where μ is the magnetic permeability, ε is the (real) electric permittivity and σ is the electrical conductivity of the material the wave is travelling through (corresponding to the imaginary component of the permittivity multiplied by omega). In the equation, i is the imaginary unit, and ω is the angular frequency of the wave. In the case of an ideal dielectric (where the conductivity is zero), the equation reduces to the real number