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The electric field of a luminous region is much more regular than is generally realized. Phases vary periodically and predictably, and the nature of the polarization remains approximately constant, for times of the order of the reciprocal of the line width in cycles per second. In a plane at right angles to the mean direction of propagation the light wave is coherent over an area approximately λ2/Ω, where Ω is the solid angular spread in the light. These ideas, when applied to the problem of producing beats between lightwaves, lead to a signal amplitude which has been verified experimentally.
A. Theodore Forrester (Sun,) studied this question.