![]() ![]() Most of the world’s supply of precious opal comes from the Coober Pedy and Andamooka fields in South Australia. Potch accounts for 80% to 90% of all opal, and has no commercial value. Common opal, sometimes called "potch," is usually colorless or white, but the presence of fine-grained impurities may color it gray, brown, yellow, or red. The "fire opal" is a transparent or translucent opal ranging in color from yellow to orange to bright red. ![]() Precious opal is identified by the defining "play of color," or the way in which colors change within a particular stone as it is rotated and tilted. Opals of gemstone quality usually contain 6% to 10% water. Opal is an amorphous form of silica (SiO 2.nH 2O), chemically similar to quartz (SiO 2), but containing 3% to 21% water within its mineral structure. Throughout the world, silica gel precipitates at low temperatures to form layers or nodules of opal in fissures, veins, and cavities of volcanic and sedimentary rocks. Opal is formed from silica-bearing waters and can be found inside any type of rock. Later, the Latin word "opalus" came to mean "precious stone." The name evolved from the Greek "opallus," which refers to a change in color. They are the subject of much folklore, and have been considered lucky in some cultures, and thought of as unlucky in others. As the number of slits increases, the intensity maxima become very sharp and narrow. Light passing through the grating at selected angles is in phase, and constructive interference creates light intensity maxima. The diffraction angle varies with the wavelength of the light. Light rays that pass through a diffraction grating are bent as a result of diffraction. The distance between the slits or grooves is typically close to the wavelength of light. Diffraction is a property relating to the wave nature of light.Ī diffraction grating is an array of thousands of narrow parallel slits or grooves. Different wavelengths of light are diffracted at different angles, so different colors are seen.ĭiffraction is the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Diffraction is thus a function of refractive index, wavelength, spacing of the slits, and angle of incidence. Diffraction occurs when the conditions in the Bragg equation n =2dsin are satisfied. When light rays pass through multiple slits, they are diffracted and then interfere constructively and destructively to produce the arrangements as shown above. ![]()
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