I am dealing with the classic problem of a potential barrier of finite width, with a particle tunneling through, in the case of E < V. I am to calculate the transmission/reflection coefficients, and we first start off with the wavefunctions for the three regions. Before the barrier, we have Aexp(ikx) + Bexp(-ikx) In the barrier, we have Cexp(k1x) + Dexp(-k1x) After the barrier, we have Fexp(kx). My only question about this is where the i term comes from. It is probably a very simple answer but I want to know exactly its reason before I go on past this point. In the case of E > V, the i term appears in all the exponentials. Thank you.