Null energy condition for the electromagnetic field
= Null energy condition for the electromagnetic field
For every null vector $k$, put $q^\rho=k_\mu F^{\mu\rho}$. Antisymmetry gives $q\cdot k=0$, so $q$ has nonnegative norm and
$$
T^{\mu\nu}k_\mu k_\nu=\frac1{\mu_0}q^\rho q_\rho\geq0.
$$