Question
In an $E.M.$ wave the average energy density is associated with
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Answer
Energy density due to electric field $=\frac{1}{2} \epsilon E^{2}$
Energy density due to magnetic field $=\frac{1}{2} \frac{B^{2}}{\mu}$
since $\frac{E}{B}=c=\frac{1}{\sqrt{\epsilon \mu}}$
Energy density is equally contributed to by electric and magnetic fields.
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