Quantum theory of light explains the characteristics of light. According to which the light has dual character. Light acts as wave in some experiments like diffraction, interference. Whereas in some experiments light is considered to be having particle character as in photoelectric effect.
In photoelectric effect ,the wave-like property of light could not used to explain the phenomenon. But, Einstein said that light is made of particles called photons each having energy such that each photon knocks out 1 electron.The wave nature of light could explain that light travels in a straight line as an electromagnetic wave with a speed of and it could also travel through vacuum. Therefore, quantum theory of light could explain all the properties of light.