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A particle on the classical view is a concentration of energy and other properties in space and time, whereas a wave is spread out over a larger region of space and time. The question whether light are streams of particles (corpuscles) or waves is a very old one. This "either - or" formulation was classically natural and alien to the advanced "both - and" even the "neither - nor" solution of today. Early in the nineteenth century experiments were suggested and made to show that light is a wave motion. A key figure in this endeavour was Thomas Young, one of the most intelligent and clever scientists ever to live, who studied diffraction and interference of light already in 1803 with results that gave strong support to the wave theory of Christian Huygens as opposed to the particle or corpuscular theory of Isaac Newton. Further contributions were made by many other researchers, among them Augustin Jean Fresnel, who showed that light is a transverse wave

Experiments with beams of light or of electrons have been made such that both aspects - waves and particles - are observed. For interference to occur it is among other things also necessary for the beam to have available more than one path from source to detector (e.g. a screen). Interference is explained by the wave picture. When the beam intensity is sufficiently low and the detector suitable the impact of particles one by one can be observed. The energy quanta are then localised as if particles in space and time.