The formula to calculate the energy confined within the electric component of an electromagnetic wave can be written as

...(i)

Let's assume that the equation of the electric component is given by

E = E 0 sin ...(ii)

Substitute the expression for the electric field component from equation (ii) into equation (i) and simplify to obtain the expression for the energy confined within the small volume dV.

...(iii)

From equation (iii), it can be concluded that the frequency of oscillation of the energy confined within a small volume is double the frequency of the wave.