Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, therefore lower energy, than the absorbed radiation; the most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can be seen only when exposed to UV light. Fluorescent materials cease to glow nearly when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after. Fluorescence has many practical applications, including mineralogy, medicine, chemical sensors, fluorescent labelling, biological detectors, cosmic-ray detection, most fluorescent lamps. Fluorescence occurs in nature in some minerals and in various biological states in many branches of the animal kingdom.
An early observation of fluorescence was described in 1560 by Bernardino de Sahagún and in 1565 by Nicolás Monardes in the infusion known as lignum nephriticum. It was derived from the wood of Pterocarpus indicus and Eysenhardtia polystachya; the chemical compound responsible for this fluorescence is matlaline, the oxidation product of one of the flavonoids found in this wood. In 1819, Edward D. Clarke and in 1822 René Just Haüy described fluorescence in fluorites, Sir David Brewster described the phenomenon for chlorophyll in 1833 and Sir John Herschel did the same for quinine in 1845. In his 1852 paper on the "Refrangibility" of light, George Gabriel Stokes described the ability of fluorspar and uranium glass to change invisible light beyond the violet end of the visible spectrum into blue light, he named this phenomenon fluorescence: "I am inclined to coin a word, call the appearance fluorescence, from fluor-spar, as the analogous term opalescence is derived from the name of a mineral." The name was derived from the mineral fluorite, some examples of which contain traces of divalent europium, which serves as the fluorescent activator to emit blue light.
In a key experiment he used a prism to isolate ultraviolet radiation from sunlight and observed blue light emitted by an ethanol solution of quinine exposed by it. Fluorescence occurs when an orbital electron of a molecule, atom, or nanostructure, relaxes to its ground state by emitting a photon from an excited singlet state: Excitation: S 0 + h ν e x → S 1 Fluorescence: S 1 → S 0 + h ν e m + h e a t Here h ν is a generic term for photon energy with h = Planck's constant and ν = frequency of light; the specific frequencies of exciting and emitted lights are depended on the particular system. S0 is called the ground state of the fluorophore, S1 is its first excited singlet state. A molecule in S1 can relax by various competing pathways, it can undergo non-radiative relaxation in which the excitation energy is dissipated as heat to the solvent. Excited organic molecules can relax via conversion to a triplet state, which may subsequently relax via phosphorescence, or by a secondary non-radiative relaxation step.
Relaxation from S1 can occur through interaction with a second molecule through fluorescence quenching. Molecular oxygen is an efficient quencher of fluorescence just because of its unusual triplet ground state. In most cases, the emitted light has a longer wavelength, therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; the emitted radiation may be of the same wavelength as the absorbed radiation, termed "resonance fluorescence". Molecules that are excited through light absorption or via a different process can transfer energy to a second'sensitized' molecule, converted to its excited state and can fluoresce; the fluorescence quantum yield gives the efficiency of the fluorescence process. It is defined as the ratio of the number of photons emitted to the number of photons absorbed. Φ = Number of photons emitted Number of photons absorbed The maximum possible fluorescence quantum yield is 1.0.
Compounds with quantum yields of 0.10 are still considered quite fluorescent. Another way to define the quantum yield of fluorescence is by the rate of excited state decay: Φ = k f ∑ i k i where k f is the rate constant of spontaneous emission of radiation and ∑ i k i is the sum of all rates of
United States twenty-dollar bill
The United States twenty-dollar bill is a denomination of U. S. currency. The seventh U. S. President, Andrew Jackson, has been featured on the front side of the bill since 1928; as of December 2013, the average circulation life of a $20 bill is 7.9 years before it is replaced due to wear. About 11% of all notes printed in 2009 were $20 bills. Twenty-dollar bills are delivered by Federal Reserve Banks in violet straps. 1861: A demand note with Lady Liberty holding a sword and shield on the front, an abstract design on the back. The back is printed green. 1862: A note, similar, the first $20 United States note. The back is different, with several small variations extant. 1863: A gold certificate $20 note with an Eagle vignette on the face. The reverse has various abstract elements; the back is orange. 1865: A national bank note with "The Battle of Lexington" and Pocahontas's marriage to John Rolfe in black, a green border. 1869: A new United States note design, with Alexander Hamilton on the left side of the front and Victory holding a shield and sword.
