Diffraction refers to various phenomena that occur when a wave encounters an obstacle or a slit. It is defined as the bending of waves around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle. In classical physics, the diffraction phenomenon is described as the interference of waves according to the Huygens–Fresnel principle that treats each point in the wave-front as a collection of individual spherical wavelets; these characteristic behaviors are exhibited when a wave encounters an obstacle or a slit, comparable in size to its wavelength. Similar effects occur when a light wave travels through a medium with a varying refractive index, or when a sound wave travels through a medium with varying acoustic impedance. Diffraction has an impact on the acoustic space. Diffraction occurs with all waves, including sound waves, water waves, electromagnetic waves such as visible light, X-rays and radio waves. Since physical objects have wave-like properties, diffraction occurs with matter and can be studied according to the principles of quantum mechanics.
Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1660. While diffraction occurs whenever propagating waves encounter such changes, its effects are most pronounced for waves whose wavelength is comparable to the dimensions of the diffracting object or slit. If the obstructing object provides multiple spaced openings, a complex pattern of varying intensity can result; this is due to the addition, or interference, of different parts of a wave that travel to the observer by different paths, where different path lengths result in different phases. The formalism of diffraction can describe the way in which waves of finite extent propagate in free space. For example, the expanding profile of a laser beam, the beam shape of a radar antenna and the field of view of an ultrasonic transducer can all be analyzed using diffraction equations; the effects of diffraction are seen in everyday life. The most striking examples of diffraction are those.
This principle can be extended to engineer a grating with a structure such that it will produce any diffraction pattern desired. Diffraction in the atmosphere by small particles can cause a bright ring to be visible around a bright light source like the sun or the moon. A shadow of a solid object, using light from a compact source, shows small fringes near its edges; the speckle pattern, observed when laser light falls on an optically rough surface is a diffraction phenomenon. When deli meat appears to be iridescent, diffraction off the meat fibers. All these effects are a consequence of the fact. Diffraction can occur with any kind of wave. Ocean waves diffract around other obstacles. Sound waves can diffract around objects, why one can still hear someone calling when hiding behind a tree. Diffraction can be a concern in some technical applications; the effects of diffraction of light were first observed and characterized by Francesco Maria Grimaldi, who coined the term diffraction, from the Latin diffringere,'to break into pieces', referring to light breaking up into different directions.
The results of Grimaldi's observations were published posthumously in 1665. Isaac Newton attributed them to inflexion of light rays. James Gregory observed the diffraction patterns caused by a bird feather, the first diffraction grating to be discovered. Thomas Young performed a celebrated experiment in 1803 demonstrating interference from two spaced slits. Explaining his results by interference of the waves emanating from the two different slits, he deduced that light must propagate as waves. Augustin-Jean Fresnel did more definitive studies and calculations of diffraction, made public in 1815 and 1818, thereby gave great support to the wave theory of light, advanced by Christiaan Huygens and reinvigorated by Young, against Newton's particle theory. In traditional classical physics diffraction arises because of the way; the propagation of a wave can be visualized by considering every particle of the transmitted medium on a wavefront as a point source for a secondary spherical wave. The wave displacement at any subsequent point is the sum of these secondary waves.
When waves are added together, their sum is determined by the relative phases as well as the amplitudes of the individual waves so that the summed amplitude of the waves can have any value between zero and the sum of the individual amplitudes. Hence, diffraction patterns have a series of maxima and minima. In the modern quantum mechanical understanding of light propagation through a slit every photon has what is known as a wavefunction which describes its path from the emitter through the slit to the screen; the wavefunction is determined by the physical surroundings such as slit geometry, screen distance and initial conditions when the photon is created. In important experiments the existence of the photon's wavef
A micrograph or photomicrograph is a photograph or digital image taken through a microscope or similar device to show a magnified image of an object. This is opposed to a macrograph or photomacrograph, an image, taken on a microscope but is only magnified less than 10 times. Micrography is the art of using microscopes to make photographs. A micrograph contains extensive details of microstructure. A wealth of information can be obtained from a simple micrograph like behavior of the material under different conditions, the phases found in the system, failure analysis, grain size estimation, elemental analysis and so on. Micrographs are used in all fields of microscopy. A light micrograph or photomicrograph is a micrograph prepared using an optical microscope, a process referred to as photomicroscopy. At a basic level, photomicroscopy may be performed by connecting a camera to a microscope, thereby enabling the user to take photographs at reasonably high magnification. Scientific use began in England in 1850 by Prof Richard Hill Norris FRSE for his studies of blood cells.
