Particles on  the classical view are concentration of energy and other properties in space and  time, whereas waves are normally spread out over a larger region of space and  time. The question whether light are streams of particles (corpuscles)
or waves is not new.

·  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, 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.

·   O n the other hand, some phenomena also
demonstrate the particle nature of light:
1) Albert Einstein’s photoelectric  effect and 2) the Compton scattering of  X-rays and 3) Planck’s energy  quanta.

It was Einsten who in 1921 said  concluded that light also behaves like  particles.
The Key Idea

  Albert Einstein (1879-1955)