The back design is green. 1875: As above, except with a different reverse. 1878: A silver certificate $20 note with a portrait of Stephen Decatur on the right side of the face. The back design is black. 1882: A new gold certificate, with a portrait of James Garfield on the right of the face. The back features an eagle. 1882: A new national bank note. The front is similar. 1886: A new silver certificate $20 note, with Daniel Manning on the center of the face. 1890: A treasury note with John Marshall on the left of the face. Two different backs exist both with abstract designs. 1902: A new national bank note. The front features Hugh McCulloch, the back has a vignette of an allegorical America. 1905: A new gold certificate $20 note, with George Washington on the center of the face. The back design is orange. Andrew Jackson first appeared on the $20 bill in 1928. Although 1928 coincides with the 100th anniversary of Jackson's election as president, it is not clear why the portrait on the bill was switched from Grover Cleveland to Jackson..
According to the U. S. Treasury, "Treasury Department records do not reveal the reason that portraits of these particular statesmen were chosen in preference to those of other persons of equal importance and prominence."The placement of Jackson on the $20 bill may be a historical irony. In his farewell address to the nation, he cautioned the public about paper money. 1914: Began as a large-sized note, a portrait of Grover Cleveland on the face, and, on the back, a steam locomotive and an automobile approaching from the left, a steamship approaching from the right. 1918: A federal reserve banknote with Grover Cleveland on the front, a back design similar to the 1914 Federal Reserve Note. 1928: Switched to a small-sized note with a portrait of Andrew Jackson on the face and the south view of the White House on the reverse. The banknote is redeemable in silver at any Federal Reserve Bank. 1933: With the U. S. having abandoned the gold standard, the bill is no longer redeemable in gold, but rather in "lawful money", meaning silver.
1942: A special emergency series, with brown serial numbers and "HAWAII" overprinted on both the front and the back, is issued. These notes are designed to circulate on the islands and be deemed invalid in the event of a Japanese invasion. 1948: The White House picture was updated to reflect renovations to the building itself, including the addition of the Truman Balcony, as well as the passage of time. Most notably, the trees are larger; the change occurred during production of Series 1934C. 1950: Design elements like the serial numbers are reduced in size and moved around subtly for aesthetic reasons. 1963: "Will Pay To The Bearer On Demand" is removed from the front of the bill below the portrait, the legal tender designation is shortened to "This note is legal tender for all debts and private" Also, "In God We Trust" is added above the White House on the reverse. These two acts are coincidental if their combined result is implemented in one redesign. Several design elements are rearranged, less perceptibly than the change in 1950 to make room for the rearranged obligations.
1969: The new treasury seal appears on all denominations, including the $20. 1977: A new type of serial-number press results in a different font. The old presses are retired, old-style serial numbers appear as late as 1981 for this denomination. 1992: Anti-counterfeiting features are added: microprinting around the portrait, a plastic strip embedded in the paper. Though the bills read Series 1990, the first bills were printed in April 1992. 1994: The first notes at the Western Currency Facility are printed in January late during production of Series 1990. September 24, 1998: Received a new appearance to further deter counterfeiting. A larger, off-center portrait of Jackson was used on the front, several anti-counterfeiting features were added, including color-shifting ink, a watermark; the plastic strip now glows green under a black light. The bills were first printed in Jun
Plastic is material consisting of any of a wide range of synthetic or semi-synthetic organic compounds that are malleable and so can be molded into solid objects. Plasticity is the general property of all materials which can deform irreversibly without breaking but, in the class of moldable polymers, this occurs to such a degree that their actual name derives from this specific ability. Plastics are organic polymers of high molecular mass and contain other substances, they are synthetic, most derived from petrochemicals, however, an array of variants are made from renewable materials such as polylactic acid from corn or cellulosics from cotton linters. Due to their low cost, ease of manufacture and imperviousness to water, plastics are used in a multitude of products of different scale, including paper clips and spacecraft, they have prevailed over traditional materials, such as wood, stone and bone, metal and ceramic, in some products left to natural materials. In developed economies, about a third of plastic is used in packaging and the same in buildings in applications such as piping, plumbing or vinyl siding.
