ABSTRACT
All electromagnetic fields have fluctuations associated with them, which can be described in statistical terms. Thus, the theory of coherence, in a way, is concerned with the statistical description of electromagnetic field. We cannot measure these field fluctuations directly as they oscillate much faster than the time resolution of any detector. However, we can deduce the existence of fluctuation by using suitable experimental arrangements, and by measuring correlations between them at two or more space–time points. Consider the optical region of the electromagnetic spectrum. The basic experimental phenomenon that describes the correlation between optical fields is interference effects that arise when two light beams, originating from the same source, are superposed.