Roman Vishniac was a pioneer in the field of photomicroscopy, specializing in the photography of living creatures in full motion. He made major developments in light-interruption photography and color photomicroscopy. Photomicrographs may be obtained using a USB microscope attached directly to a home computer or laptop. An electron micrograph is a micrograph prepared using an electron microscope. Micrographs have micron bars, or magnification ratios, or both. Magnification is a ratio between the size of an object on its real size. Magnification can be a misleading parameter as it depends on the final size of a printed picture and therefore varies with picture size. A scale bar, or micron bar, is a line of known length displayed on a picture; the bar can be used for measurements on a picture. When the picture is resized the bar is resized making it possible to recalculate the magnification. Ideally, all pictures destined for publication/presentation should be supplied with a scale bar. All but one of the micrographs presented on this page do not have a micron bar.
The microscope has been used for scientific discovery. It has been linked to the arts since its invention in the 17th century. Early adopters of the microscope, such as Robert Hooke and Antonie van Leeuwenhoek, were excellent illustrators. After the invention of photography in the 1820s the microscope was combined with the camera to take pictures instead of relying on an artistic rendering. Since the early 1970s individuals have been using the microscope as an artistic instrument. Websites and traveling art exhibits such as the Nikon Small World and Olympus Bioscapes have featured a range of images for the sole purpose of artistic enjoyment; some collaborative groups, such as the Paper Project have incorporated microscopic imagery into tactile art pieces as well as 3D immersive rooms and dance performances. Close-up Digital microscope Macro photography Microphotograph Microscopy USB microscope Make a Micrograph – This presentation by the research department of Children's Hospital Boston shows how researchers create a three-color micrograph.
Shots with a Microscope – a basic, comprehensive guide to photomicrography Scientific photomicrographs – free scientific quality photomicrographs by Doc. RNDr. Josef Reischig, CSc. Micrographs of 18 natural fibres by the International Year of Natural Fibres 2009 Seeing Beyond the Human Eye Video produced by Off Book - Solomon C. Fuller bio Charles Krebs Microscopic Images Dennis Kunkel Microscopy Andrew Paul Leonard, APL Microscopic Cell Centered Database - Montage Nikon Small World Olympus Bioscapes Other examples
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, soil, bones, or metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a diamond ground optically flat, it is mounted on a glass slide and ground smooth using progressively finer abrasive grit until the sample is only 30 μm thick. The method involved using the Michel-Lévy interference colour chart. Quartz is used as the gauge to determine thickness as it is one of the most abundant minerals; when placed between two polarizing filters set at right angles to each other, the optical properties of the minerals in the thin section alter the colour and intensity of the light as seen by the viewer. As different minerals have different optical properties, most rock forming minerals can be identified. Plagioclase for example can be seen in the photo on the right as a clear mineral with multiple parallel twinning planes.
The large blue-green minerals are clinopyroxene with some exsolution of orthopyroxene. Thin sections are prepared in order to investigate the optical properties of the minerals in the rock; this work helps to reveal the origin and evolution of the parent rock. A photograph of a rock in thin section is referred to as a photomicrograph. Under thin section, in plane polarized light, quartz is colorless with no cleavage, its habit is either equant or anhedral if it infills around other minerals as a cement. Under cross polarized light quartz displays low interference colors and is the defining mineral used to determine if the thin section is at standardized thickness of 30 microns as quartz will only display up to a pale yellow interference color and no further at that thickness, it is common in most rocks so it will be available to judge the thickness. In thin section, quartz grain provenance in a sedimentary rock can be estimated. In crossed polarized light, the quartz grain can go extinct all at once, called monocrystalline quartz, or in waves, called polycrystalline quartz.
The extinction in waves is called undulose extinction and indicates dislocation walls in mineral grains. Dislocation walls are where dislocations, intracrystalline deformation via movement of a dislocation front within a plane, organize themselves into planes of sufficient quantity, they change the crystallographic orientation across the walls, so for example in quartz, the two sides of the wall will have different extinction angles and thus result in undulose extinction. Since undulose extinction requires dislocation walls to have developed, these occur more at higher pressures and temperatures, quartz grains with undulose extinction indicate metamorphic rock provenance for that grain; those grains that are monocrystalline quartz are more to have been formed by igneous processes. Differing sources suggest the extent; some note the trend for immature sandstones to have less polycrystalline quartz grains compared to mature sandstones, which have grains that have passed through many sedimentary cycles.
