Then substituting the conductivity in the equation of n , gives n = 1 + χ χ e which is a more familiar form. For the region z < 0 , one imposes the condition of a wave traveling to the left. By setting the conductivity in this region σ σ = 0 , we obtain the reflected wave E ( z ) = E R exp ⁡ ⁡ ( − − i k z ) , traveling at the speed of light.