A wave represented by the given equation y = a cos(kx-ωt) is superposed with another wave to form a stationary wave such that the point x = 0 is a node. The equation for the other wave is

Since the point x = 0 is a node and reflection is taking place from point x = 0. This means that reflection must be taking place from the fixed end and hence the reflected ray must suffer an additional phase change of π or a path change of λ2 .So, if yincident = a cos (kx-ωt)⇒yincident = acos(-kx-ωt+π) = -a cos(ωt+kx)