  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 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 of Wolf shifts and its application
in the context of pulse propagation in a dispersive medium.
