Now, if you look at this diagram what happens is that, if I call this height say, we have to
keep on introducing some mathematical terms now, and if I call to be the mean surface,
this is my mean surface, this writing is too small, is it? Then what happen, that xi can be
expressed as a cos function, cos of… I explained to this terms later on. See if you take a
function xi as xi a cos k x minus omega t, this is just a sine function, a sinusoidal
function, when x represents the x axis, this is my x and t of course is time.