question_answer 33)

  Poynting vector  is defined as a vector whose magnitude is equal to the wave intensity and whose direction is along the direction of wave propogation. Mathematically, it is given by   Show the nature of S vs t graph.

Answer:

                  In em wave let  be varying along y-axis,  is along z-axis and propagation of wave be along x-axis. Then will tell the direction of propagation of energy flow in em wave, along x-axis.                 Let  ;                                                     The variation of  with time t will be as shown in Fig. 5(EP).l by thick solid curve.