The transmission coefficient is the complex ratio between transmitted and incident wave amplitudes. Flux transmission may also contain an impedance or normal-wavenumber factor and therefore need not equal .
The transmitted-to-incident compressional pressure or normal-stress amplitude ratio. At normal incidence, continuity of normal stress gives , with pressure-amplitude reflection coefficient . The transmitted energy fraction is , not alone.
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The transmission coefficient is a measure used in various fields of physics and engineering, particularly in wave mechanics, optics, and quantum mechanics, to describe the fraction of an incident wave (such as light, sound, or particles) that passes through a barrier or interface.