Show that average value of radiant flux density ‘S’ over a single period ‘T’ is given by .

The radiation flux density is given by,

But,

…(1)

Assume that the electromagnetic waves are propagated in X direction. So, the vector form of electric and magnetic field is given by,

Taking cross product of magnetic field and electric field we get,

…(2)

Substituting equation 2 in equation 1 we get,

Thus, average value of radiant flux density ‘S’ over a single period ‘T’ is given by

(Substituting value of c in these equation)