When a dust grain, composed of discrete electric charges, is illuminated by an electromagnetic wave, the electric field of the incident electromagnetic wave will set the electric charges in the dust into oscillatory motion. These accelerated electric charges radiate electromagnetic energy in all directions, at the same frequency as that of the incident wave. This process, known as "scattering" (to be more precise, elastic scattering), removes energy from the incident beam of electromagnetic radiation. Absorption also arises as the excited charges transform part of the incident electromagnetic energy into thermal energy. The combined effect of absorption and scattering, known as "extinction", is the total energy loss of the incident wave [6].
The scattering of light by dust depends on the size, shape, and chemical composition of the dust and the direction at which the light is scattered. This can be qualitatively understood, as schematically shown in Figure 1, by conceptually subdividing the dust into many small regions, in each of which a dipole moment will be induced when illuminated by an incident electromagnetic wave. These dipoles oscillate at the frequency of the incident wave and therefore scatter secondary radiation in all directions. The total scattered field of a given direction (e.g. P1, P2) is the sum of the scattered wavelets, with their phase differences taken into account. Since these phase relations change for a different scattering direction, the scattered field varies with scattering direction. 1 The phase relations among the scattered wavelets also change with the size and shape of the dust. On the other hand, the amplitude and phase of the induced dipole moment for a given frequency depend on the material of which the dust is composed. Therefore, the scattered field is sensitive to both the size, shape and chemical composition of the dust [6].
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Figure 1. A conceptual illustration of the scattering of light by dust. The dust is conceptually subdivided into many small regions. Upon illuminated by an incident electromagnetic wave, in each small region a dipole moment (of which the amplitude and phase depend on the composition the dust) will be induced. These dipoles oscillate at the frequency of the incident wave and scatter secondary radiation in all directions. The total scattered field of a given direction (e.g. P1, P2) is the sum of the scattered wavelets, where due account is taken of their phase differences. Since the phase relations among the scattered wavelets change with the scattering direction and the size and shape of the dust, the scattering of light by dust depends on the size, shape, and chemical composition of the dust and the direction at which the light is scattered. |
1 An exception to this is the dust in the Rayleigh regime (i.e. with its size being much smaller than the wavelength) which scatters light nearly isotropically, with little variation with direction since for this dust all the secondary wavelets are approximately in phase. Back.