The Electron Double Slit Experiment proved the duality of matter: fundamental particles have particle-like properties, as well as wave-like properties.
In the experiment, when electrons were shot through an apparatus with 2 slits, it caused an interference pattern, suggesting that electrons behave like waves, with properties like position and momentum represented by a probability wave.
Double Slit Experiment - Wikipedia
This simulation contains abstractions and an implementation of the double slit experiment in which electrons behave like particles only. It goes to show that duality is needed to observe a diffraction pattern.
