Pointing vectors `vec(S)` is defined as a vector whose magnitude is equal to the wave intensity and whose direction is along the direction of wave propagation. Mathematically, it is given by `vec(S)=(1)/(mu_(0))(vec(E )xx vec(B))`. Show the nature of `vec(S)` vs graph.

Explanation

In electromagnetic wave let `vec(E )` is in y - direction `vec(B)` is in z - direction and electromagnetic wave propagate in x - direction energy propagating will be in direction of `vec(E )xx vec(B)` (in x - direction).
`vec(E )=E_(0)sin(omega t-kx)hat(j)`
`vec(B)=B_(0)sin(omega t-kx)hat(k)`
`therefore vec(S)=(1)/(mu_(0))(vec(E )xx vec(B))=(1)/(mu_(0))E_(0)B_(0)sin^(2)(omega t-kx)(hat(j)xx hat(k))`
`therefore vec(S)=(E_(0)B_(0))/(mu_(0))sin^(2)(omega t-kx)hat(i)[because hat(j)xx hat(k)=hat(i)]`
Variation in magnitude of `|vec(S)|` with time is shown in figure shown below.