Anomalous diffraction theory
Anomalous diffraction theory (also van de Hulst approximation, eikonal approximation, high energy approximation, soft particle approximation) is an approximation developed by Dutch astronomer van de Hulst describing light scattering for optically soft spheres. The anomalous diffraction approximation for extinction efficiency is valid for optically soft particles and large size parameter, x = 2πa/λ: Q e x t = 2 − 4 p sin p + 4 p 2 ( 1 − cos p ) {\displaystyle Q_{ext}=2-{\frac {4}{p}}\sin {p}+{\frac {4}{p^{2}}}(1-\cos {p})} , where Q e x t = Q a b s + Q s c a = Q s c a {\displaystyle Q_{ext}=Q_{abs}+Q_{sca}=Q_{sca}} in this derivation since the refractive index is assumed to be real, and thus there is no absorption ( Q a b s = 0 {\displaystyle Q_{abs}=0} ).
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