Other uses include automobiles and toys. In the developing world, the applications of plastic may differ—42% of India's consumption is used in packaging. Plastics have many uses in the medical field as well, with the introduction of polymer implants and other medical devices derived at least from plastic; the field of plastic surgery is not named for use of plastic materials, but rather the meaning of the word plasticity, with regard to the reshaping of flesh. The world's first synthetic plastic was bakelite, invented in New York in 1907 by Leo Baekeland who coined the term'plastics'. Many chemists have contributed to the materials science of plastics, including Nobel laureate Hermann Staudinger, called "the father of polymer chemistry" and Herman Mark, known as "the father of polymer physics"; the success and dominance of plastics starting in the early 20th century led to environmental concerns regarding its slow decomposition rate after being discarded as trash due to its composition of large molecules.
Toward the end of the century, one approach to this problem was met with wide efforts toward recycling. The word plastic derives from the Greek πλαστικός meaning "capable of being shaped or molded" and, in turn, from πλαστός meaning "molded"; the plasticity, or malleability, of the material during manufacture allows it to be cast, pressed, or extruded into a variety of shapes, such as: films, plates, bottles, amongst many others. The common noun plastic should not be confused with the technical adjective plastic; the adjective is applicable to any material which undergoes a plastic deformation, or permanent change of shape, when strained beyond a certain point. For example, aluminum, stamped or forged exhibits plasticity in this sense, but is not plastic in the common sense. By contrast, some plastics will, in their finished forms, break before deforming and therefore are not plastic in the technical sense. Most plastics contain organic polymers; the vast majority of these polymers are formed from chains of carbon atoms,'pure' or with the addition of: oxygen, nitrogen, or sulfur.
The chains comprise many repeat units, formed from monomers. Each polymer chain will have several thousand repeating units; the backbone is the part of the chain, on the "main path", linking together a large number of repeat units. To customize the properties of a plastic, different molecular groups "hang" from this backbone; these pendant units are "hung" on the monomers, before the monomers themselves are linked together to form the polymer chain. It is the structure of these side chains; the molecular structure of the repeating unit can be fine tuned to influence specific properties in the polymer. Plastics are classified by: the chemical structure of the polymer's backbone and side chains. Plastics can be classified by: the chemical process used in their synthesis, such as: condensation and cross-linking. Plastics can be classified by: their various physical properties, such as: hardness, tensile strength, resistance to heat and glass transition temperature, by their chemical properties, such as the organic chemistry of the polymer and its resistance and reaction to various chemical products and processes, such as: organic solvents and ionizing radiation.
In particular, most plastics will melt upon heating to a few hundred degrees celsius. Other classifications are based on qualities that are relevant for product design. Examples of such qualities and classes are: thermoplastics and thermosets, conductive polymers, biodegradable plastics and engineering plastics and other plastics with particular structures, such as elastomers. One important classification of plastics is by the permanence or impermanence of their form, or whether they are: thermoplastics or thermosetting polymers. Thermoplastics are the plastics that, when heated, do not undergo chemical change in their composition and so can be molded again and again. Examples include: polyethylene, polypropylene and polyvinyl chloride. Common thermoplastics range from 20,000 to 500,000 amu, while thermosets are assumed to have infinite molecular weight. Thermosets, or thermosetting polymers, can melt and take shape only once: after they have solidified, they stay solid. In the thermosetting process, a chemical reaction occurs, irreversible.
