There are four main processes, by which the path of electromagnetic radiation or light can change direction, when passing through a medium containing a suspended particulate phase, see ELSD scattering mechanisms.
The importance of each of these processes depends on the radius of the particle (r) compared to the wavelength (λ) of the incident light. Rayleigh scattering is predominant when r/λ is < 5·10-2 . When particle dimensions are greater than λ/20 they no longer behave as point sources, and Mie scattering becomes predominant. Once particle size approaches the wavelength of incident light then reflection and refraction begin to prevail.
The relative importance of refraction and reflection can be understood by examining the effects of the incident light on a single spherical particle whose equilateral axis lies in the same plane as the photodetector and light source. With this configuration, refraction is of greater significance than reflection. Most organic compounds have refractive indices between 1.3 and 1.5 . Changes in the refractive index within this range will not greatly affect the quantity of light reaching the detector. This accounts for similarities in the sensitivity of the instrument to various compounds.
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