Wave particle Dualism
Classical physics discriminates between particles and waves. The former have matter and are affected by forces in their behaviour, the latter do not have mass and have the exclusive ability to superimpose with one another. An experiment that shows the superposition effect of wave is the double-slit experiment, where waves impinge on a surface with two openings (slits) and their intensities are measured on a plane behind the slits. Due to the superposition of hills and valleys of the waves there are areas on the measuring plane where high intensity can be observed and other areas where there is no wave intensity at all. This phenomenon is called interference pattern. Conducting the same experiment with particles under certain conditions it was found that these also show this interference pattern, even if only one particle at a time passes the two slits. As classical mechanics could not explain this behaviour of the particles, quantum mechanics predicted that the particles also have wave characteristics. There is another experiment, the so-called photo-electric effect, where light is shone upon a metal, which caused electrons to be emitted from the metal.
Contrary to the expectation that the higher the intensity of the light wave the higher the energy of the electrons that come out of the metal, it is just the amount of electrons that increase. This is an indicator for the particle characteristic of light, although light also shows an interference pattern in a double-slit experiment. Quantum theory comes to the conclusion that everything in the universe is both, particle and wave, hence called the wave-particle dualism of matter.