Interference and diffraction of light are elementary topics in optics. The aim of the work presented here is to develop an accurate and cheap optical-system simulation software that provides a virtual laboratory for studying the effects of propagation in both time and space for the near- and far-field regions. In laboratory sessions, this software can let optical engineering undergraduates simulate many optical systems based on thin slits. The numerical method used is the finite-difference time-domain (FDTD) method that has been successfully applied in many engineering fields. Using this numerical method, the irradiance distribution can be successfully evaluated in different planes far from the simulation grid without degrading performance. In addition, an easy-to-use MATLAB GUI handles all the parameters of the FDTD simulation and computes theoretical values of irradiance for both the Fresnel and Fraunhofer regions. Therefore, by using this software, the student is able to analyze the behavior of the Fresnel and Fraunhofer expressions as a function of the distance. This distance is defined as the space between the slits plane and the plane that contains the virtual screen on which the irradiance pattern is represented. (Contains 1 table and 7 figures.)