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Scattering crosssectionThe scattering crosssection, σ_{scat}, relates the scattering of light or other radiation to the number of particles present. Its SI unit is the square metre, m², although smaller units are usually used in practice. The name crosssection arises because it has the dimensions of area. The definition of the scattering crosssection is derived from the BeerLambert law expressed in terms of the number concentration of particles, C,
where E(λ) is the extinction at a given wavelength, λ, and l is the path length. The extinction of the radiation is the logarithm (decadic or, more usually, natural) of the ratio of the incident spectral intensity, I_{0} to the transmitted spectral intensity, I,
and, in general, has three independant componants, due to absorption, scattering and luminescence:
Additional recommended knowledge
Relation to physical sizeThere is no simple relationship between the scattering crosssection and the physical size of the particles, as the scattering crosssection depends on the wavelength of radiation used. This can be seen when driving in foggy weather: the droplets of water (which form the fog) scatter red light less than they scatter the shorter wavelengths present in white light, and the red rear fog light can be distinguished more clearly than the white headlights of an approaching vehicle. That is to say that the scattering crosssection of the water droplets is smaller for red light than for light of shorter wavelengths, even though the physical size of the particles is the same. Meteorological rangeThe scattering crosssection is related to the meteorological range, L_{V}, by the theoretical expression
The quantity Cσ_{scat} is sometimes denoted b_{scat}. UnitsWhen the scattered radiation is visible light, it is conventional to measure the path length in centimetres. To avoid the need for conversion factors, the scattering crosssection is expressed in cm² (1 cm² = 10^{−4} m²) and the number concentration in cm^{−3} (1 cm^{−3} = 10^{6} m^{−3}). The measurement of the scattering of visible light is known as nephelometry, and is effective for particles of 2–50 µm in diameter: as such, it is widely used in meterology and in the measurement of atmospheric pollution. Scattering crosssections are also widely used in the description of the scattering of neutrons by atomic nuclei. In this case, the conventional unit is the barn, b, where 1 b = 10^{−28} m² = 100 fm².^{[1]} The scattering of Xrays can also be described in terms of scattering crosssections, in which case the square ångström, Å², is a convenient unit: 1 Å² = 10^{−20} m² = 10^{4} pm². However, in crystallography, it is more usual to express Xray scattering in terms of structure factors, f. See also
References


This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Scattering_crosssection". A list of authors is available in Wikipedia. 