This chapter focuses on the numerical simulation of light diffraction by single or multiple slits, which serves to illustrate key principles of wave physics and interference. Students will become acquainted with numerical differentiation and quadrature procedures, particularly in relation to grid parameter convergence. The physics background emphasises wave physics elements, such as the superposition principle and phase difference, as well as their practical applications in real systems. Concepts such as optical paths and coherence are addressed. To understand diffraction phenomena, the Huygens principle is introduced, leading to the diffraction integral formulation for infinite slits. The chapter then explores numerical methods based on local approximations of functions, such as the two-, three-, and five-point schemes for derivatives. This study culminates in the presentation of quadrature schemes, the application of power series expansions for numerical differentiation, and the Simpson algorithm for accurate numerical integration.