Effect of the imaginary part of the refractive index on light scattering by spheres

For spheres of arbitrary radius R and complex index of refraction n+iκ, we identify a new parameter that indicates when the imaginary part κ seriously affects the scattering by a sphere. The parameter is κkR, where k is the wave vector magnitude, such that when κkR grows larger than one, the scattering is affected and, when κkR>3, the effects saturate. The effects are uncovered and demonstrated with Q-space analysis. The physical basis for κkR lies in the fact that it is the ratio of the radius to the optical skin depth.