Optical softness implies that the relative refractive index of particle is close to that of the surrounding medium.
The approximation holds for particles of arbitrary shape that are relatively small but can be larger than Rayleigh scattering limits.
The second condition is a statement of the Born approximation, that is, that the incident field is not greatly altered within one particle so that each volume element is considered to be illuminated by an intensity and phase determined only by its position relative to the incident wave, unaffected by scattering from other volume elements.
[1] The particle is divided into small volume elements, which are treated as independent Rayleigh scatterers.
denotes the phase difference due to each individual element,[3] and the fraction in parentheses is the electric polarizability as found from the refractive index using the Clausius–Mossotti relation.
Per the optical theorem, absorption cross section is given as: which is independent of the polarization[dubious – discuss].
[6] The theory was also applied to anisotropic spheres for nanostructured polycrystalline alumina and turbidity calculations on biological structures such as lipid vesicles[7] and bacteria.
[8] A nonlinear Rayleigh−Gans−Debye model was used to investigate second-harmonic generation in malachite green molecules adsorbed on polystyrene particles.