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Bronze Mirrors Unearthed in China Still Reflect After 2,000 Years

Bronze Mirrors Unearthed in China Still Reflect After 2,000 Years

More than 80 exquisite bronze mirrors in excellent condition have been discovered in a large Han Dynasty tomb in China, after being hidden underground for over two millennia. Not only do some of them still have their original reflective quality, but experts are learning from the 2,000-year-old inscriptions and symbols that adorn them.

Bronze Mirror Image of the Mighty Han Dynasty

Ruling for just over 400 years, between 202 BC and 220 AD, the Han Dynasty was the second of the imperial dynasties in ancient Chinese history. Their rule is considered transformative in Chinese history, being dubbed the “Golden Age” of China. Now, in an article published in the Heritage Science Journal , Jiafang Lian and Quentin Parker from the University of Hong Kong write about the discovery of 80 exquisite bronze mirrors, discovered on-site in a large-scale ancient tomb in Western China.

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The expedition was carried out by archaeologists from the Shaanxi Provincial Institute of Archaeology at a large cemetery in Dabaozi Village, Xianyang City, Shaanxi Province . To their surprise and amazement, after a bit of cleaning most of these mirrors still display reflectivity, even after 2,000 years. Not only that, but the wiping off of the dirt revealed symbols and ancient Chinese scriptures.

The back surface of the bronze mirrors shoes the detail of the decoration, which included symbols and ancient Chinese scriptures. (Jiafang Liang & Quentin Parker / Heritage Science )

Bronze Mirrors and Other Artifacts Unearthed at Gaozhuang

The mirrors varied in length – between 7 centimeters and 22 centimeters (3 to 8 inches respectively), and were generally buried near the head or around the upper body within the tombs. What was instantly clear from the graveyard at Gaozhuang Township – made up of 400 separate tombs – was that this was a burial ground for the Han elite.

This hypothesis was confirmed with the discovery of the artifacts unearthed within the tombs, including fine works of pottery, jade, iron and bronze that were buried with the bodies. Some of the mirrors show four Chinese characters “ jia chang fu gi ” which translated into “home of prosperity”, leaving no doubts about the class of those who had been buried.

The Han Dynasty , like several other dynasties across ancient, medieval and modern history, ruled with an emperor at the pinnacle of Han society. The emperor presided over their government, but shared power with the nobility and appointed ministers, who formed the gentry and scholarly class. It is this class of people whose graves were uncovered at Gaozhuang.

One of the archaeologists from the dig told the Global Times that “the newly discovered mirrors are great references for archaeologists to further study the material culture of the early and middle periods of the Western Han Dynasty . They are also excellent examples of the aesthetic taste of ancient Chinese and possess both historical and artistic values.”

The mirrors varied in length – between 7 centimeters and 22 centimeters (3 to 8 inches). ( China News Service / Zhang Yuan)

The Chinese and Their Mirrors: Xuan Xi

Lian and Parker write, rather poignantly, that the “modern scholar Liang thought the ancient Chinese got the inspiration of creating a reflective surface to see the world from looking at still water in a lake or pond.” This, in its essence, provides the spiritual and philosophical lens through which mirrors were theorized. The shimmering glow was obtained from an ancient Chinese method of rubbing quicksilver (tin and mercury paste) and polishing it with white felt – a process called xuan xi .

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“The earliest recognizable Chinese bronze mirror was unearthed in Gansu Province and has been dated to the Neolithic period’s Qijia culture (2200 BC – 1600 BC)”, write Lian and Parker of the first discovery from the Bronze Age culture. China’s next 4,000 years have three distinct and most important phases of mirror history: the Warring States (475 to 221 BC), the Han (202 BC to 220 AD) and the Tang (618 to 907 AD).

These three periods witnessed a gradual refinement of the bronze technique, and diverse artistic styles, as well as delicate and intricate decorations, during the Warring period. During the Han, though the quality of designs dipped, the quantities of production began at a mass scale – mirrors remain one of the most significant archaeological remains from this period. During the Tang, this technique and design became even more advanced and sophisticated as lacquer and mother of pearl included in the finish. Even then, the two scholars argue, the design quality could not surpass that of the Warring period.

Chinese art

Chinese art is visual art that, whether ancient or modern, originated in or is practiced in China or by Chinese artists. The Chinese art in the Republic of China (Taiwan) and that of overseas Chinese can also be considered part of Chinese art where it is based in or draws on Chinese heritage and Chinese culture. Early "Stone Age art" dates back to 10,000 BC, mostly consisting of simple pottery and sculptures. After this early period Chinese art, like Chinese history, is typically classified by the succession of ruling dynasties of Chinese emperors, most of which lasted several hundred years.

Chinese art has arguably the oldest continuous tradition in the world, and is marked by an unusual degree of continuity within, and consciousness of, that tradition, lacking an equivalent to the Western collapse and gradual recovery of classical styles. The media that have usually been classified in the West since the Renaissance as the decorative arts are extremely important in Chinese art, and much of the finest work was produced in large workshops or factories by essentially unknown artists, especially in Chinese ceramics.