A foil is a thin sheet of metal made by hammering or rolling. Foils are most made with malleable metals, such as aluminium, copper and gold. Foils bend under their own weight and can be torn easily; the more malleable a metal, the thinner foil can be made with it. For example, aluminium foil is about 1/1000 inch, whereas gold can be made into foil only a few atoms thick, called gold leaf. Thin foil is called metal leaf. Leaf tears easily and must be picked up with special brushes. Foil is used in household applications, it is useful in survival situations, because the reflective surface reduces the degree of hypothermia caused by thermal radiation. Aluminium foil Tin foil Gold leaf Metal leaf
Engraving is the practice of incising a design onto a hard flat surface by cutting grooves into it with a burin. The result may be a decorated object in itself, as when silver, steel, or glass are engraved, or may provide an intaglio printing plate, of copper or another metal, for printing images on paper as prints or illustrations. Engraving is one of the oldest and most important techniques in printmaking. Wood engraving is not covered in this article. Engraving was a important method of producing images on paper in artistic printmaking, in mapmaking, for commercial reproductions and illustrations for books and magazines, it has long been replaced by various photographic processes in its commercial applications and because of the difficulty of learning the technique, is much less common in printmaking, where it has been replaced by etching and other techniques. "Engraving" is loosely but incorrectly used for any old black and white print. Many old master prints combine techniques on the same plate, further confusing matters.
Line engraving and steel engraving cover use for reproductive prints, illustrations in books and magazines, similar uses in the 19th century, not using engraving. Traditional engraving, by burin or with the use of machines, continues to be practised by goldsmiths, glass engravers and others, while modern industrial techniques such as photoengraving and laser engraving have many important applications. Engraved gems were an important art in the ancient world, revived at the Renaissance, although the term traditionally covers relief as well as intaglio carvings, is a branch of sculpture rather than engraving, as drills were the usual tools. Other terms used for printed engravings are copper engraving, copper-plate engraving or line engraving. Steel engraving is the same technique, on steel or steel-faced plates, was used for banknotes, illustrations for books and reproductive prints and similar uses from about 1790 to the early 20th century, when the technique became less popular, except for banknotes and other forms of security printing.
In the past, "engraving" was used loosely to cover several printmaking techniques, so that many so-called engravings were in fact produced by different techniques, such as etching or mezzotint. "Hand engraving" is a term sometimes used for engraving objects other than printing plates, to inscribe or decorate jewellery, trophies and other fine metal goods. Traditional engravings in printmaking are "hand engraved", using just the same techniques to make the lines in the plate; each graver has its own use. Engravers use a hardened steel tool called a burin, or graver, to cut the design into the surface, most traditionally a copper plate. However, modern hand engraving artists use burins or gravers to cut a variety of metals such as silver, steel, gold and more, in applications from weaponry to jewellery to motorcycles to found objects. Modern professional engravers can engrave with a resolution of up to 40 lines per mm in high grade work creating game scenes and scrollwork. Dies used in mass production of molded parts are sometimes hand engraved to add special touches or certain information such as part numbers.
In addition to hand engraving, there are engraving machines that require less human finesse and are not directly controlled by hand. They are used for lettering, using a pantographic system. There are versions for the insides of rings and the outsides of larger pieces; such machines are used for inscriptions on rings and presentation pieces. Gravers come in a variety of sizes that yield different line types; the burin produces a unique and recognizable quality of line, characterized by its steady, deliberate appearance and clean edges. The angle tint tool has a curved tip, used in printmaking. Florentine liners are flat-bottomed tools with multiple lines incised into them, used to do fill work on larger areas or to create uniform shade lines that are fast to execute. Ring gravers are made with particular shapes that are used by jewelry engravers in order to cut inscriptions inside rings. Flat gravers are used for fill work on letters, as well as "wriggle" cuts on most musical instrument engraving work, remove background, or create bright cuts.
Knife gravers are for line engraving and deep cuts. Round gravers, flat gravers with a radius, are used on silver to create bright cuts, as well as other hard-to-cut metals such as nickel and steel. Square or V-point gravers are square or elongated diamond-shaped and used for cutting straight lines. V-point can be anywhere depending on purpose and effect; these gravers have small cutting points. Other tools such as mezzotint rockers and burnishers are used for texturing effects. Burnishing tools can be used for certain stone setting techniques. Musical instrument engraving on American-made brass instruments flourished in the 1920s and utilizes a specialized engraving technique where a flat graver is "walked" across the surface of the instrument to make zig-zag lines and patterns; the method for "walking" the graver may be referred to as "wriggle" or "wiggle" cuts. This technique is necessary due to the thinness of metal used to make musical instruments versus firearms or jewelry. Wriggle cuts are found on
Paper is a thin material produced by pressing together moist fibres of cellulose pulp derived from wood, rags or grasses, drying them into flexible sheets. It is a versatile material with many uses, including writing, packaging, decorating, a number of industrial and construction processes. Papers are essential in non-legal documentation; the pulp papermaking process is said to have been developed in China during the early 2nd century CE as early as the year 105 CE, by the Han court eunuch Cai Lun, although the earliest archaeological fragments of paper derive from the 2nd century BCE in China. The modern pulp and paper industry is global, with China leading its production and the United States right behind it; the oldest known archaeological fragments of the immediate precursor to modern paper date to the 2nd century BCE in China. The pulp paper-making process is ascribed to a 2nd-century CE Han court eunuch. In the 13th century, the knowledge and uses of paper spread from China through the Middle East to medieval Europe, where the first water powered paper mills were built.
