The propagation of electromagnetic waves has a vectorial character, its basic equations being the vectorial differential equations of Maxwell. In Gaussian units the latter read: $${\begin{array}{*{20}{c}} {{\text{curl }}E + \frac{1}{c}\frac{{\partial B}}{{\partial t}} = 0;{\text{ curl }}H - \frac{1}{c}\frac{{\partial D}}{{\partial t}} = \frac{{4\pi }}{c}I,} \\ {{\text{div }}D = 4\pi \varrho ;{\text{ div }}B = 0,} \end{array}}$$ (1.1) in which occur, apart from the field vectors (E = electric field, H = magnetic field, D = dielectric polarization, B = magnetic induction), the electrostatic densities ϱ and I of the distributed charge and current respectively. The right-hand sides, insofar as differing from zero, represent the sources producing the electromagnetic field in question.