The image of a particle is always affected by the properties of the medium in which they are mounted. This is the case for a particle in air or any other mounting medium. It is the image of the particle that is seen, and if the mounting medium is changed, then the particle image is altered. A (colorless) particle in a fixed mount is visible as a result of the size, shape, and differences between the refractive index of the particle and the refractive index of the mounting medium. The properties of the mounting medium become the reference by which the properties of the particle are measured. If the particle is already fixed in an adhesive layer, as is the case with a sticky-tape lift, then knowing and matching the refractive index of the adhesive may be a consideration to minimize artifacts that may be introduced by the adhesive. A collection of loose individual particles may be prepared in a permanent mounting medium selected for the properties of the medium itself, e.g. dispersion staining, wherein the medium may be selected to match the refractive index of a particle of interest at some wavelength. Or, it may be selected to have a significantly different refractive index to optimize morphological features, as might be the case with pollen identification. For electrically conductive particles, the refractive index is equal to the square root of the product of the dielectric constant times the magnetic permeability. The imaginary part of this calculation is the part of interest, because it is the difference in the imaginary refractive index that creates a visible effect. The selection of the mounting medium is part of the analysis, and it controls or limits the interfacial optical properties that can be seen.
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