Light is a dual nature particle it demonstrates both practical as well as wave nature.
The smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as quantum. The energy of the radiation absorbed or emitted is directly proportional to the frequency of the radiation. Light consists of photon and the practical nature of light explains the photoelectric effect.
The wave nature of light was demonstrated convincingly for the first time in 1801 by Thomas Young by a wonderfully simple experiment. He let a ray of sunlight into a dark room, placed a dark screen in front of it, pierced with two small pinholes, and beyond this, at some distance a white screen. He then saw two darkish lines at both sides of a bright line, which gave him sufficient encouragement to repeat the experiment, this time with spirit flame as light source, with a little salt in it, to produce the bright yellow sodium light. This time he saw a number of dark lines, regularly spaced; the first clear proof that light added to light can produce darkness. This phenomenon is called interference. Thomas Young had expected it because he believed in the wave theory of light.
Definition from: Advanced physics