An oscillating quantity a can be written as a= ao cos(wt), where ao is the amplitude, w is the angular fre- q ueno cy, f=w/ 2T is the frequency, and T=1/f is the period a plane wave a =a(t-0)travelling along the x-axis can be written as a= an coslwlt The quantity w(t-i) in the bracket is the phase The wave is also unattenuated since an is a constant