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Optical Properties of Cometary Dust

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Scientific rept. no. 5

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Cometary dust is observed in a variety of ways scattered light, thermal emission, stellar occultations, dust coma and tail morphology, radar echos, and meteors associated with comets. To interpret these observations with respect to the physical parameters of the dust size, shape, composition and the properties of the dust emitting region, the physics of how electromagnetic radiation interacts with small particles must be understood. To understand the interaction of electromagnetic radiation with matter, the measurement and interpretation of optical constants of bulk materials must be understood Classically, a material can be approximated by a series collection of bound charges with a number of resonant frequencies. The optical constants index of refraction or dielectric constant of a material are a measure of the ability of the material to vibrate in response to an incoming electromagnetic wave. The laboratory measurement of optical constants is very difficult, and many published optical constants may be in error. Care must be taken in choosing a set of optical constants which best represent the material being used for modelling cometary dust. Scattering theory is usually synonymous with Mie theory, although Mie theory pertains only to spherical particles. In many cases spheres may be good approximations to the particles in the coma, but an understanding of non-spherical scattering effects is essential to determine the limitations of the spherical approximation. A variety of methods exist which, although computationally intensive, provide insight into shape effects.

Subject Categories:

  • Astronomy
  • Optics

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