Emil Wolf discovered how the spatial coherence characteristics of the source affect the spectrum of the radiation in the far zone. In particular, the spatial coherence of the source can result either in red or blue shifts in the measured spectrum. His predictions have been verified in a large number of different classes of systems. Wolf and coworkers usually assume a given form of source correlations and study its consequence. In this paper we consider the microscopic origin of spatial coherence and radiation from a system of atoms. We discuss how the radiation is different from that produced from an independent system of atoms. We show that the process of radiation itself is responsible for the creation of spatial correlations within the source. We present different features of the spectrum and other statistical properties of the radiation, which show strong dependence on the spatial correlations. We show the existence of a new type of two-photon resonance that arises as a result of such spatial correlations. We further show how the spatial coherence of the field can be used in the context of radiation generated by nonlinear optical processes.We conclude by demonstrating the universality ofWolf shifts and its application in the context of pulse propagation in a dispersive medium.
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