Hyper–Rayleigh scattering Optical Activity (/ˈreɪli/ RAY-lee), (a form of Chiroptical harmonic scattering) is a nonlinear optical physical effect whereby chiral scatterers (such as nanoparticles or molecules) convert light (or other electromagnetic radiation) to higher frequencies via harmonic generation processes, in a way that the intensity of generated light depends on the chirality of the scatterers. "Hyper–Rayleigh scattering" is a nonlinear optical counterpart to Rayleigh scattering. "Optical activity" refers to any changes in light properties (such as intensity or polarization) that are due to chirality.