228 With the current development of optoelectronic applications the need to adjust the refractive index of polymeric materials is higher than ever before. Most of the conventional polymers show refractive indices between 1.3 and 1.7 only few polymers exhibit higher refractive indices such as polythiophene with n = 2.12. The refractive index is one of the important optical properties in focus for nanocomposite research at present. Leuteritz, in Polymer Science: A Comprehensive Reference, 2012 8.08.4.7 High and Low Refractive Index in Polymer Nanocomposites The Becke line appears to move toward the medium of higher refractive index when the distance between specimen and objective is increased, thus indicating whether the mounting liquid has a higher or lower refractive index than the fragment.
Either of these methods can be aided by observation of the Becke line, a bright fringe that appears just inside or outside the boundary of the fragment when it is almost in focus. Because in general the refractive index of the sample changes more slowly than that of the liquid as the temperature rises, a specific ‘match temperature’ can be determined. A development of this method involves the use of a microscope hot stage to raise the temperature of the liquid and sample gradually. The simplest of these is the examination of samples mounted in a range of liquids of which the refractive index is known until the sample–liquid interface becomes invisible. The refractive indices of glasses, minerals, plastics, and many other samples may be measured by a range of microscopical methods. Evennett, in Encyclopedia of Analytical Science (Second Edition), 2005 Refractive Index Measurement The match point of a glass fragment can be detected automatically, although the image of the fragment is displayed on a screen throughout the measuring process.Ĭ. At this point, known as the match point temperature, the bright line around the fragment disappears. The temperature of the hotstage is varied until the oil has the same RI as the glass. Monochromatic light from a sodium lamp or filtered light of a narrow range of wavelength produces a bright line around the edge of the fragment. The glass fragment is viewed by transmitted light through a phase contrast microscope. This enables the RI of the oil to be calculated for any particular hotstage temperature. The oil is calibrated by glass standards of known RI. Typical RI/temperature coefficients are 4×10 −4 C −1 for a liquid, and 1×10 −6 C −1 for a glass.Ī fragment of glass is immersed in a colorless oil and placed on a hotstage that is programmed to change temperature with time at a linear rate. The inherent variation in RI within the bulk of a single sheet of glass is of the order of 0.00005–0.0001, so greater accuracy and precision of measurement is unlikely to increase the discriminating power of this technique.Ĭurrent techniques of RI measurement are based on the fact that the RI of a liquid changes with temperature at a much greater rate than that of a solid.
As RI measurement can discriminate glasses with RI differences of 0.0001, glasses can be divided into a large number of groups by this technique. The RI of glasses examined in forensic science laboratories generally falls in the narrow range of 1.51–1.54, with the exception of borosilicate glass from motor vehicle headlamp lenses, which typically has an RI of 1.475–1.480. The RI of a glass depends upon the combination of raw materials used, together with the nature of the manufacturing process. It can be measured to a high degree of accuracy and precision, which is essentially independent of fragment size. Refractive index is the most useful physical property in forensic glass characterization. Lambert, in Encyclopedia of Analytical Science (Second Edition), 2005 Refractive Index