Flux densities of the Lyα (top panel) and Lyβ (bottom panel) lines of atomic hydrogen obtained by applying the exact radiative-transfer algorithm of Appendix A on an interstellar shock. The flux densities are computed in the backward direction, that is in the direction of the preshock material, at the position z = 0. The model displayed corresponds to a shock at 50 km s⁻¹ propagating in a medium with a density of 1 cm⁻³ and a magnetic field strength of 1 μG, and irradiated by an external UV radiation field with a scaling factor G₀ = 1 (in Mathis units).