# What are electromagnetic waves?

In vacuum, they propagate with a velocity approximately given as $c = 3 \cdot {10}^{8} m {s}^{- 1}$
In presence of material medium their velocity is smaller than this and is given as $v = {\left(\epsilon \mu\right)}^{- \frac{1}{2}}$ where $\epsilon$ is the electrical permittivity and $\mu$ is the magnetic permeability.