It has been known for some time that the spectrum of light may change on propagation, even in free space. The theory of this phenomenon was developed within the framework of scalar theory. We generalize it to electromagnetic beams, generated by planar, secondary, stochastic sources. We also derive an electromagnetic analog of the so-called scaling law. When this law is satisfied the normalized spectrum of the beam is the same throughout the far zone and across the source.
Pu, Jixiong; Korotkova, Olga; and Wolf, Emil, "Invariance and Noninvariance of the Spectra of Stochastic Electromagnetic Beams on Propogation" (2006). Physics Articles and Papers. 16.