English: Amplitude of a scalar plain wave illuminating a circular obstacle at normal incidence (Dirichlet boundary condition). Formation of the spot of Arago is visible in the shadow. Computed by superposing a plain wave with a wave radiated by 300 concentric rings of Green's function point sources across the obstacle. The amplitudes for the sources found from the condition that the total amplitude is zero at the obstacle. Wavelength equals 6 pixels (1/120 of width of the image).