Light scattering from large particles

We seek here a description of the scattering from large particles compared with the wavelength of light. In this limit the dominant scattering occurs at small angles. Furthermore, the major contribution to the scattering field arises from interactions with the particle boundary. 

From Rayleigh theory, the intensity of light scattered from each particle depends largely on its size and shape and on the difference in refractive index between the particle and the medium. For a suspension, each spherical particle scatters light at an intensity Id at a distance x from the particle, according to the following relationship ... 

S.R. Aragon, R. Pecora, Theory of dynamic light scattering from large anisotropic particles. 

S.R. Aragon, R. Pecora, Theory of dynamic light scattering from large anisotropic particles. J. Chem. Phys. 66(6), 2506-2516 (1977). doi 10.1063/1.434246... 

Aragon, S. R., Pecora, R., Theory of Dynamic Light Scattering from Large Anisotropic Particles, J. Chem. Phys., 1977,66,2506-2516. 

Figure 3.7 (a) Scattering from a relatively large particle, (b) Radiation envelope for light scattered from a spherical particle (jc = 0.8, m = 1.25). See text and Figure 3.6 for explanation... 

Fraunhofer diffraction theory combines the above results to compute the light scattered at small angles from large particles. Such a particle is pictured in Figure 4.15. 

The measurement does not require particles to pass through the laser beam one at a time. In fact, there are normally large numbers of particles in the beam at any point in time. Each particle forms its own diffraction pattern, and the system sums the results from these patterns over the total measurement period. It is important that the particulate concentration be low enough so that multiple scattering does not occur. This happens when light which is scattered from one particle strikes another particle and is scattered further before it is collected. This phenomenon is easily detected, and the loading range is usually quite broad. 

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