In the Newtonian era of nineteenth century, an electromagnetic radiation is believed to be exclusively a waveform, as it has no mass. Reflection, refraction etc., of the electromagnetic radiations was explained by the wave nature. But it could not explain other properties like photoelectric effect, black body radiations.
Max Plank proposed for the first time that the electromagnetic radiations are made-up of small units like particles making up a matter. He called the basic units of radiations as Quanta and explained the black body radiations. Plank gave an expression relating the energy of the quanta to the frequency of the electromagnetic wave radiations. The hypothesis of particle nature of electromagnetic radiations is known as Plank’s Quantum theory and the expression is known as Plank’s Law.
Einstein applied the particle nature to explain the photoelectric effect of metals.