The solutions of this equation are rising and falling exponentials in the form of evanescent waves. When M (x) varies with position, the same difference in behaviour occurs, depending on whether M(x) is negative or positive. It follows that the sign of M (x) determines the nature of the medium, with negative M (x) corresponding to medium A and positive M (x) corresponding to medium B. It thus follows that evanescent wave coupling can occur if a region of positive M (x) is sandwiched between two regions of negative M (x), hence creating a potential barrier.