Because paper was introduced to the West through the city of Baghdad, it was first called bagdatikos. In the 19th century, industrialization reduced the cost of manufacturing paper. In 1844, the Canadian inventor Charles Fenerty and the German F. G. Keller independently developed processes for pulping wood fibres. Before the industrialisation of paper production the most common fibre source was recycled fibres from used textiles, called rags; the rags were from hemp and cotton. A process for removing printing inks from recycled paper was invented by German jurist Justus Claproth in 1774. Today this method is called deinking, it was not until the introduction of wood pulp in 1843 that paper production was not dependent on recycled materials from ragpickers. The word "paper" is etymologically derived from Latin papyrus, which comes from the Greek πάπυρος, the word for the Cyperus papyrus plant. Papyrus is a thick, paper-like material produced from the pith of the Cyperus papyrus plant, used in ancient Egypt and other Mediterranean cultures for writing before the introduction of paper into the Middle East and Europe.
Although the word paper is etymologically derived from papyrus, the two are produced differently and the development of the first is distinct from the development of the second. Papyrus is a lamination of natural plant fibres, while paper is manufactured from fibres whose properties have been changed by maceration. To make pulp from wood, a chemical pulping process separates lignin from cellulose fibres; this is accomplished by dissolving lignin in a cooking liquor, so that it may be washed from the cellulose. Paper made from chemical pulps are known as wood-free papers–not to be confused with tree-free paper; the pulp can be bleached to produce white paper, but this consumes 5% of the fibres. There are three main chemical pulping processes: the sulfite process dates back to the 1840s and it was the dominant method extent before the second world war; the kraft process, invented in the 1870s and first used in the 1890s, is now the most practiced strategy, one of its advantages is the chemical reaction with lignin, that produces heat, which can be used to run a generator.
Most pulping operations using the kraft process are net contributors to the electricity grid or use the electricity to run an adjacent paper mill. Another advantage is that this process reuses all inorganic chemical reagents. Soda pulping is another specialty process used to pulp straws and hardwoods with high silicate content. There are two major mechanical pulps: groundwood pulp. In the TMP process, wood is chipped and fed into steam heated refiners, where the chips are squeezed and converted to fibres between two steel discs. In the groundwood process, debarked logs are fed into grinders where they are pressed against rotating stones to be made into fibres. Mechanical pulping does not remove the lignin, so the yield is high, >95%, however it causes the paper thus produced to turn yellow and become brittle over time. Mechanical pulps have rather short fibres. Although large amounts of electrical energy are required to produce mechanical pulp, it costs less than the chemical kind. Paper recycling processes can use mechanically produced pulp.
Most recycled paper contains a proportion of virgin fibre for the sake of quality. There are three main classifications of recycled fibre:. Mill broke or internal mill waste – This incorporates any substandard or grade-change paper made within the paper mill itself, which goes back into the manufacturing system to be re-pulped back into paper; such out-of-specification paper is not sold and is therefore not classified as genuine reclaimed recycled fibre, however most paper mills have been reusing their own waste fibre for many years, long before recycling became popular. Preconsumer waste – This is offcut and processing waste, such as guillotine trims and envelope blank waste.
