Around 1700, when Mechanics was made for the first time by Sir Isaac Newton, he said about the nature of the light that it resembles thiny particles. Somewhat latter , around 1865, Maxwell discovered that the Electromagnetic waves have the same speed as the speed of light. From this resemblance he deduced that light should be an Electromagnetic wave. Latter than this, at around 1900, when trying to explain some peculiarities of the Photoelectric effect, Eistein postulated that light behaves like thiny particles (the older Newton Theory from 1700).
Today we say that LIGHT HAS A DUAL CHARACTER. It is also a wave and a particle. In some experiments light behaves like an Electromagnetic wave (diffraction, reflection, refraction), in other experiments light behaves like thiny particles (quanta): the Photoelectric effect and the Compton effect.