  Excitation of molecules by incident incoherent electromagnetic radiation,
such as sunlight, is described in detail and contrasted with the effect of
coherent (e.g. laser) light. The nature of the quantum coherences induced by
the former, relevant to transport processes in nature and in technology, is
emphasized. Both equilibrium and steady state scenarios are discussed, Three
examples: simple models, calcium excitation in polarized light, and the
isomerization of retinal in rhodopsin are used to expose the underlying
qualitative nature of the established coherences.