Ultraviolet designates a band of the electromagnetic spectrum with wavelength from 10 nm to 400 nm, shorter than that of visible light but longer than X-rays. UV radiation is present in sunlight, contributes about 10% of the total light output of the Sun, it is produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, black lights. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce; the chemical and biological effects of UV are greater than simple heating effects, many practical applications of UV radiation derive from its interactions with organic molecules. Suntan and sunburn are familiar effects of over-exposure of the skin to UV, along with higher risk of skin cancer. Living things on dry land would be damaged by ultraviolet radiation from the Sun if most of it were not filtered out by the Earth's atmosphere.
More energetic, shorter-wavelength "extreme" UV below 121 nm ionizes air so that it is absorbed before it reaches the ground. Ultraviolet is responsible for the formation of bone-strengthening vitamin D in most land vertebrates, including humans; the UV spectrum thus has effects both harmful to human health. The lower wavelength limit of human vision is conventionally taken as 400 nm, so ultraviolet rays are invisible to humans, although some people can perceive light at shorter wavelengths than this. Insects and some mammals can see near-UV. Ultraviolet rays are invisible to most humans; the lens of the human eye blocks most radiation in the wavelength range of 300–400 nm. Humans lack color receptor adaptations for ultraviolet rays; the photoreceptors of the retina are sensitive to near-UV, people lacking a lens perceive near-UV as whitish-blue or whitish-violet. Under some conditions and young adults can see ultraviolet down to wavelengths of about 310 nm. Near-UV radiation is visible to insects, some mammals, birds.
Small birds have a fourth color receptor for ultraviolet rays. "Ultraviolet" means "beyond violet", violet being the color of the highest frequencies of visible light. Ultraviolet has a higher frequency than violet light. UV radiation was discovered in 1801 when the German physicist Johann Wilhelm Ritter observed that invisible rays just beyond the violet end of the visible spectrum darkened silver chloride-soaked paper more than violet light itself, he called them "oxidizing rays" to emphasize chemical reactivity and to distinguish them from "heat rays", discovered the previous year at the other end of the visible spectrum. The simpler term "chemical rays" was adopted soon afterwards, remained popular throughout the 19th century, although some said that this radiation was different from light; the terms "chemical rays" and "heat rays" were dropped in favor of ultraviolet and infrared radiation, respectively. In 1878 the sterilizing effect of short-wavelength light by killing bacteria was discovered.
By 1903 it was known. In 1960, the effect of ultraviolet radiation on DNA was established; the discovery of the ultraviolet radiation with wavelengths below 200 nm, named "vacuum ultraviolet" because it is absorbed by the oxygen in air, was made in 1893 by the German physicist Victor Schumann. The electromagnetic spectrum of ultraviolet radiation, defined most broadly as 10–400 nanometers, can be subdivided into a number of ranges recommended by the ISO standard ISO-21348: A variety of solid-state and vacuum devices have been explored for use in different parts of the UV spectrum. Many approaches seek to adapt visible light-sensing devices, but these can suffer from unwanted response to visible light and various instabilities. Ultraviolet can be detected by suitable photodiodes and photocathodes, which can be tailored to be sensitive to different parts of the UV spectrum. Sensitive ultraviolet photomultipliers are available. Spectrometers and radiometers are made for measurement of UV radiation.
Silicon detectors are used across the spectrum. Vacuum UV, or VUV, wavelengths are absorbed by molecular oxygen in the air, though the longer wavelengths of about 150–200 nm can propagate through nitrogen. Scientific instruments can therefore utilize this spectral range by operating in an oxygen-free atmosphere, without the need for costly vacuum chambers. Significant examples include 193 nm photolithography equipment and circular dichroism spectrometers. Technology for VUV instrumentation was driven by solar astronomy for many decades. While optics can be used to remove unwanted visible light that contaminates the VUV, in general, detectors can be limited by their response to non-VUV radiation, the development of "solar-blind" devices has been an important area of research. Wide-gap solid-state devices or vacuum devices with high-cutoff photocathodes can be attractive compared to silicon diodes. Extreme UV is characterized by a transition in the physics of interaction with matter. Wavelengths longer than about 30 nm interact with the outer valence electrons of atoms, while wavelengths shorter than that interact with inner-shell electrons and nuclei.
The long end of the EUV spectrum is set by a prominent He+ spectr