Much of the best work in ceramics, textiles, carved lacquer, and other techniques was produced over a long period by the various Imperial factories or workshops, which as well as being used by the court was distributed internally and abroad on a huge scale to demonstrate the wealth and power of the Emperors. In contrast, the tradition of ink wash painting, practiced mainly by scholar-officials and court painters especially of landscapes, flowers, and birds, developed aesthetic values depending on the individual imagination of and objective observation by the artist that are similar to those of the West, but long pre-dated their development there. After contacts with Western art became increasingly important from the 19th century onwards, in recent decades China has participated with increasing success in worldwide contemporary art.

Bronze Mirrors Unearthed in China Still Reflect After 2,000 Years - History

The findings, uncovered at Lajia Village, Minhe County, in Northwest China's Qinghai Province, could date back as far as 2250 BC, and are believed to belong to the Qijia Culture.

The human remains at the site have surprised archaeologists from the Archaeology Institute of the Chinese Academy of Social Sciences (CASS) and the Qinghai Provincial Archaeology Institute.

In the ruins of two of four half-underground cellar-like houses excavated in the 400-home village, human remains dating back 3,500 to 4,000 years have been found. At the site of one house, remains of 14 human beings in groups of three to five shocked every excavator.

"It is the first time in China that the remains of so many ancients have been found in a single house," said Zhao Zhijun, a doctor in archaeology and a research fellow with the CASS Archaeology Institute.

Although Chinese archaeologists have clues to the existence of Qijia Culture ruins in Qinghai Province from as early as the 1950s, they did not actually find the site until 30 years later.

Even after the first excavation was conducted near Lajia Village in 1999, when some large jade articles such as pendants and knives typical of the Qijia Culture were unearthed, archaeologists were not sure what else was awaiting them.

The 14 sets of human bones were found buried irregularly around the house, covering a floor space of roughly 14 square meters. Some lie on one side, others are entwined, as if in an embrace, while still others seem to crawl.

Near the round-shaped cooking stove in the center of the house is a skeleton which appears to be an adult, with hands raised over the head and two legs arched together. It is possible the person was killed before the body hit the ground.

Zhao described the scene. "I have never seen anything like this before," he said.

He thought the ancient victims in this particular house were in three groups, each with two to four children led by an adult.

"Something enormously extraordinary must have happened to these ancients," he said. "The young and strong have run for life, leaving behind children and the elderly who then appeared to have hid in places they thought were safe to shelter in," said Zhao, who returned from the excavation site recently.

A cluster of the remains of five ancients were found in the southwestern section of the house, appearing to be four children under the protection of one senior person.

Near the east wall, one could see the skeleton of an elder sitting against the wall, propped up with the right hand, the left hand holding a baby to its bosom, its face close to the head. The baby's two tiny hands are fastened around the elder at the waist, giving an impression of pain and horror.

"This," said Zhao, "is the most pitiful and horrible scene."

In another house about two meters away, the remains of an elder and a baby in a similar posture were found, assumed to have died at about the same time as the 14 victims in the other house.

The cause of the prehistoric tragedy remained a mystery, the only thing certain that the massive deaths were abnormal and sudden, according to Zhao.

Zhao, who is a specialist in palaeothrobotany (the study of the ancients), is inclined to believe there may have been a disastrous incident such as a big flood, based on the evidence of the group deaths and surrounding geological conditions.

The entire site, with its center several hundred meters away from the Yellow River, covers an area of 200,000 square meters and is surrounded by a large moat dozens of meters wide and five meters deep. The moat is now dry, but archaeologists have found cobbles in it.

Zhao has taken back some tiny spiral shells and snails from the unearthed houses for further examination.

He feels they indicate that the site was soaked in water as it was buried.

"What we have to determine is whether the shells and snails are tiny by nature, or whether there wasn't enough time for them to grow," he said.

Other archaeologists tend to relate the deaths of the ancient villagers to pestilence or plague. Another proposal points to religious action.

Although final conclusions could not be drawn yet, the phenomenon itself was worth studying, said Wang Renxiang, head of the excavation team and research fellow of the CASS Archaeology Institute.

"It is the rare site of a prehistoric disaster," he said.

"Most suggestions about the root cause of the disaster, such as flood or pestilence, suggest a relationship between man and nature at the time of the tragedy," Wang said. "If they are verified, that means the area had already witnessed some environmental and ecological deterioration as much as 4,000 years back."

Whatever the cause of the ancient tragedy, one thing was clear. The findings would provide much evidence of family composition in ancient times, said Zhao Zhijun, adding, "it will help us understand the population increase, the rate of survival and population control in the Neolithic Age."

How the ancient Chinese managed to control their family size is a controversial topic.

There are suggestions of infanticide, but a lack of evidence to support them. Should DNA tests determine that each group of the ancients found at Lajia Village was of one family, Zhao said, the data would be of huge demographic significance.

