Photon : the quantisation of light
To understand this part of the Quantum Mechanics I hope you know the basic theory of Quantum Mechanics. If you are not familiar then I would like to recommend you to read the Introduction to Quantum Mechanics and First Quantum Theory : Black-Body Radiation by Max Planck. Moving on!
The Photon is the fundamental particle of visible light. In other words a photon is a discrete bundle (or quantum) of electromagnetic energy. And those are always in the motion. Photon have a constant speed of light (c = 2.998 x 108 m/s) in a vacuum.
Well, Back in 1905, one of the great scientist on the planet Albert Einstein took new step into Quantum Mechanics.. According to him the quantisation was not just a mathematical construct, but that the energy in a beam of light actually occurs in individual packets, which are now called photons. He added the energy of a single photon is given by its frequency multiplied by Planck’s constant:
Of course! Here all things are just messed up. For so many years scientists debated over 2 possible theories of light. Whether was it a wave or did it instead comprise a stream of tiny particles? In 19th century, after many debates and discussion the debate has been settled in favor of the wave theory. Because the wave theory was able to explain the refraction, diffraction, interference and polarization.
In addition another scientist James Clerk Maxwell shown electricity and light are all manifestations of the same phenomenon: the electromagnetic field. The Maxwell’s equation describe light as waves: a combination of oscillating electric and magnetic fields. So, but obvious the favor must goes into the wave theory.
So, the Einstein’s photon theory become suspicion. But not only that, the the photon model became popular later on. Because it is able to explain several properties of the photoelectric effect. And the wave theory.
The Photoelectric effect refers to the emission, or ejection, of electrons from the surface of, generally, a metal in response to incident light. In other words under the right circumstances light can be used to free electrons, from the surface of a solid. This process is called Photoelectric Effect.
The wave theory remained essential because it was able to understand other characteristics of light such as diffraction, refraction and interference.