the wave will be reflected back to the source. Because the voltage on a transmission line is always the sum of the forward and reflected waves at that point, if the incident wave amplitude is 1, and the reflected wave is , then the amplitude of the forward wave must be sum of the two waves or . The value for is uniquely determined from first principles by noting that the incident power on the discontinuity must equal the sum of the power in the reflected and transmitted waves: . Solving the quadratic for leads both to the reflection coefficient: , and to the transmission coefficient: . The probability that a portion of a communications system, such as a line, circuit, channel or trunk, will meet specified performance criteria is also sometimes called the "transmission coefficient" of that portion of the system. The value of the transmission coefficient is inversely related to the quality of the line, circuit, channel or trunk. Quantum mechanics In non-relativistic quantum mechanics, the transmission coefficient and related reflection coefficient are used to describe the behavior of waves incident on a barrier. The transmission coefficient represents the probability flux of the transmitted wave relative to that of the incident wave. This coefficient is often used to describe the probability of a particle tunneling through a barrier. The transmission coefficient is defined in terms of the incident and transmitted probability current density J according to: where is the probability current in the wave incident upon the barrier with normal unit vector and is the probability current in the wave moving away from the barrier on the other side. The reflection coefficient R is defined analogously: Law of total probability requires that , which in one dimension reduces to the fact that the sum of the transmitted and reflected currents is equal in magnitude to the incident current. For sample calculations, see rectangular potential barrier. WKB approximation Using the WKB approximation, one can obtain a tunnelling coefficient that looks like where are the two classical turning points for the potential barrier. In the classical limit of all other physical parameters much larger than Planck's constant, abbreviated as , the transmission coefficient goes to zero. This classical limit would have failed in the situation of a square potential. If the transmission coefficient is much less than 1, it can be approximated with the following formula: where is the length