Consider a wave guide built from two parallel slabs at $x=0, x=L$. between the slabs there is a dielectric material with $\mu=\epsilon=1$ and electric conductivity with a frequency of $\sigma(\omega)=\frac{\omega^2}{4\pi \omega_0}$. an electric field that moves to the $\hat z$ direction and linearly polarized at the $\hat y$ enters the waveguide.
my prof. used the following wave equation:
$\nabla^2 E=\frac{1}{c^2}\frac{\partial^2E }{\partial t^2}+\frac{4\pi\sigma(\omega)}{c^2}\frac{\partial E }{\partial t}$
I don't understand why this equation is used and I did not find any info about it.