Scattering processes in an optical microcavity are investigated for the case of silicon nanocrystals embedded in an ultra-high-Q toroid microcavity. Using a novel measurement technique based on the observable mode splitting, we demonstrate that light scattering is highly preferential: more than 99.8% of the photon flux is scattered into the original doubly degenerate cavity modes. The large capture efficiency is shown to result from the Purcell enhancement of the optical density of states over the free space value, an effect that is more typically associated with spontaneous emission. The experimentally determined Purcell factor has a lower bound of 171.