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 w(t-x/c). what is x/c negative? Update : http://www.animations.physics.unsw.edu.au/jw/travelling_sine_wave.htm I understand this derivation apart from x' = x-vt. Shouldnt it be x' = x-vt since x' is moving relative to x?