Planck’s constant defines the boundary between the macrocosm, where the laws of Newtonian mechanics operate, and the microcosm, where the laws of quantum mechanics operate.
Max Planck – one of the founders of quantum mechanics – came to the ideas of quantizing energy, trying to theoretically explain the process of interaction between recently discovered electromagnetic waves (cm. Maxwell’s equations) and atoms and, thus, solve the problem of blackbody radiation. He realized that to explain the observed emission spectrum of atoms, one must take for granted that atoms emit and absorb energy in portions (which the scientist called quanta) and only at individual wave frequencies. The energy carried by one quantum is equal to:
E = hv
Where v Is the radiation frequency, and h – elementary quantum of action, which is a new universal constant, which soon received the name Planck’s constant… Planck was the first to calculate its value based on experimental data h = 6,548 × 10^{–34} J s (in SI system); according to modern data h = 6,626 × 10^{–34} J s. Accordingly, any atom can emit a wide range of interconnected discrete frequencies, which depends on the orbits of the electrons in the atom. Soon, Niels Bohr will create a coherent, albeit simplified, model of the Bohr atom, consistent with the Planck distribution.
Having published his results at the end of 1900, Planck himself – and this is evident from his publications – at first did not believe that quanta are a physical reality, and not a convenient mathematical model. However, when five years later, Albert Einstein published an article explaining the photoelectric effect based on energy quantization radiation, in scientific circles Planck’s formula began to be perceived not as a theoretical game, but as a description of a real physical phenomenon at the subatomic level, proving the quantum nature of energy.
Planck’s constant appears in all equations and formulas of quantum mechanics. It, in particular, determines the scale from which the Heisenberg uncertainty principle comes into force. Roughly speaking, Planck’s constant indicates to us the lower limit of spatial quantities, after which quantum effects cannot be ignored. For grains of sand, say, the uncertainty in the product of their linear size and speed is so insignificant that it can be neglected. In other words, Planck’s constant draws the border between the macrocosm, where the laws of Newtonian mechanics operate, and the microcosm, where the laws of quantum mechanics come into force. Having been obtained only for the theoretical description of a single physical phenomenon, Planck’s constant soon became one of the fundamental constants of theoretical physics, determined by the very nature of the universe.
1859

Spectroscopy

1899

Photoelectric effect
