A transition is a process or a period of change from one state to another. In physics, a transition is the passage of a system from one quantum state to another by the absorption or emission of energy in the form of radiation. Transitions can also be classified according to the symmetry properties of the initial and final states. Some transitions are forbidden by certain selection rules, while others are allowed but may be very improbable.
The study of transitions and their rates is important in many areas of physics, including astrophysics and nuclear physics. In atoms and molecules, transitions between energy levels give rise to spectral lines that provide information about the structure and dynamics of these objects. The rates of atomic and molecular transitions are also important in chemical kinetics and lasers.
In condensed matter physics, phase transitions are changes in the physical properties of materials as a result of changes in temperature or pressure. Phase transitions can be first-order (characterized by a discontinuous change in one or more physical quantities) or second-order ( characterized by a continuous change in one or more physical quantities). Examples of first-order phase transitions include melting, freezing, sublimation, vaporization, and condensation. Second-order phase transitions include ferromagnetism–paramagnetism and superconductivity–normal conducting phases.