The energy contained in a small volume through which an electromagnetic wave is passing oscillates with

Energy contained per unit volume through which electromagnetic wave is propagating is sum of energy of electric field and energy of magnetic field. U=UE+UB=12∈o E2+12B2μ0Where E and B are Sinusoidal functionE=E0sinω(t−XC) &  B=Bosinω(t−XC)⇒UE=12∈0E2=12E 02sin2ω(t−XC)  ⇒UE=12×E021−cos2ω(t−XC)2Hence the frequency of UE is double than that of electromagnetic wave. Similarly the frequency of UB is double than that of electromagnetic wave.