Quantum Physics Study Materials

Quantum Physics Introduction to quantum mechanics Quantum physics is the physics theory that describes the behavior of matter, radiation and all their interactions seen as both wave and particle phenomena. This is the main difference with classical or Newtonian physics which, for example, sees light only as a wave and electrons only as a particle. One of the main experiments showed that two waves of the same amplitude can be in phase and, if they interfere, they originate a sinusoidal wave that is the sum of its sinusoidal components; however, they can be in counterphase and, if they interfere, they originate a zero wave. In certain situations, as highlighted by Einstein with the hypothesis of the photon in the photoelectric effect, light behaved decisively as composed of particles. The photoelectric effect is the phenomenon that occurs with the emission of electrically charged particles from an objects exposed to light waves or electromagnetic radiation of various frequencies. According to Planck, light radiation is composed of corpuscular particles (photons). In order to be able to rip an electron from a metal surface, the photon's energy must be greater than the electron's binding energy in the metal, thus generating a current flow. If, on the other hand, the photon's energy is lower, there is no photoelectric effect and no current flow. Albert Einstein instead theorized the photoelectric effect with the hypothesis that light rays carried particles, called photons, whose energy is directly proportional to the frequency of the corresponding wave: affecting the surface of a metal object, the photons give part of their energy to the free electrons of the conductor, causing the emission. Then the energy of the released electron depends only on the energy of the photon, while the magnitude of the radiation is directly related to the number of photons carried by the wave. The real demonstration of the wave-particle duality of matter came with Davisson and Germer, with the experiment of the two cracks. Later Niels Bohr also introduced the principle of complementarity, according to which the two aspects, corpuscular and undulatory, cannot be observed at the same time because they are mutually exclusive. The Planck constant Planck's constant, also known as action constant, is indicated by the letter h and is a fundamental constant in quantum mechanics and, more generally, in atomic physics and has the size of an energy for a time. In the system of units of measure called 'Atomic units' it is the unit of measure of the angular momentum. The Planck constant is a characteristic constant of the atomic and molecular world whose numerical value corresponds to: The Planck constant is linked to the quantization of the dynamic quantities linked to the microscopic world. For example, we find Planck's constant in the energy value associated with a frequency quantum v. This value, according to Einstein's well-known equation, is valid The constant h is also found in the relation known as Heisenberg's uncertainty principle according to which the uncertainties in the determination of the values of the position (∆x) and the momentum (Ap) of an electron must respect the following relation: Curiosity: the value of the Planck constant, from 20 May 2019, is the constant used to define the kilogram Bosons and Fermions Bosons are the particles that obey Bose-Einstein's statistics and (according to the spin-statistics theorem) have therefore whole spin. The Bose-Einstein statistic determines the statistical distribution of energy states to the thermal equilibrium of a boson system. In practice, any number of bosons can occupy the same sub-level of a given energy state. While the fermions, which have semi-internal spin, obey the law of exclusion of Pauli (according to which a single quantum state cannot be occupied by more than one particle), the bosons are free to crowd in large numbers the same quantum state. Pauli's exclusion law is a law of quantum mechanics which states that two identical fermions cannot occupy the same quantum state simultaneously. Photons The photon is indicated with the Greek letter y and is associated with any electromagnetic radiation. The electromagnetic radiation also has a quantized nature that allows it to be described as a stream of photons. The photon is a particle that has infinite life, it can in fact be created and destroyed by interaction with other particles. Although it has no mass, it is influenced by gravity and has energy; in vacuum it moves at the speed of light, while in matter it behaves differently and its speed drops because, when it interacts with other particles, it acquires mass and no longer moves at the speed of light. An atom can only emit an electromagnetic wave when an electron moves from an orbit with higher energy to an orbit with lower energy. The energy of the electromagnetic wave emitted is the difference in energy between the two orbitals. Since this energy has well-defined values, the energy of the electromagnetic radiation emitted by the atom must also have a defined value. Einstein calculated the energy associated with the emission of a photon and saw that it was proportional to the frequency of the electromagnetic wave. The photon shows a dual nature and is a perfect example of wave-particle dualism. The radiation in fact behaves like a wave when it propagates in space, while it behaves like a particle when it interacts with matter.