InterferenceSimulator is a program demonstrating quantal two-slit Fresnel interference patterns with one, the other, or both slits open. A magnetic flux situated between the two slits allows demonstration of the Aharonov–Bohm effect. Simulations with short de Broglie wavelengths illustrate the classical limit of quantum mechanics. Because of the universality of wave phenomena, this program can also demonstrate the geometrical-optics limit of wave optics for small wavelengths.