So, at t equal to 0, you will find that this becomes xi a cos 2 pi T by t, basically xi a here. Again you will find that it goes like that. And this time is T equal to 0 and this is T equal to T. In other words, in a period of this t it has repeated itself. So, when you write them as two pi by lambda X minus 2 pi by capital T into T. This is time, this is distance, it
turns out that this curve represents a sinusoidal wave form which has a wave length,
which repeats itself at every lambda and also at every t. At every time t it repeats and
every distance lambda it repeats.