Asymmetry Parameter and Radiation Pressure

The asymmetry parameter, which was defined in Section 3.4 as the average cosine of the scattering angle, depends, in general, on the polarization state of the incident light. However, the asymmetry parameter for a spherical particle is clearly independent of polarization and is given by 

Therefore, to obtain the asymmetry parameter, we must evaluate the integrals 

The derivation above of the efficiency for radiation pressure is heuristic a rigorous derivation of this result, which was first obtained by Debye (1909), entails integrating the stress tensor of the electromagnetic field over a spherical surface surrounding the particle. 

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