We model analytically light harvesting in realistic solar cells by extending a formalism suggested by Deckman et al. [Appl. Phys. Lett. 42, 110968 (1983)], based on tracing of an average ray of light. Arbitrary light scattering schemes and parasitic absorption are implemented in the model, and we validate our approach by comparing with experimental measurements from microcrystalline silicon devices. The intuitive understanding obtained with this extended model is discussed. This approach enables identifying parasitic absorption as main limitation of state-of-the-art light harvesting schemes, and highlights that a better light trapping requires improving the first scattering events.