Discovery of quantization.

Discovery of quantization in quantum physics.

Quantization is probably familiar to you, but you wouldn't have encountered it in a classical physics book. The observable of macroscopic systems are not quantized. The energy of a ball on a string, the linear momentum of a hockey puck, and the angular momentum of an astronaut in a centrifuge-all can take on any of a continuum of values, subject only to whatever constraints are imposed by forces acting on the system. In the macroscopic world nature is continuous.

Not so in the microworld, where there is abundant evidence that nature is inherently discrete . Physicists have been aware of this fact since pre-quantum days. By 1900, for example, spectroscopes knew that radiation was emitted by atoms and molecules at discrete frequencies. But the first direct evidence of quantization of energy appeared in collision experiments performed in the early 1900's by James Franck (1882-1964) and  Gustav Hertz (1887-1975).

For several years, Franck and Hertz had been measuring the ionization potentials of atoms, and this experience influenced their interpretation of the results of their collision experiments.r In these experiments, they scattered electrons from various atoms; for example, in their most famous experiment, Franck and Hertz accelerated electrons emerging from a heated platinum wire, and sent the resulting beam through a gas of mercury vapor. By measuring the current in the scattered electron beam as a function of the energy of the incident electrons, Franck and Hertz could study the energy loss suffered by the electrons in their collisions with atoms of the gas. They found that the scattered current exhibited a series of sharp drops, each occurring at an incident energy equal to an integral multiple of 4.9 e.V. Franck and Hertz were initially led astray by their prior experience studying ionization; they concluded that 4.9 e.V is the ionization potential of mercury. But Neils Bohr,

on learning of Franck and Hertz's results, realized that the electrons were not ionizing the mercury atoms but rather were exciting them-losing energy to the atoms through inelastic collisions. Drawing on his model of the atom, Bohr further deduced that the energy spacing at which the current drops occur, 4.9 e.V, is the separation of two discrete energy levels of the mercury atom.

In 1914, when Franck and Hertz reported their work, physicists thought that it supported the Bohr theory of the atom-which has since been superseded by quantum theory. Nevertheless, the Franck-Hertz experiment stands as one of the first demonstrations of energy quantization in atoms, a striking example of non-classical shenanigans in the microworld.

The source: 

Michael A. Morrison - Understanding Quantum Physics.  

By. Fady Tarek