Effective refractive index determination and light propagation mechanisms in natural scattering media

We present a new mixing rule for calculating the effective refractive index beyond the limit of small particles; a restriction found in common effective medium approaches such as the Maxwell-Garnett or Bruggeman theory. Subsequently, these findings are used to distinguish the diffusive and coherent portion of light transport in ultrathin, strongly scattering white beetle scales. It is shown that weakly localized random photonic modes contribute about one third of the overall scattered light, revealing their significance for the brilliant whiteness.