Particles of different densities either sink or float

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Particles of different densities either sink or float, depending on whether they are more or less dense than the liquid In order to distinguish between the normal internal density variations of a single sheet of glass and those of glasses of different origins, it is advisable to let the comparative density approach but not exceed a sensitivity value of 0.0003 g/ml - The flotation method meets this requirement and can adequately distinguish glass particles that differ in density by 0.001 g/ml Once glass has been distinguished by a density determination, differ origins are immediately concluded. The determination is best accomplished by the immersion method. - Glass particles are immersed in a liquid medium whose refractive index is adjusted until it equals that of the glass particles. - At this point, known as the match point, the observer notes the disappearance of the Becke line and minimum contrast between the glass and liquid medium.
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Becke line is a bright halo that is observed near the border of a particle immersed in a liquid of a different refractive index. The halo disappears when the medium and fragment have similar refractive indices. The refractive index of an immersion fluid is best adjusted by changing the temperature of the liquid - One approach to this procedure is to heat the liquid in a special apparatus known as a hot stage - Increasing the temperature of the liquid has a negligible effect on the refractive index of glass, whereas the liquid’s index decreases at the rate of approximately 0.0004 per degree Celsius The hot stage is designed to be used in conjunction with a microscope, through which the examiner can observe the disappearance of the Becke line on minute glass particles that illuminated with sodium d light or other wavelengths of light The examiner can determine the refractive index value of the immersion fluid as it changes with temperature. The exact numerical value of the glass refractive index can be calculated at the match point temperature The FBI laboratory has collected density and refractive index values from glass submitted to it for examination - The wide distribution of values clearly demonstrate that the refractive index is a highly distinctive property of glass and is thus useful for defining its frequency of occurrence and hence its evidential value Introduction of a technique that aims a high energy laser pulse to vaporize a microscopic amount of glass, raising its temperature by thousands of degrees - As a result, the elements present in the glass are induced to emit light whose wavelengths correspond to the identity of the elements present - The distinction between tempered and nontempered glass particles can be made by slowly heating and then cooling the glass (a process known as annealing).
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  • Fall '19
  • Light, Wavelength, refractive index

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