Optical simulation of Huygens’s principle

The two-dimensional stationary-wave equation (Helmholtz equation) can be solved by a simple linear-optical filtering experiment for arbitrary source distributions. The experiment can be considered as direct visualization of Huygens’s principle, i.e., each point of an initial wave front can be regarded as the source of an elementary wave. Many phenomena can be visualized: diffraction, interference (including Talbot effect, Bragg effect) with monochromatic and polychromatic light, birefringence, etc. Optical filtering can be applied to other linear differential equations with constant coefficients too, such as the Poisson equation and the diffusion equation.