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It has been known that for all wavelengths of light, the index of refraction of a material is not the same. A good demonstration of this is by passing a white light through a prism. The more strongly refracted light is the light at the violet end of the spectrum. While the light at the red end of the spectrum is considered as the less refracted ones. The normal dispersion of the refractive indices is the relationship, in which indices of refraction decrease for increasing wavelengths of light. But there can be also instances where certain wavelength bands may have abnormal dispersion of the refractive indices. This case exhibits indices of refraction that increase for the increasing wavelengths. But unfortunately, almost all materials show abnormal dispersion at certain wavelength. But these wavelengths may be found outside the spectrum. This is why the terms mentioned above are considered somewhat misleading.

 

            The consequence of the interaction of light with the natural resonant frequencies of the electron clouds around each atom is called dispersion. What causes the electron cloud around each atom to resonate at the frequency of the light is the electric vector of the light. The light is then re-emitted by the atom. Only that, it is not in phase with the incident light. It should be noted that the degree to which the re-emitted light is out of phase is very much dependent on the degree to which the frequency of the incident light differs from the natural resonant frequency of the electron clouds. It can be shown, through a complex set of equations that the index increases with the increasing frequency and of course decreasing wavelength, producing normal dispersion if the frequency of the light is significantly different from a resonant frequency of the electron clouds. The light is considered strongly absorbed and the index of refraction sharply decreases with the increasing frequency and decreasing wavelength if the frequency of light is nearly the same as one of the natural resonant frequencies of the electron clouds. This thereby produces abnormal dispersion.

 

            It is also necessary to report the index of refraction at several wavelengths when describing the dispersion of the material. The indices that are usually reported for light by convention are those of light 486nm, 589nm, and 656nm. These wavelengths are used because they happen to correspond to certain wavelengths called Fraunhoper lines. These Fraunhoper lines are absorption lines in the spectrum of the sun. Also, 589nm is actually the wavelength produced by a sodium vapor lamp and it is almost near the middle of the visible spectrum. The large coefficient dispersion means that the material shows a large change of index as a function of wavelength. This can be described through the indices of lights having wavelengths 486nm and 656nm. Another related term is the dispersive power. The large value for dispersive power usually means that the mineral exhibits a large change of index as a function of wavelength.



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Wednesday, February 27th, 2008 at 3:46 am
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Optical Mineralogy
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