The children in each group were definitely aged between two and 10, the findings have shown. The two elders found holding children in their arms were both thought to be female and aged between 30 and 35 when they died, according to Wang Renxiang.

Apart from the remains of human beings, archaeologists also unearthed a 4,000-year-old rectangular stone, which scientists say was a type of percussion instrument. The finely cut and well-polished instrument, 96 centimeters long, 66 centimeters wide and about 10 centimeters thick, is dark blue and still produces a loud and clear sound.

Wang Renxiang discovered it at the home of a Lajia villager.

Experts say this is the first such instrument ever found in the history of China's archaeology. They say the discovery may reverse conventional theory that ancient percussion instruments were triangular or square shaped.

A number of delicate jade articles used in primitive religious rituals and some pottery relics were also found at the site. Judging by the jade texture and quality, experts assume they were originally from Hetian in Xinjiang, about 2,000 kilometers to the west, which is well known for its jade deposits.

The findings indicate cultural exchanges in ancient times.

The excavated moat has also drawn attention from archaeologists. At this stage they cannot say whether there was a city around Lajia Village more than 30 centuries ago, but the moat does suggest that possibility, according to Wang.

He believes the discovery may at least have been the center of an administrative region.

First discovered in 1924 at Qijiaping in Guanghe, Gansu Province, the Qijia Culture spread around the upper reaches of the Taohe, Daxia and Weihe rivers in Gansu and the Huangshui basin in the upper reaches of the Yellow River in Qinghai, during the transitional period from the Neolithic Age to the Bronze Age (2250-1900 BC). The culture was at a same time as the Longshan Culture (2500-2000 BC), which was widespread in the central plains in the middle and lower reaches of the Yellow River, and is characterized by very fine unpainted ceramics and simple tools.

Tools were mainly of stone, although copper articles had made an appearance. Pottery included red fine clay and a grey type of coarse sand.

A cast bronze mirror has also been found, suggesting that some elements of early Chinese bronze casting may have originated in western China - and may even have been linked to the bronze casting of Central Asia and the Iranian area.

The Longshan Culture, discovered largely in East and Central China, represents a critical period for the origin of civilization in China, with the appearance of city sites as its significant symbol. Up to now, dozens of sites confirmed to be ancient cities have been unearthed in the central plains and southern areas of China, whereas sites of Qijia Culture have to date produced no such city.

Bronze Mirrors Unearthed in China Still Reflect After 2,000 Years - History

A bronze mirror shows exquisite patterns after cleaning in Xixian New Area, Shaanxi Province, northwest China, April 13, 2021. /Xinhua

The Dabaozi Cemetery is home to more than 400 tombs. Excavation work and archaeological clearance started at the ruins in May 2020 and led to the discovery of over 2,000 pieces of cultural relics, such as pottery and bronze ware, from the Western Han Dynasty (202 BC to 8 AD).

The archaeologists found over 80 bronze mirrors of various sizes and styles during a recent excavation at the cemetery, which spans the late Warring States Period (475 to 221 BC) to the late Western Han Dynasty.

Due to the difference in casting techniques in various periods, this batch of bronze mirrors vary a lot in their sizes, with the diameter ranging from 8 centimeters to 22.1 centimeters, said Zhu Yingpei, head of the archaeological team at the cemetery.

Most of the mirrors are well-preserved with exquisite patterns and different inscriptions and some still have high gloss. Among them, the archaeologists found one that still reflects clear images.

According to archaeologists, most of the mirrors were placed close to the head or around the upper body of the tomb owners, with inscriptions showing people's wishes for a better life. And their owners were not only women but also men.


Prehistory Edit

The first mirrors used by humans were most likely pools of dark, still water, or water collected in a primitive vessel of some sort. The requirements for making 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. [4] Examples of obsidian mirrors found in Anatolia (modern-day Turkey) have been dated to around 6000 BC. [5] Mirrors of polished copper were crafted in Mesopotamia from 4000 BC, [5] and in ancient Egypt from around 3000 BC. [6] Polished stone mirrors from Central and South America date from around 2000 BC onwards. [5]

Bronze Age to Early Middle Ages Edit

By the Bronze Age most cultures were using mirrors made from polished discs of bronze, copper, silver, or other metals. [4] [7] 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. [8] In China, bronze mirrors were manufactured from around 2000 BC, [9] [ citation needed ] some of the earliest bronze and copper examples being produced by the Qijia culture. Such metal mirrors remained the norm through to Greco-Roman Antiquity and throughout the Middle Ages in Europe. [10] During the Roman Empire silver mirrors were in wide use even by maidservants. [11]

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. [12] Mirrors of speculum metal or any precious metal were hard to produce and were only owned by the wealthy. [13]

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. [14] : p.11 These defects explain the New Testament reference in 1 Corinthians 13 to seeing "as in a mirror, darkly."

