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Becke Line Test
The Becke line test is a technique in optical mineralogy that helps determine the relative refractive index of two materials. It is done by lowering the stage (increasing the focal distance) of the petrographic microscope and observing which direction the light appears to move. This movement will always go into the material of higher refractive index. This index is determined by comparing two minerals directly, or comparing a mineral to a reference material such as Canada Balsam or an oil of known refractive index (oil immersion). When permanently mounted to a slide under a cover slip, the mounting medium is normally chosen to have the same refractive index as Canada Balsam (''n''=1.55) to avoid confusion when comparing with previously made slides. If a different mounting medium is used, it's refractive index should be recorded ''on the slide'', to avoid loss of the information. Media used for impregnating a specimen before sectioning (either for mechanical strength, or to pick ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Optical Mineralogy
Optical mineralogy is the study of minerals and rocks by measuring their optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrographic microscope. Optical mineralogy is used to identify the mineralogical composition of geological materials in order to help reveal their origin and evolution. Some of the properties and techniques used include: * Refractive index * Birefringence * Michel-Lévy Interference colour chart * Pleochroism * Extinction angle * Conoscopic interference pattern (Interference figure) * Becke line test * Optical relief * Sign of elongation (Length fast vs. length slow) * Wave plate History William Nicol, whose name is associated with the creation of the Nicol prism, is likely the first to prepare thin slices of mineral substances, and his methods were applied by Henry Thronton Maire Witham (1831) to the study of plant petrifactions. This method, of significant im ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Refractive Index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or refracted, when entering a material. This is described by Snell's law of refraction, , where ''θ''1 and ''θ''2 are the angle of incidence and angle of refraction, respectively, of a ray crossing the interface between two media with refractive indices ''n''1 and ''n''2. The refractive indices also determine the amount of light that is reflected when reaching the interface, as well as the critical angle for total internal reflection, their intensity ( Fresnel's equations) and Brewster's angle. The refractive index can be seen as the factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values: the speed of light in a medium is , and similarly the wavelength in that medium is , where ''Π... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Petrographic Microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks. The method is called "polarized light microscopy" (PLM). __TOC__ Description Depending on the grade of observation required, petrological microscopes are derived from conventional brightfield microscopes of similar basic capabilities by: * Adding a Nicol prism polarizer filter to the light path beneath the sample slide * Replacing the normal stage with a circular rotating stage (typically graduated with vernier scales for reading orientations to better than 1 degree of arc) * Adding a second rotatable and removable Nicol prism filter, called the analyzer, to the light path between objective and eyepiece * Adding a phase telescope, also known as a Bertrand lens, which allows the viewer to see ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Oil Immersion
In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index, thereby increasing the numerical aperture of the objective lens. Without oil, light waves reflect off the slide specimen through the glass cover slip, through the air, and into the microscope lens (see the colored figure to the right). Unless a wave comes out at a 90-degree angle, it bends when it hits a new substance, the amount of bend depending on the angle. This distorts the image. Air has a very different index of refraction from glass, making for a larger bend compared to oil, which has an index more similar to glass. Specially manufactured oil can have nearly exactly the same refractive index as glass, making an oil immersed lens nearly as effective as having entirely glass to the sample (which would be impractical). Immersion oils are transparent oils t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Friedrich Johann Karl Becke
Friedrich Johann Karl Becke (31 December 1855, in Prague – 18 June 1931, in Vienna) was an Austrian mineralogist and petrographer. Biography After studying at the University of Vienna, where he specialized in the natural sciences, he became there a lecturer on geology. In 1882 he was appointed professor at the University of Czernowitz. Eight years later he received a similar appointment at Charles University in Prague, Prague, but soon after went to Vienna, where he became professor of mineralogy, succeeding Gustav Tschermak von Seysenegg as such, of whose periodical ''Mineralogische und Petrographische Mittheilungen'' he became editor. He published many papers on the science of geology and mineralogy, but he was best known on account of his researches in the field of rock-forming minerals and how they may be determined by means of their light refraction, light-refractive properties. The results of these studies were published by the Vienna Academy (1893). His doctoral students ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
