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Chapter 16:
Qualitative Description of theWolf Effect and Differences between the Doppler and the Wolf Shifts
In the papers of Emil Wolf a statement that at first seems paradoxical but becomes evident after simple analysis, was formulated. The statement is that the spectrum of a source depends on its spatial coherence properties. This phenomenon was later called the “Wolf effect.” In this paper we present a simple physical picture explaining the influence of spatial coherence on the spectrum, and on the basis of this explanation we analyze the differences between the Doppler and the Wolf shifts. For the simplest illustration of the role of spatial coherence, in this paper we compare two limiting cases: the completely spatially coherent and completely spatially incoherent cases. This paper is not a review, but rather a qualitative physical picture of the Wolf effect, based on the ideas in Ref. [3]. The models considered in this paper are rather simple, perhaps even oversimplified, and do not pretend to be a comprehensive description of the subject, which can be found in the review paper and in the book. In Sect. 16.2 we consider the Young experiment, but starting with Sect. 16.3 we analyze the case of radiation, which is produced by the plane circular disk. We consider the cases of completely spatially coherent sources, and the cases of wide, narrow, and discrete spectral lines. Sect. 16.4 is devoted to a qualitative explanation of increasing spatial coherence in the process of propagation. In Sect. 16.5 the differences between the Doppler shift and the Wolf shift are discussed. We do not discuss here dynamic scattering (see [5]) related to the time-dependent fluctuations of dielectric permittivity. The reason for this restriction is that dynamic scattering includes the real Doppler effect.
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