Polynting vectors `vecS` 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 `vecS=1/(mu_0) vecExxvecB`. Show the nature of `S` vs `t` graph

Explanation

Consider and electromagnetic waves. Let E varying along y-axis B is along z-axis and propagation of wave be along x-axis . Then `Exx B` will tell the direction of propagation of energy flow in electomegnetic wave along x-axis.
`E=E_(0) " sin"(omegat - kx)hagj`
`B=B_(0)" sin "(omegat-kx)hatk`
`S=1/mu_(0) (Exx B) = (1)/(mu_(0))E_(0)B_(0)" sin"^(2)(omegat -kx)[hatjxxhatk]`
`(E_(0)B_(0))/(mu_(0)) "sin"^(2)(omegat -kx)hati`
The variation of `|S|` with time t will be as given in the figure below