(a) Spectral index between 5 GHz and 7 GHz as a function of the inclination angle for κ₀ = 0.6. The green square represents the angle where the emission becomes optically thin. Red and blue circles are the same as in Figs. 5 and 6. (b),(c) Positions of the jet surface where optical depth τ = 0.08 for 5 GHz and 7 GHz, respectively, with κ₀ = 0.6. l_{τ = 0.08} = 1 represents the position on the jet after which the emission becomes optically thin all along the jet, i.e., when τ ≪ 1.