (Most modern MBES systems use an integrated motion-sensor and position system that measures yaw as well as the other dynamics and position.) A boat-mounted Global Positioning System (GPS) (or other Global Navigation Satellite System (GNSS)) positions

11300-594: The side of a boat, "pinging" a beam of sound downward at the seafloor or from remote sensing LIDAR or LADAR systems. The amount of time it takes for the sound or light to travel through the water, bounce off the seafloor, and return to the sounder informs the equipment of the distance to the seafloor. LIDAR/LADAR surveys are usually conducted by airborne systems. Starting in the early 1930s, single-beam sounders were used to make bathymetry maps. Today, multibeam echosounders (MBES) are typically used, which use hundreds of very narrow adjacent beams (typically 256) arranged in

11413-401: The soundings with respect to the surface of the earth. Sound speed profiles (speed of sound in water as a function of depth) of the water column correct for refraction or "ray-bending" of the sound waves owing to non-uniform water column characteristics such as temperature, conductivity, and pressure. A computer system processes all the data, correcting for all of the above factors as well as for

11526-402: The specific method used depends upon the scale of the area under study, financial means, desired measurement accuracy, and additional variables. Despite modern computer-based research, the ocean seabed in many locations is less measured than the topography of Mars . Seabed topography (ocean topography or marine topography) refers to the shape of the land ( topography ) when it interfaces with

11639-565: The subtle variations in sea level caused by the gravitational pull of undersea mountains, ridges, and other masses. On average, sea level is higher over mountains and ridges than over abyssal plains and trenches. In the United States the United States Army Corps of Engineers performs or commissions most surveys of navigable inland waterways, while the National Oceanic and Atmospheric Administration (NOAA) performs

11752-928: 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 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

11865-439: The surface of transparent media, such as water or glass . In the diagram, a light ray PO strikes a vertical mirror at point O , and the reflected ray is OQ . By projecting an imaginary line through point O perpendicular to the mirror, known as the normal , we can measure the angle of incidence , θ i and the angle of reflection , θ r . The law of reflection states that θ i = θ r , or in other words,

11978-409: 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 the illusion that those objects are behind the mirror, or (sometimes) in front of it . When

12091-656: 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

12204-423: The width of the scan. In 1957, Marie Tharp , working with Bruce Charles Heezen , created the first three-dimensional physiographic map of the world's ocean basins. Tharp's discovery was made at the perfect time. It was one of many discoveries that took place near the same time as the invention of the computer . Computers, with their ability to compute large quantities of data, have made research much easier, include

12317-460: Was also affected by water movement–current could swing the weight from the vertical and both depth and position would be affected. This was a laborious and time-consuming process and was strongly affected by weather and sea conditions. There were significant improvements with the voyage of HMS Challenger in the 1870s, when similar systems using wires and a winch were used for measuring much greater depths than previously possible, but this remained

12430-431: 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

12543-425: 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

12656-462: 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,

12769-414: Was limited to relatively shallow depths. Single-beam echo sounders were used from the 1920s-1930s to measure the distance of the seafloor directly below a vessel at relatively close intervals along the line of travel. By running roughly parallel lines, data points could be collected at better resolution, but this method still left gaps between the data points, particularly between the lines. The mapping of

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