[en] The interaction of electromagnetic waves with a plasma is derived in the geometric optics approximation. In particular, the force exerted on the plasma is determined by the plasma conductivity tensor, Σ. The current in the plasma that is induced by the wave is written in four notation, j_w^μ(x) = ∫Α^μν(x+s/2,k)A_ν(x+s)exp(iks)d⁴sd⁴k/(2π)⁴. The total four-current j^μ is the sum of the current induced by the wave and the current in external sources, such as antennas j_x^μ. The external currents j_x^μ are related to the four-potential A_ν by the tensor Β^μν. The tensors Α^μν and Β^μν obey the constraints k_μΑ^μν = 0 from ∂_μj_w^μ = 0 and Α^μνk_ν = 0 from gauge invariance. In the radiation gauge, E = -(∂A/∂t)/c, one needs only the three-tensor Α, which is closely related to the plasma conductivity, Α = iωΣ/c and the three tensor β = -(c/4π)[kk-(k·k)I+(ω/c)²I]-Α. The three polarizations of waves ω(k,x,t) are given by the vanishing of one of the eigenvalues β of Β_h, the Hermitian part of Β, and the eigenvectors give the polarizations of E. A direct calculation of the force on the plasma is made using the Wigner tensor 4J_μν(x,k). The wave action J, the momentum and energy of the wave, and the three-force on the plasma are calculated