On the Size Parameter for Thermally-Emitting Particles

Details On the Size Parameter for Thermally-Emitting Particles On the Size Parameter for Thermally-Emitting Particles

May 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999aas...194.4409l&link_type=abstract

American Astronomical Society, 194th AAS Meeting, #44.09; Bulletin of the American Astronomical Society, Vol. 31, p.885

Physics

Optics

Scientific paper

The conventional means of quickly determining the scattering regime of a particle is the size parameter X = 2pi a/lambda where a is the particle radius and lambda is the wavelength of light being scattered. As arguments to the Bessell functions in Mie theory, however, X is multiplied by m = N_p/N where N_p = n +ik is the complex index of the particle and N is the index of the surrounding medium (= 1.0 for a vacuum). The quantity mX is complex (mX = 2pi a/lambda +i2pi ka/lambda ) For thermal emission from particles, we show that the opacity parameter Omega = 4pi ka/lambda = 2kX is a more accurate means of distinguishing between ``small" and ``large" particles. Here ``large" particles emit in a black body-like manner and ``small" particles emit with spectral structure indicative of their optical constants. Omega is approximately equal to the particle's mean optical depth and is equal to two times the imaginary part of mX. Omega can be derived from both scattering theory and geometrical optics.

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