Increased incoherent backscattering from a liquid-solid interface at the Rayleigh angle

The experimentally observed peak in the backward radiation from a liquid-solid interface at the Rayleigh angle is caused by a sharp increase in the otherwise relatively weak incoherent scattering from the inherent inhomogeneities of the solid. The increase of the incoherent backscattering is essentially a specular effect in contrast with the nonspecular, finite-beam effect predicted to cause the much weaker coherent reflection. The iincoherent scattering can be caused by either geometrical irregularity or material inhomogeneity (e.g., surface roughness or polycrystalline grain structure). This paper presents analytical results showing that, regardless of the physical nature of the scattering inhomogeneity, there is a distinct peak in the backscattered intensity around the Rayleigh angle. The angular dependence of the incoherent backscattering is determined by the average p[opcrtics of the liquid and the solid. The peak always occurs at the Rayleigh angle and the width of the peak is determined by the density ratio between the solid and the liquid. The maximum backscattering atthe peak is essentially independent of the average material properties and is determined solely by the scattering inhomogeneity.