The Greek philosopher Socrates, of "know thyself" fame, 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. [14] : p.106

Glass began to be used for mirrors in the 1st century CE, with the development of soda-lime glass and glass blowing. [15] 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. [16] [17] [18] [14] : p.12 [19] However, there is no archeological evidence of glass mirrors before the third century. [20]

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. [14] : p.10 Due to their poor quality, high cost, and small size, solid-metal mirrors, primarily of steel, remained in common use until the late nineteenth century. [14] : p.13

Silver-coated metal mirrors were developed in China as early as 500 CE. The bare metal was coated with an amalgam, then heated it until the mercury boiled away. [21]

Middle Ages and Renaissance Edit

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 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. [14] : p.11 Venetian glassmakers also adopted lead glass for mirrors, because of its crystal-clarity and its easier workability. By the 11th century, glass mirrors were being produced in Moorish Spain. [22]

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 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. [14] : p.16 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 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. [23] However, by the end of that century the secret was leaked through to 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. [24]

Industrial Revolution Edit

The invention of the ribbon machine in the late Industrial Revolution allowed modern glass panes to be produced in bulk. [14] The Saint-Gobain factory, founded by royal initiative in France, was an important manufacturer, and Bohemian and German glass, often rather cheaper, was also important.

The invention of the silvered-glass mirror is credited to German chemist Justus von Liebig in 1835. [25] 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 the greater availability of affordable mirrors.

Contemporary technologies Edit

Currently 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. [26]

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 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 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 aluminum-coated telescope mirrors in the 1930s. [27] The first dielectric mirror was created in 1937 by Auwarter using evaporated rhodium. [15]

The metal coating of glass mirrors is usually protected from abrasion and corrosion by 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 the Schott Glass company, Walter Geffcken invented the first dielectric mirrors to use multilayer coatings. [15]

Burning mirrors Edit

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. [28] Ptolemy conducted a number of experiments with curved polished iron mirrors, [2] : p.64 and discussed plane, convex spherical, and concave spherical mirrors in his Optics. [29]

Parabolic mirrors were also described by the Caliphate mathematician Ibn Sahl in the tenth century. [30] The scholar Ibn al-Haytham discussed concave and convex mirrors in both cylindrical and spherical geometries, [31] carried out a number of experiments with mirrors, and solved the problem of finding the point on a convex mirror at which a ray coming from one point is reflected to another point. [32]

Mirrors can be classified in many ways including by shape, support and reflective materials, manufacturing methods, and intended application.

By shape Edit

Typical mirror shapes are planar, convex, and concave.

The surface of curved mirrors is often a part of a sphere. Mirrors that are meant to precisely concentrate parallel rays of light into 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 a circular cylinder or of a parabolic cylinder. [ citation needed ]

By structural material Edit

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.

Back-silvered mirrors Edit

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 is usually soda-lime glass, but lead glass may be used for decorative effects, and other transparent materials may be used for specific applications. [ citation needed ]

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 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. [ citation needed ]

Front-silvered mirrors Edit

Mirrors which are reflective on the front surface (the same side of the incident and reflected light) may be made of any rigid material. [33] The supporting material does not necessarily need to be transparent, but telescope mirrors often use glass anyway. Often 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. [ citation needed ]

Flexible mirrors Edit

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 aluminum between two thin layers of transparent plastic. [ citation needed ]

By reflective material Edit

In common mirrors, the reflective layer is usually some metal like silver, tin, nickel, or chromium, deposited by a wet process or aluminum, [26] [34] 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 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 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 a small fraction of the rays are reflected. [35] 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. [36]

Nonlinear optical mirrors Edit

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. [37] [38] [39]

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.

  • When parallel beams of light are reflected on a plane surface, the reflected rays will be parallel too.
  • If the reflecting surface is concave, the reflected beams will be convergent, at least to some extent and for some distance from the surface.
  • A convex mirror, on the other hand, will reflect parallel rays towards divergent directions.

More specifically, 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 the other focus. [40]

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 the other focus. [40]

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. [40]

A similar aberration occurs with parabolic mirrors when 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 objet 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. [40]

Mirror images Edit

Mirrors reflect an image to 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 a different image in the same mirror. Thus, the images observed in a mirror depends 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 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 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. [41]

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 a person's head still appears above their body). [42] However, a mirror does not usually "swap" left and right any more than it swaps top and bottom. A mirror typically reverses the forward-backward axis. To be precise, it reverses the object in the direction perpendicular to the mirror surface (the normal). Because left and right are defined relative to front-back and top-bottom, the "flipping" of front and back results in the perception of a left-right reversal in the image. (i.e.: 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 image appears to be facing them. If they stand side-on to a mirror, the mirror really does reverse left and right, that is, objects that are physically closer to the mirror always appear closer in the virtual image, and objects farther from the surface always appear symmetrically farther away regardless of angle.)

