2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/185576It is shown that the amplitude for reflection of a Dirac particle with arbitrarily low momentum incident on a potential of finite range is -1 and hence the transmission coefficient T=0 in general. If however the potential supports a half-bound state at k=0 this result does not hold. In the case of an asymmetric potential the transmission coefficient T will be non-zero whilst for a symmetric potential T=1.12 pages; revised to include additional references; to be published in J Phys AHigh Energy Physics - Theorytext