Photosynthesis is a process where light is absorbed or harvested by pigment protein complexes which are able to turn sunlight into energy. Absorption of a photon by a molecule takes place when pigment protein complexes harvest sunlight leading to electronic excitation delivered to the reaction centre where the process of charge separation can take place. when the energy of the captured photon matches that of an electronic transition. The fate of such excitation can be a return to the ground state or another electronic state of the same molecule. When the excited molecule has a nearby neighbour molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another. This process is called resonance energy transfer, and the rate depends strongly on the distance between the energy donor and energy acceptor molecules. Before an excited molecule can transition back to ground state, the energy needs to be harvested. This excitation is transferred among chromophores where it is delivered to the reaction centre. Light-harvesting complexes have their pigments specifically positioned to optimize these rates.