Refractive indices particulate scattering

An approach widely used by atmospheric scientists is to infer the imaginary part of the refractive index k from measurements of the absorption coefficient a of particulate samples. Diffuse reflection, the photoacoustic effect, and integrating plates have been used for determining absorption even in the presence of considerable scattering these methods are discussed briefly in the following section. The relation (2.52) between a and k, a - 4nk/, is, of course, strictly valid only for homogeneous media. But under some circum-... 

For particulate materials with high real indices, the reported size depends less on the complex component (7). Also, as the size of particles increases, the total scattering phenomenon simplifies to Fraunhofer diffraction, and becomes a very weak function of refractive index. 

Refractive index influences light scattering in fillers and pigments. A correct choice in the refractive index of the particulate material and binder permits a formulation of transparent materials containing fillers (for further information on light scattering see Chapter 2, especially section on titanium dioxide and Section 5.3). Table 5.16 gives an overview of refractive indices of various fillers. 

Figure 12. Solid curve the refractive index of water showing a simple, monotonic dispersion curve. Dotted curve the contribution of the 189 nm band of iV-methylacetamide at a 1 M concentration added on to the water curve using equation 19. The point to be made is that there is no way to match the background curve of a good solvent to the anomalous dispersion of a chromophoric system under study. Such matching is commonly attempted to remove light scattering problems which depend on the difference in refractive index of the particle, n, with that of the solvent, n, i.e., (tip—nf). The dashed curve adds the second dispersion term in equation 25. Also included is the calculated refractive index of particulate poly-L-glutamic acid (PGA) (see section 4(cKii))-...

Another variant on the refractive index detector arose from the work of Christiansen on crystal filters.If a cell is packed with particulate material having the same refractive index as the mobile phase passing through it, light will pass through the cell with little or no refraction or scattering. If, however, the refractive index of the mobile phase... 

As dry powders, several common fillers appear white, because they scatter light strongly, and it might therefore be supposed that they would make good pigments for polymers. This is usually not the case however, because the amount of scattering is determined by the difference in refractive index between the particulate and continuous phases. This... 

Submicron particle size analysis employs a scatter theory not completely described by Fraunhofer diffraction. The small particle range down to about 0.1 micrometer in diameter utilizes a combination of Fraunhofer diffraction and Mie theory for the forward scattered light and 90-degree Mie scatter at three (3) wavelengths and two (2) polarizations of each wavelength. Because of its need for a technology more involved than diffraction theory, submicron measurements are influenced by the index of refraction of the material making up the particulates to be sized. 

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