The name laser is an acronym for "light amplification by stimulated emission of radiation". One can understand the principles of laser operation on the basis of the concepts of photons and atomic energy levels. If a quantum system with two levels interacts with radiation, three different processes can occur. 1. Absorption of a photon. In this case, an atom goes from a lower to an upper (excited) state. 2. Spontaneous emission occurs when an excited atom returns to the lower state by emitting a photon. The direction and the phase of the emitted photon are random, and the outcome of spontaneous emission is therefore unpredictable. The frequency of absorbed and emitted photons is determined by the energy difference between the two levels: hν = En - Em. 3. Stimulated emission from excited to lower state can also occur. Einstein predicted this process in 1916, and it constituted the foundation of laser physics. In stimulated emission, an incident photon encounters an excited atom. By virtue of a certain resonance effect, the photon forces the atom to emit another photon with the same frequency, phase, and polarization, and moving in the same direction as the incident photon. All three abovementioned processes occur simultaneously when an external resonant beam of light passes through a two - level system. The intensity of this beam decreases as some of the excited-state atoms lose their energy and come to the lower state without emitting a photon (for example, due to inelastic collisions with other atoms). In order for the beam?s intensity to increase, an enhancement of the number of atoms in excited states by some external source of energy needs to occur. This can be achieved in a variety of ways, but all such processes are impossible in a two-level system. The supply of energy must take place via a third, higher level. This creates a non-equilibrium situation, whereupon the number of excited atoms is larger than the number of lower-state atoms. Such condition is called population inversion. When the number of excited atoms is large enough, the rate of energy radiation by stimulated emission exceeds the rate of absorption. The system begins to act as a source of radiation. For a laser to work like this, the system must be located between two parallel mirrors with a high reflection index. In this instance, the increasingly intensive light passes through the medium after a multitude of reflections, causing an avalanche of emitted photons. Thus emerges laser emission.