The Birth of Quantum Mechanics: Arriving of the Photon Concept

Before the twentieth century, the concepts of particleParticle and waveWave were entirely independent. The particleParticle is a localized point mass, while the waveWave is a spread-out oscillation of a medium. The concepts of “particleParticle” and “waveWave” thus are totally incompatible. At the end of the nineteenth century, physicists discovered evidence that this conventional thinking might not be true. It was discovered that light could behave like particlesParticle. This discovery was due to the work of Max PlanckPlanck, M.. In order to explain the spectrum of black-body radiationBlack-body radiation, PlanckPlanck, M. postulated that the radiation energy is transmitted in package (“quantum”). The size of the quantum energy was proportional to the frequencyFrequency ν of the radiation waveWave. EinsteinEinstein, A. later studied the photoelectric effectPhotoelectric effect and found that the energy of light is transferred in small “quantum”, as predicted in the Planck's relationPlanck’s relation. From then on, people started to realize that light is composed of discrete packages of energy, each of which could be regarded as a particleParticle. This particle-like energy quantum is now called “photonPhoton”.