In this paper we consider dynamics of a molecular system subjected to external pumping by a light source. Within a completely quantum mechanical treatment, we derive a general formula, which enables to asses effects of different light properties on the photo-induced dynamics of a molecular system. We show that once the properties of light are known in terms of certain two-point correlation function, the only information needed to reconstruct the system dynamics is the reduced evolution superoperator. The later quantity is in principle accessible through ultrafast non-linear spectroscopy. Considering a direct excitation of a small molecular antenna by incoherent light we find that excitation of coherences is possible due to overlap of homogeneous line shapes associated with different excitonic states. In Markov and secular approximations, the amount of coherence is significant only under fast relaxation, and both the populations and coherences between exciton states become static at long time. We also study the case when the excitation of a photosynthetic complex is mediated by a mesoscopic system. We find that such case can be treated by the same formalism with a special correlation function characterizing ultrafast fluctuations of the mesoscopic system. We discuss bacterial chlorosom as an example of such a mesoscopic mediator and propose that the properties of energy transferring chromophore-protein complexes might be specially tuned for the fluctuation properties of their associated antennae. ; Comment: 12 pages