Looking at an image of oneself with the front-back axis flipped results in the perception of an image with its left-right axis flipped. When reflected in the mirror, a person's right hand remains directly opposite their real right hand, but it is perceived by the mind as the left hand in the image. When a person looks into a mirror, the image is actually front-back reversed, which is an effect similar to the hollow-mask illusion. Notice that a mirror image is fundamentally different from the object and cannot be reproduced by simply rotating the object.

For things that may be considered as two-dimensional objects (like text), front-back reversal cannot usually explain the observed reversal. An image is a two-dimensional representation of a three-dimensional space, and because it exists in a two-dimensional plane, an image can be viewed from front or back. In the same way that text on a piece of paper appears reversed if held up to a light and viewed from behind, text held facing a mirror will appear reversed, because the image of the text is still facing away from the observer. Another way to understand the reversals observed in images of objects that are effectively two-dimensional is that the inversion of left and right in a mirror is due to the way human beings perceive their surroundings. A person's reflection in a mirror appears to be a real person facing them, but for that person to really face themselves (i.e.: twins) one would have to physically turn and face the other, causing an actual swapping of right and left. A mirror causes an illusion of left-right reversal because left and right were not swapped when the image appears to have turned around to face the viewer. The viewer's egocentric navigation (left and right with respect to the observer's point of view i.e.: "my left. ") is unconsciously replaced with their allocentric navigation (left and right as it relates another's point of view ". your right") when processing the virtual image of the apparent person behind the mirror. Likewise, text viewed in a mirror would have to be physically turned around, facing the observer and away from the surface, actually swapping left and right, to be read in the mirror. [41]

Reflectivity Edit

The reflectivity of a mirror is determined by the percentage of reflected light per the total of the incident light. The reflectivity may vary with wavelength. All or a portion of the light not reflected is absorbed by the mirror, while in some cases a portion may also transmit through. Although some small portion of the light will be absorbed by the coating, the reflectivity is usually higher for first-surface mirrors, eliminating both reflection and absorption losses from the substrate. The reflectivity is often determined by the type and thickness of the coating. When the thickness of the coating is sufficient to prevent transmission, all of the losses occur due to absorption. Aluminum is harder, less expensive, and more resistant to tarnishing than silver, and will reflect 85 to 90% of the light in the visible to near-ultraviolet range, but experiences a drop in its reflectance between 800 and 900 nm. Gold is very soft and easily scratched, costly, yet does not tarnish. Gold is greater than 96% reflective to near and far-infrared light between 800 and 12000 nm, but poorly reflects visible light with wavelengths shorter than 600 nm (yellow). Silver is expensive, soft, and quickly tarnishes, but has the highest reflectivity in the visual to near-infrared of any metal. Silver can reflect up to 98 or 99% of light to wavelengths as long as 2000 nm, but loses nearly all reflectivity at wavelengths shorter than 350 nm. Dielectric mirrors can reflect greater than 99.99% of light, but only for a narrow range of wavelengths, ranging from a bandwidth of only 10 nm to as wide as 100 nm for tunable lasers. However, dielectric coatings can also enhance the reflectivity of metallic coatings and protect them from scratching or tarnishing. Dielectric materials are typically very hard and relatively cheap, however the number of coats needed generally makes it an expensive process. In mirrors with low tolerances, the coating thickness may be reduced to save cost, and simply covered with paint to absorb transmission. [43]

Surface quality Edit

Surface quality, or surface accuracy, measures the deviations from a perfect, ideal surface shape. Increasing the surface quality reduces distortion, artifacts, and aberration in images, and helps increase coherence, collimation, and reduce unwanted divergence in beams. For plane mirrors, this is often described in terms of flatness, while other surface shapes are compared to an ideal shape. The surface quality is typically measured with items like interferometers or optical flats, and are usually measured in wavelengths of light (λ). These deviations can be much larger or much smaller than the surface roughness. A normal household-mirror made with float glass may have flatness tolerances as low as 9–14λ per inch (25.4 mm), equating to a deviation of 5600 through 8800 nanometers from perfect flatness. Precision ground and polished mirrors intended for lasers or telescopes may have tolerances as high as λ/50 (1/50 of the wavelength of the light, or around 12 nm) across the entire surface. [44] [43] The surface quality can be affected by factors such as temperature changes, internal stress in the substrate, or even bending effects that occur when combining materials with different coefficients of thermal expansion, similar to a bimetallic strip. [45]

Surface roughness Edit

Surface roughness describes the texture of the surface, often in terms of the depth of the microscopic scratches left by the polishing operations. Surface roughness determines how much of the reflection is specular and how much diffuses, controlling how sharp or blurry the image will be.

For perfectly specular reflection, the surface roughness must be kept smaller than the wavelength of the light. Microwaves, which sometimes have a wavelength greater than an inch (

25 mm) can reflect specularly off a metal screen-door, continental ice-sheets, or desert sand, while visible light, having wavelengths of only a few hundred nanometers (a few hundred-thousandths of an inch), must meet a very smooth surface to produce specular reflection. For wavelengths that are approaching or are even shorter than the diameter of the atoms, such as X-rays, specular reflection can only be produced by surfaces that are at a grazing incidence from the rays.

