Basic Concepts of Quantum Mechanics
The appearance of vacuum apparatus, the development of radio and television techniques, and the improvement of other technical aids to physical phenomena led at the end of the nineteenth century to the discovery of electrons, X-rays, and radioactivity. The possibility arose of studying separate atoms and molecules. It then turned out that classical physics was not able to explain the properties of atoms and molecules and their inter-actions with electromagnetic radiation. A study of the conditions of equilibrium between matter and electromagnetic radiation by Planck in 1900 and of photoelectric phenomena by Einstein in 1905 led to the conclusion that electromagnetic radiation possessed both a wave character and a corpuscular character. It was established that electromagnetic radiation is absorbed and emitted in separate portions which now-a days called photons.
On the other hand, interference and diffraction phenomena, widely used in a number of optical apparatuses, Show the wave character oi electromagnetic radiation. It turned out to be impossible to consider the wave properties of radiation as‘a manifestation of the collective motions of a large number of photons, in the same way as sound waves correspond to the motion of a large number of molecules of the air, of a liquid, or of a solid.
Franck and Hertz showed in 1914 the discrete character of the atomic energy states in their experiments on the ionization potentials of gases. The discrete character of atomic levels became also apparent from the study of the optical spectra of atoms. Stern and Gerlach showed in 1922 in their experiments on the deviation of a beam of atoms in an inhomogeneous magnetic field that the component of angular momentum along the direction of a magnetic field takes on discrete values.
The year 1913 saw the first successful attempt to explain the properties of the hydrogen atom by Niel Bohr, who introduced some special postulates . These postulates were in essential contradiction to the established rules of classical physics.
The experiments of Davisson and Germor in 1927, in which diffraction of electrons on reflection by thin metallic foils was observed, were very important for an explanation of the properties of electrons. These experiments confirmed de Broglie’s hypothesis in 1924 about the wave properties of all particles of small mass.
The theory explaining the basic properties of atomic and nuclear phenomena is quantum mechanics which was founded by the work of Bohr, Schrodinger, Heisenberg, Dirac, Fock and Pauli. Quantum mechanics is the theory which is the basis of the explanation of the properties of atoms, molecules, and atomic nuclei, that is, phenomena’occurring in volume elements of linear dimension; cannot directly be perceived with our sense organs. We can only investigate them by using “apparatus”, that is, by using macroscopic systems which translate the action of micro-objects into a macroscopic language.
Quantum mechanics shows that many other concepts of classical physics also have a limited domain of applicability. For example, it turned out that it is impossible to define the velocity of a particle as the derivative dr/dt. Quantum mechanics often merely gives probability statements. It enables us to evaluate the probability of the influence of atomic object under well- defined macroscopic conditions upon macroscopic apparatus. Quantum mechanics is a new vigorously developed branch of theoretical physics.
In conclusion, we see that, it is clearly unsatisfactory to have both radiation and matter being treated sometimes as waves and sometimes as particles is an apparently arbitrary manner, and discrete energy levels in atoms being produced by ad-hoc rules. What is needed is a basic reformulation of the theory in such a way that both classical concepts which remain correct,and the Planck-Bohr-de Broglie rules, shall appear as natural consequence.