To simplify, consider only waves propagating in one dimension (extension to three dimensions is straightforward). Then the general solution is u ( x , t ) = A e i ( k x − − ω ω t ) + B e − − i ( k x + ω ω t ) , in which we may take ω = kc. The first term represents a wave propagating in the positive x -direction since it is a function of x − ct only; the second term, being a function of x + ct, represents a wave propagating in the negative x -direction.