Surface roughness is typically measured in microns, wavelength, or grit size, with

Transmissivity Edit

Transmissivity is determined by the percentage of light transmitted per the incident light. Transmissivity is usually the same from both first and second surfaces. The combined transmitted and reflected light, subtracted from the incident light, measures the amount absorbed by both the coating and substrate. For transmissive mirrors, such as one-way mirrors, beam splitters, or laser output couplers, the transmissivity of the mirror is an important consideration. The transmissivity of metallic coatings are often determined by their thickness. For precision beam-splitters or output couplers, the thickness of the coating must be kept at very high tolerances to transmit the proper amount of light. For dielectric mirrors, the thickness of the coat must always be kept to high tolerances, but it is often more the number of individual coats that determine the transmissivity. For the substrate, the material used must also have good transmissivity to the chosen wavelengths. Glass is a suitable substrate for most visible-light applications, but other substrates such as zinc selenide or synthetic sapphire may be used for infrared or ultraviolet wavelengths. [48] : p.104–108

Wedge Edit

Wedge errors are caused by the deviation of the surfaces from perfect parallelism. An optical wedge is the angle formed between two plane-surfaces (or between the principle planes of curved surfaces) due to manufacturing errors or limitations, causing one edge of the mirror to be slightly thicker than the other. Nearly all mirrors and optics with parallel faces have some slight degree of wedge, which is usually measured in seconds or minutes of arc. For first-surface mirrors, wedges can introduce alignment deviations in mounting hardware. For second-surface or transmissive mirrors, wedges can have a prismatic effect on the light, deviating its trajectory or, to a very slight degree, its color, causing chromatic and other forms of aberration. In some instances, a slight wedge is desirable, such as in certain laser systems where stray reflections from the uncoated surface are better dispersed than reflected back through the medium. [43] [49]

Surface defects Edit

Surface defects are small-scale, discontinuous imperfections in the surface smoothness. Surface defects are larger (in some cases much larger) than the surface roughness, but only affect small, localized portions of the entire surface. These are typically found as scratches, digs, pits (often from bubbles in the glass), sleeks (scratches from prior, larger grit polishing operations that were not fully removed by subsequent polishing grits), edge chips, or blemishes in the coating. These defects are often an unavoidable side-effect of manufacturing limitations, both in cost and machine precision. If kept low enough, in most applications these defects will rarely have any adverse effect, unless the surface is located at an image plane where they will show up directly. For applications that require extremely low scattering of light, extremely high reflectance, or low absorption due to high energy-levels that could destroy the mirror, such as lasers or Fabry-Perot interferometers, the surface defects must be kept to a minimum. [50]

Mirrors are usually manufactured by either polishing a naturally reflective material, such as speculum metal, or by applying a reflective coating to a suitable polished substrate. [51]

In some applications, generally those that are cost-sensitive or that require great durability, such as for mounting in a prison cell, mirrors may be made from a single, bulk material such as polished metal. However, metals consist of small crystals (grains) separated by grain boundaries that may prevent the surface from attaining optical smoothness and uniform reflectivity. [15] : p.2,8

Coating Edit

Silvering Edit

The coating of glass with a reflective layer of a metal is generally called "silvering", even though the metal may not be silver. Currently the main processes are electroplating, "wet" chemical deposition, and vacuum deposition [15] Front-coated metal mirrors achieve reflectivities of 90–95% when new.

Dielectric coating Edit

Applications requiring higher reflectivity or greater durability, where wide bandwidth is not essential, use dielectric coatings, which can achieve reflectivities as high as 99.997% over a limited range of wavelengths. Because they are often chemically stable and do not conduct electricity, dielectric coatings are almost always applied by methods of vacuum deposition, and most commonly by evaporation deposition. Because the coatings are usually transparent, absorption losses are negligible. Unlike with metals, the reflectivity of the individual dielectric-coatings is a function of Snell's law known as the Fresnel equations, determined by the difference in refractive index between layers. Therefore, the thickness and index of the coatings can be adjusted to be centered on any wavelength. Vacuum deposition can be achieved in a number of ways, including sputtering, evaporation deposition, arc deposition, reactive-gas deposition, and ion plating, among many others. [15] : p.103,107

Shaping and polishing Edit

Tolerances Edit

Mirrors can be manufactured to a wide range of engineering tolerances, including reflectivity, surface quality, surface roughness, or transmissivity, depending on the desired application. These tolerances can range from wide, such as found in a normal household-mirror, to extremely narrow, like those used in lasers or telescopes. Tightening the tolerances allows better and more precise imaging or beam transmission over longer distances. In imaging systems this can help reduce anomalies (artifacts), distortion or blur, but at a much higher cost. Where viewing distances are relatively close or high precision is not a concern, wider tolerances can be used to make effective mirrors at affordable costs.

