The intensity of an electromagnetic wave is given by which expression?

$I = \frac{c\mu_0 B_0^2}{2}$
$I = \frac{c\varepsilon_0 B_0^2}{2}$
$I = \frac{B_0^2}{c\mu_0}$
$I = \frac{E_0^2}{2c\varepsilon_0}$

Correct answer: $I = \frac{B_0^2}{c\mu_0}$

Solution

For a plane electromagnetic wave in vacuum, the average intensity is $I = \frac{1}{2}c\varepsilon_0E_0^2 = \frac{cB_0^2}{2\mu_0}$. Using the relation $E_0 = cB_0$, this becomes $I = \frac{B_0^2}{c\mu_0}$.

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