In Quantum Mechanics we are interested in solutions to the Schrödinger Equation that are Renormalizable. One class of functions that does this is the complex exponentials, and we can write a solution to the Schrödinger Equation as the sum of two complex exponentials
which is the superposition of waves, one traveling from the left, the other from the right. k is the wave number which is in units of 1/m and is usually given by where L is determined by boundary conditions.
Let us consider a situation in which there is a wave traveling from the left to the right. On the interval (-,0) V(x)<E(x)and from [0, ) V(x)>E(x) and the solutions of the Schrödinger Equation are of the form
on the first interval and
on the second interval
Even though at first glance our wave equation looks like a superposition of normal exponentials, it is still a complex wave though because the term inside of the square root is negative and this allows us to always have renormalizable solutions.