Quartz grains derived from previous sedimentary sources are determined by looking for authigenic, or grown in place, overgrowths of silica cement over the grain. The above descriptions of quartz in thin section is enough to identify it. Minerals with similar appearance may include plagioclase, although it can be distinguished by the distinctive twinning in crossed polarized light and cleavage in plane polarized light, cordierite, although it can be distinguished by twinning or inclusions in the grain. However, for certainty, other distinguishing features of quartz include the fact that it is uniaxial, it has a positive optic sign, length-slow sign of elongation, zero degree extinction angle. Fine-grained rocks those containing minerals of high birefringence, such as calcite, are sometimes prepared as ultra-thin sections. An ordinary 30 μm thin section is prepared as described above but the slice of rock is attached to the glass slide using a soluble cement such as Canada balsam to allow both sides to be worked on.
The section is polished on both sides using a fine diamond paste until it has a thickness in the range of 2-12 μm. This technique has been used to study the microstructure of fine-grained carbonates such as the Lochseitenkalk mylonite in which the matrix grains are less than 5 μm in size; this method is sometimes used in the preparation of mineral and rock specimens for transmission electron microscopy and allows greater accuracy in comparing features using both optical and electron imaging. Ceramography: thin sections of ceramics Shelley, D. Optical Mineralogy, Second Edition. University of Canterbury, New Zealand. Thin sections of soils. Collection of Prof. Kubiëna
Conoscopy is an optical technique to make observations of a transparent specimen in a cone of converging rays of light. The various directions of light propagation are observable simultaneously. A conoscope is an apparatus to carry out conoscopic observations and measurements realized by a microscope with a Bertrand lens for observation of the direction's image; the earliest reference to the use of conoscopy for evaluation of the optical properties of liquid crystalline phases is in 1911 when it was used by Mauging to investigate the alignment of nematic and chiral-nematic phases. A beam of convergent light is known to be a linear superposition of many plane waves over a cone of solid angles; the raytracing of Figure 1 illustrates the basic concept of conoscopy: transformation of a directional distribution of rays of light in the front focal plane into a lateral distribution appearing in the back focal plane. The incoming elementary parallel beams are converging in the back focal plane of the lens with the distance of their focal point from the optical axis being a function of the angle of beam inclination.
This transformation can be deduced from two simples rules for the thin positive lens: the rays through the center of the lens remain unchanged, the rays through the front focal point are transformed into parallel rays. The object of measurement is located in the front focal plane of the lens. In order to select a specific area of interest on the object an aperture can be placed on top of the object. In this configuration only rays from the measuring spot hit the lens; the image of the aperture is projected to infinity while the image of the directional distribution of the light passing through the aperture is generated in the back focal plane of the lens. When it is not considered appropriate to place an aperture into the front focal plane of the lens, i.e. on the object, the selection of the measuring spot can be achieved by using a second lens. An image of the object is generated in the back focal plane of the second lens; the magnification, M, of this imaging is given by the ratio of the focal lengths of the lenses L1 and L2, M = f2 / f1.
A third lens transforms the rays passing through the aperture into a second directions image which may be analyzed by an image sensor. The functional sequence is as follows: the first lens forms the directions image, the second lens together with the first projects an image of the object, the aperture allows selection of the area of interest on the object, the third lens together with the second images the directions image on a 2-dimensional optical sensor; this simple arrangement is the basis for all conoscopic devices. It is not straight forward however to design and manufacture lens systems that combine the following features: maximum angle of light incidence as high as possible, diameter of measuring spot up to several millimeters, achromatic performance for all angles of inclination, minimum effect of polarization of incident light. Design and manufacturing of this type of complex lens system requires assistance by numerical modelling and a sophisticated manufacturing process. Modern advanced conoscopic devices are used for rapid measurement and evaluation of the electro-optical properties of LCD-screens.
