This is about wave reflection and transmission. For an infinite string with a density change at x=0, consider an incident wave propagating to the right from x = -∞. The most general form is W = A cos(w(t-x/c)+θ), with amplitude A, angular freuqency w, time t, distance x (from origin), wave speed c and phase θ. I do not understand the term (t-x/c). Shouldn't it be just t since we're starting from infinity? Thanks!