Misplaced 2,000-year-old ring discovered in Jerusalem’s City of David

Amanda Borschel-Dan is The Times of Israel's Jewish World and Archaeology editor.

Some 2,000 years ago, a Jewish penitent misplaced a bronze ring during his climb of a 600-meter-long (about 2,000 feet) pilgrims’ thoroughfare leading to the Temple Mount. While the recently recovered ring is today heavily corroded, its central blue semi-precious stone still sparkles.

The ring was recently discovered at the City of David’s Sifting Project in Emek HaTsurim, in a bucket of dirt excavated from a structure on the side of the broad 7.5-meter (24-feet) -wide road that is thought to have housed a ritual bath, or mikveh. According to City of David archaeologists, the worshiper likely lost the ring when fresh from ritual purification prior to his ascent to the Temple Mount.

For the past seven years at the City of David National Park in Jerusalem, archaeologists have been excavating a now-subterranean stairway that once served as a main artery to the Temple Mount, beginning at the intersection of the Kidron and Ben Hinnom Valleys.

“Every step on this street brought the pilgrims closer to the Temple,” said City of David archaeologist Nahshon Szanton, in a recent video tour of the site. “Imagine to yourselves the joy, the songs, the prayers, the spiritual journey that these people experience when they know they are just meters away from reaching the gates of the Temple,” he added, while climbing the monumental staircase.

לסיור בדרך עולי הרגל – הקליקו >>

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לאחר 6 שנים של חפירות מרתקות ב"דרך עולי הרגל" וחשיפת גרם המדרגות האדיר אשר שימש את עולי הרגל בדרכם אל בית המקדש, תוך עשרות גילויים מפתיעים ומרגשים מתולדות ירושלים הקדומה נחשון זנטון – אשר ניהל את החפירה המרשימה הזו, יוצא לכתיבת דוקטורט במטרה לסכם את כל הממצאים החשובים שנאספו עד כה. עיני העולם כולו נשואות לחפירות הללו ולתגליות הרבות והמשמעותיות שנחשפו עד כה ושממתינות עדיין להיחשף. אנו שמחים שאדם כמו נחשון היווה חלק משמעותי מפרויקט כה חשוב ומיוחד ומאחלים לו הצלחה רבה!

Posted by ‎עיר דוד- ירושלים הקדומה‎ on Monday, November 20, 2017

The pilgrims’ road, which ascends from the Pool of Siloam to the Jewish Temple, dates to no earlier than 30-31 CE, during the time of the notorious Roman governor Pontius Pilate. In the short video, Szanton emphasized that this was the period when Jesus was sentenced to death.

According to the City of David, the Herodian road was lined with shops and businesses to serve the thousands of pilgrims to Jerusalem on the major holidays.

The broad road is a monumental achievement: Szanton estimates that some 10,000 tons of quarried rock was used in its construction. The road was built above a complex drainage system, which rebels hid in 40 years after the Pilgrims’ Path’s construction as the Second Temple was destroyed by the Romans in 70 CE.

The drainage channel “was essentially a manmade tunnel,” according to the City of David, and was built underneath the Herodian Road. Its ceiling is made of the rectangular paving stones of the pilgrims road above.

The ring is perhaps a testament to a final period of peace, in which pilgrims could still safely climb the path to the Temple Mount and freely worship.

In a statement released by the City of David, archaeologists Szanton, Moran Hajbi, Ari Levy, and Dr. Joe Uziel said, “Just like today, it would appear that in the past, rings and jewelry were removed before bathing, and sometimes forgotten. This phenomenon, perhaps, is behind the discovery of the ring in what appears to be a ritual bath.”

The ring is a very human reminder of the people who ascended the path prior to the temple’s destruction, said the archaeologists.

“This ring allows us to personally connect with an individual’s personal story from 2,000 years ago. The ring, along with other finds, can shed light and expose the lives of people during the Second Temple period,” they said.

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The Principal Concept in Ancient Chinese Cosmology

Ancient Chinese armillary sphere

Entering the East Han Dynasty (25-220) and the Three Kingdoms period (220–280), Chinese cosmology came to the second golden age.

By that time, Chinese discovered the moving paths of five major planets (i.e. Jupiter, Mars, Saturn, Venus and Mercury), the causes of the solar and lunar eclipses and sunspots, and the understanding about the 28 constellations became more comprehensive.

Although it was during the Warring States in the 4th Century BC when two men named Shi Shen and Gan Shi created the first armillary sphere, it had to wait until the East Han dynasty, a man called Zhang Heng built China’s first water-power based sphere to visually illustrate Chinese cosmological theory.

According to this theory, the cosmos is like an egg, with celestial layers wrapped the Earth core. While the Earth is made of earth, celestial layers are filled with water, and the water is supported by qi – the info-energy.

Since everything in the universe is nothing more than various expressions of qi (info-energy), with the well-ordered and fluent part rising up while the chaotic and clustered portion sinking down, ancient Chinese cosmology believes that humans and their environment are closely correlated to each other.