Pochi Yeh, Claire Gu: "Optics of Liquid Crystal Displays", John Wiley & Sons 1999, 4.5. Conoscopy, pp. 139 Hartshorne & Stuart: "Crystals and the Polarizing Microscope", London, 1970, 8: The Microscopic Examination of Crystals, Conoscopic Observations C. Burri: "Das Polarisationsmikroskop", Verlag Birkhäuser, Basel 1950 Polariscope/Conoscope - Gemology Project
Lithic fragment (geology)
Lithic fragments, or lithics, are pieces of other rocks that have been eroded down to sand size and now are sand grains in a sedimentary rock. They were first described and named by Bill Dickinson in 1970. Lithic fragments can be derived from igneous or metamorphic rocks. A lithic fragment is defined using the Gazzi-Dickinson point-counting method and being in the sand-size fraction. Sand grains in sedimentary rocks that are fragments of larger rocks that are not identified using the Gazzi-Dickinson method are called rock fragments instead of lithic fragments. Sandstones rich in lithic fragments are called lithic sandstones; these can include granular, microlitic and vitric. These correlations between composition and volcanic lithic fragment type are approximate, at best. By definition, intrusive igneous rock fragments can not be considered lithic fragments; these can include shale siltstone fragments, chert. These can include fine-grained schist and phyllite fragments, among others
A polarizer or polariser is an optical filter that lets light waves of a specific polarization pass through while blocking light waves of other polarizations. It can filter a beam of light of undefined or mixed polarization into a beam of well-defined polarization, polarized light; the common types of polarizers are circular polarizers. Polarizers are used in many optical techniques and instruments, polarizing filters find applications in photography and LCD technology. Polarizers can be made for other types of electromagnetic waves besides light, such as radio waves, X-rays. Linear polarizers can be divided into two general categories: absorptive polarizers, where the unwanted polarization states are absorbed by the device, beam-splitting polarizers, where the unpolarized beam is split into two beams with opposite polarization states. Polarizers which maintain the same axes of polarization with varying angles of incidence are called Cartesian polarizers, since the polarization vectors can be described with simple Cartesian coordinates independent from the orientation of the polarizer surface.
When the two polarization states are relative to the direction of a surface, they are termed s and p. This distinction between Cartesian and s–p polarization can be negligible in many cases, but it becomes significant for achieving high contrast and with wide angular spreads of the incident light. Certain crystals, due to the effects described by crystal optics, show dichroism, preferential absorption of light, polarized in particular directions, they can therefore be used as linear polarizers. The best known crystal of this type is tourmaline. However, this crystal is used as a polarizer, since the dichroic effect is wavelength dependent and the crystal appears coloured. Herapathite is dichroic, is not coloured, but is difficult to grow in large crystals. A Polaroid polarizing filter functions on an atomic scale to the wire-grid polarizer, it was made of microscopic herapathite crystals. Its current H-sheet form is made from polyvinyl alcohol plastic with an iodine doping. Stretching of the sheet during manufacture causes the PVA chains to align in one particular direction.
Valence electrons from the iodine dopant are able to move linearly along the polymer chains, but not transverse to them. So incident light polarized; the durability and practicality of Polaroid makes it the most common type of polarizer in use, for example for sunglasses, photographic filters, liquid crystal displays. It is much cheaper than other types of polarizer. A modern type of absorptive polarizer is made of elongated silver nano-particles embedded in thin glass plates; these polarizers are more durable, can polarize light much better than plastic Polaroid film, achieving polarization ratios as high as 100,000:1 and absorption of polarized light as low as 1.5%. Such glass polarizers perform best for short-wavelength infrared light, are used in optical fiber communications. Beam-splitting polarizers split the incident beam into two beams of differing linear polarization. For an ideal polarizing beamsplitter these would be polarized, with orthogonal polarizations. For many common beam-splitting polarizers, only one of the two output beams is polarized.
The other contains a mixture of polarization states. Unlike absorptive polarizers, beam splitting polarizers do not need to absorb and dissipate the energy of the rejected polarization state, so they are more suitable for use with high intensity beams such as laser light. True polarizing beamsplitters are useful where the two polarization components are to be analyzed or used simultaneously; when light reflects at an angle from an interface between two transparent materials, the reflectivity is different for light polarized in the plane of incidence and light polarized perpendicular to it. Light polarized in the plane is said to be p-polarized, while that polarized perpendicular to it is s-polarized. At a special angle known as Brewster's angle, no p-polarized light is reflected from the surface, thus all reflected light must be s-polarized, with an electric field perpendicular to the plane of incidence. A simple linear polarizer can be made by tilting a stack of glass plates at Brewster's angle to the beam.
