The Double Slit Experiment

The double-slit experiment is a variant of Young's experiment that allows to demonstrate the wave-particle duality of light and matter. Richard Feynman used to say that the heart of quantum mechanics can be understood by reflecting on this experiment.

Quantum Mechanics says that makes no sense to ask a priori the question about what "really" a quantum object is. This is the meaning of the superposition principle which states that wave and particle are just categories of our language by which we describe what we observe in a quantum experiment. Only the experiment shows us the wave or particle nature.

The next video shows the double-slit experiment in a fantastic and very descriptive way.