INCREASED INCOHERENT BACKSCATTERING FROM A LIQUID-SOLID INTERFACE AT THE RAYLEIGH ANGLE

The experimentally observed peak in the backward radiation from a liqu id-solid interface at the Rayleigh angle is caused by a sharp increase in the otherwise relatively weak incoherent scattering from the inher ent inhomogeneities of the solid. The increase of the incoherent backs cattering is essentially a specular effect in contrast with the nonspe cular, finite-beam effect predicted to cause the much weaker coherent reflection. The incoherent scattering can be caused by either geometri cal irregularity or material inhomogeneity (e.g., surface roughness or polycrystalline grain structure). This paper presents analytical resu lts 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 b ackscattering is determined by the average properties of the liquid an d the solid. The peak always occurs at the Rayleigh angle and the widt h of the peak is determined by the density ratio between the solid and the liquid. The maximum backscattering at the peak is essentially ind ependent of the average material properties and is determined solely b y the scattering inhomogeneity.