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

${\displaystyle \Psi (x)=Ae^{ikx}+Be^{-ikx}}$

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 ${\displaystyle 2*\pi /L}$ 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 (-${\displaystyle \infty }$,0) V(x)<E(x)and from [0, ${\displaystyle \infty }$) V(x)>E(x) and the solutions of the Schrödinger Equation are of the form

${\displaystyle \Psi (x)=Ae^{i{\sqrt {{\frac {2m}{\hbar ^{2}}}(E(x)-V(x))}}x}+Be^{-i{\sqrt {{\frac {2m}{\hbar ^{2}}}(E(x)-V(x))}}x}}$

on the first interval and

${\displaystyle \Psi (x)=Ce^{{\sqrt {{\frac {2m}{\hbar ^{2}}}(E(x)-V(x))}}x}+De^{-{\sqrt {{\frac {2m}{\hbar ^{2}}}(E(x)-V(x))}}x}}$

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.