Some of the s-polarized light is reflected from each surface of each plate. For a stack of plates, each reflection depletes the incident beam of s-polarized light, leaving a greater fraction of p-polarized light in the transmitted beam at each stage. For visible light in air and typical glass, Brewster's angle is about 57°, about 16% of the s-polarized light present in the beam is reflected for each air-to-glass or glass-to-air transition, it takes many plates to achieve mediocre polarization of the transmitted beam with this approach. For a stack of 10 plates, about 3% of the s-polarized light is transmitted; the reflected beam, while polarized, is spread out and may not be useful. A more useful polarized beam can be obtained by tilting the pile of plates at a steeper angle to the incident beam. Counterintuitively, using incident angles greater than Brewster's angle yields a higher degree of polarization of the transmitted beam, at the expense of decreased overall transmission. For angles of incidence steeper than 80° the polarization of the transmitted beam can approach 100% with as few as four plates, although the transmitted intensity is low in this case.
Adding more plates and reducing
A mineral is, broadly speaking, a solid chemical compound that occurs in pure form. A rock may consist of a single mineral, or may be an aggregate of two or more different minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are excluded, but some minerals are biogenic and/or are organic compounds in the sense of chemistry. Moreover, living beings synthesize inorganic minerals that occur in rocks. In geology and mineralogy, the term "mineral" is reserved for mineral species: crystalline compounds with a well-defined chemical composition and a specific crystal structure. Minerals without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound may occur with different crystal structures, each structure is considered different mineral species. Thus, for example and stishovite are two different minerals consisting of the same compound, silicon dioxide; the International Mineralogical Association is the world's premier standard body for the definition and nomenclature of mineral species.
As of November 2018, the IMA recognizes 5,413 official mineral species. Out of more than 5,500 proposed or traditional ones; the chemical composition of a named mineral species may vary somewhat by the inclusion of small amounts of impurities. Specific varieties of a species sometimes have official names of their own. For example, amethyst is a purple variety of the mineral species quartz; some mineral species can have variable proportions of two or more chemical elements that occupy equivalent positions in the mineral's structure. Sometimes a mineral with variable composition is split into separate species, more or less arbitrarily, forming a mineral group. Besides the essential chemical composition and crystal structure, the description of a mineral species includes its common physical properties such as habit, lustre, colour, tenacity, fracture, specific gravity, fluorescence, radioactivity, as well as its taste or smell and its reaction to acid. Minerals are classified by key chemical constituents.
Silicate minerals comprise 90% of the Earth's crust. Other important mineral groups include the native elements, oxides, carbonates and phosphates. One definition of a mineral encompasses the following criteria: Formed by a natural process. Stable or metastable at room temperature. In the simplest sense, this means. Classical examples of exceptions to this rule include native mercury, which crystallizes at −39 °C, water ice, solid only below 0 °C. Modern advances have included extensive study of liquid crystals, which extensively involve mineralogy. Represented by a chemical formula. Minerals are chemical compounds, as such they can be described by fixed or a variable formula. Many mineral groups and species are composed of a solid solution. For example, the olivine group is described by the variable formula 2SiO4, a solid solution of two end-member species, magnesium-rich forsterite and iron-rich fayalite, which are described by a fixed chemical formula. Mineral species themselves could have a variable composition, such as the sulfide mackinawite, 9S8, a ferrous sulfide, but has a significant nickel impurity, reflected in its formula.
Ordered atomic arrangement. This means crystalline. An ordered atomic arrangement gives rise to a variety of macroscopic physical properties, such as crystal form and cleavage. There have been several recent proposals to classify amorphous substances as minerals; the formal definition of a mineral approved by the IMA in 1995: "A mineral is an element or chemical compound, crystalline and, formed as a result of geological processes." Abiogenic. Biogenic substances are explicitly excluded by the IMA: "Biogenic substances are chemical compounds produced by biological processes without a geological component and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound the product can be accepted as a mineral."The first three general characteristics are less debated than the last two. Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. Recent changes have included the addition of an organic class, in both the new Dana and the Strunz classification schemes.
The organic class includes a rare group of minerals with hydrocarbons. The IMA Commission on New Minerals and Mineral Names adopted in 2009 a hierarchical scheme for the naming and classification of mineral groups and group names and established seven commissions and four working groups to review and classify minerals into an official listing of their published names. According to these new r