Answer
$\frac{1}{2}\epsilon_0~E^2 = \frac{1}{2~\mu_0}~B^2$
In an EM wave traveling in a vacuum, the electric and magnetic energy densities are equal.
Work Step by Step
We can show that the electric and magnetic energy densities are equal.
$\frac{1}{2}\epsilon_0~E^2 = \frac{1}{2}\epsilon_0~(c~B)^2$
$\frac{1}{2}\epsilon_0~E^2 = \frac{1}{2}\epsilon_0~c^2~B^2$
$\frac{1}{2}\epsilon_0~E^2 = \frac{1}{2}\epsilon_0~(\frac{1}{\sqrt{\epsilon_0~\mu_0}})^2~B^2$
$\frac{1}{2}\epsilon_0~E^2 = \frac{1}{2}\epsilon_0~(\frac{1}{\epsilon_0~\mu_0})~B^2$
$\frac{1}{2}\epsilon_0~E^2 = \frac{1}{2~\mu_0}~B^2$