In short, the environment is responsive not just to people’s actions, but thoughts and emotions.

Bronze Mirrors Unearthed in China Still Reflect After 2,000 Years - History

The house-shaped vessel that contains millet. Photo: Shaanxi News on Sina Weibo

Millet congealed together Photo: Shaanxi News on Sina Weibo

A house-shaped vessel that was once used for storing grains during China's Han Dynasty (206BC-AD220) was recently unearthed from a well-preserved tomb in Qinhan New City, Xixian New Area, Northwest China's Shaanxi Province. Upon opening the vessel, archaeologists were stunned by the fact that it was still completely filled with 2,000-year-old grains.

Despite the fact that the grains in the container had congealed together, researchers were still able to identify it as millet, one of the most common staple foods in northern China.

"Geographically speaking, millet has always been significantly consumed as a staple food by people in the north, wheat too. Likewise, people in the southern area of China make rice dishes. This discovery shows that millet has been a choice for people in the regions from ancient times till now," Xu, an archaeological expert told the Global Times, on Tuesday.

The pottery vessel is approximately 1 meter wide and 80 centimeters in height. It is huge compared to other grain vessels that have been discovered.

"The standard Han Dynasty pottery vessels that have been discovered are about a quarter of the size of this one," Tian, a botanical archaeologist in China's Northwest University, told media.

The new discovery increased netizens' "appetite" for discussion of ancient food on China's Twitter-like Sina Weibo.

Some made cheeky comments saying they had utensils ready to give the 2,000-year-old millet a go. Others were simply impressed by the "prosperity" of relics in Shaanxi, a Chinese province rich in history.

"Isn't it too frequent to find new stuff precious as such? I remember the mirror news was just a week ago," said a netizen to refer to finely restored mirrors that still appeared to be bright and very reflective after 2,000 years, which have also recently been discovered in Shaanxi Province.

Along with the newly discovered millet vessel, more than 100 relics have had been unearthed in the tomb, including precious gold and jade wares.

"Seeing such a large amount of food provides us with a clue for the further study of agricultural development of that time," Xu told the Global Times on Tuesday.

The Qin (221–206 bce ) and Han dynasties (206 bce –220 ce )

Already by late Zhou times, the more expensive medium of lacquer was often used in place of bronze. Nevertheless, some bronze vessels were still made for sacrificial rites, and other bronze objects, such as lamps and incense burners, also were made for household use. The “hill censer” (boshan xianglu) was designed as a miniature, three-dimensional mountain of the immortals, usually replete with scenes of mythic combat between man and beasts, suggesting the powerful forces of nature that only the Daoist adept could tame. Sacred vapours emanating from materials burned within were released through perforations in the lid (hidden behind the mountain peaks). Cosmic waters were depicted lapping at the base of the hills, conveying the sense of an island, and the whole was set on a narrow stem that thrust the mountain upward as if it were an axis of the universe. Such censers might have been used in ceremonial exorcisms, in funerary rites associated with the ascent of the soul, or in other varieties of Daoist religious practice.

Some Han mirrors have astronomical or astrological patterns. The most elaborate, particularly popular during the Xin dynasty (9–25 ce ), bears the so-called TLV pattern. (The TLV pattern is so called because it resembles those roman letters.) These angular shapes, ranged around the main band of decoration between a central square zone and the outer border band, are believed to be linked to a cosmological, chesslike game called liubo the decoration also may include creatures symbolic of the four directions, immortals, and other mythical beings popular in Daoist folklore. Often the mirrors carry inscriptions, varying from a simple expression of good luck to a long dedication giving the name of the maker and referring to the Shangfang, the imperial office in charge of imperial workshops. In the Eastern Han the Daoist elements dominated mirror design, which often includes the legendary Queen Mother of the West, Xiwangmu, and her royal eastern counterpart, Dongwanggong. The coming of Buddhism at the end of the Han dynasty caused a decline in the use of cosmological mirrors. Mirror making, however, was revived in the Tang dynasty (618–907).

Recognising Your Reflection?

Humans probably first started to look at their reflections in pools of water, streams and rivers. Very few animals are able to recognise their own reflections and humans share this rare ability with most of the other primates, magpies, elephants, orcas and bottlenose dolphins.

If a dog or a cat catches sight of itself in a reflective surface they will treat the reflection as another animal and probably try to chase it away. Gordon Gallup JR developed the mirror test for animals in 1970 based on observations previously made by Charles Darwin, and most human infants will not pass this test and be able to recognise their own reflection until they are at least a few months old.

We can only guess what these early humans felt as they caught sight of their reflections in a stretch of still water for the first time, and myths and superstitions about looking at your reflection grew up from the earliest times.

It was believed that gazing at your reflection was a way of entering another world and that you could divine your future from looking at yourself. But woe betide you if the image you were gazing at was distorted as this could mean death!

Watch the video: Ancient Face Mirrors Made of Bronze Are 2,000 Years Old (January 2022).