From the above expression, resonance occurs when the distance covered by the particle in traversing the well and back ( 2 L ) is an integral multiple of the De Broglie wavelength of a particle inside the potential ( λ λ = 2 π π k ). For E > V 0 , reflections at potential discontinuities are not accompanied by any phase change. Therefore, resonances correspond to the formation of standing waves within the potential barrier/well. At resonance, the waves incident on the potential at x = 0 and the waves reflecting between the walls of the potential are in phase, and reinforce each other. Far from resonances, standing waves can't be formed. Then, waves reflecting between both walls of the potential (at x = 0 and x = L ) and the wave transmitted through x = 0 are out of phase, and destroy each other by interference. The physics is similar to that of transmission in Fabry–Pérot interferometer in optics, where the resonance condition and functional form of the transmission coefficient are the same.