Explain Poisson equation and Poisson spot:
Poisson equation (S.D. Poisson): The differential form of Gauss' law, that is,
div E = rho,
Poisson spot (S.D. Poisson): Poisson initially predicted the existence of such a spot, and employed the prediction to show how the wave theory of light should be in error to generate such a counterintuitive outcome. Succeeding observation of the Arago spot given a decisive confirmation of the wave nature of light.