Description:
Huygens’ Principle (HP) contains both the principle of action-at-proximity and the superposition principle. Althoughthe propagation of sharp, non-spreading wave fronts is included in Huygens’ [1] original formulation, it can be leftout without touching those principles. The formulation of HP by means of the Chapman-Kolmogorov equation(following Feynman [2]) comprises both versions and overcomes misunderstandings like ”Huygens’ principle is notexactly obeyed in Optics” [2] and “HP is incompatible with Green’s functions” [3]. This way, HP applies not only tothe propagation of light, but also to heat and matter diffusion, Schrödinger matter waves, i. e., to virtually allpropagation phenomena, which can be described through explicit linear differential and difference equations,respectively. HP for Maxwell’s equations is discussed in terms of the Helmholtz-decomposed fields and currents. Theappearances of HP in mechanics and in fractional Fourier transformation being exploited in optics are also mentioned.