Our simple model of free–free absorption and its effect on spectral indices. Left: effect of free–free absorption on a simple power law spectrum with radio spectral index α = −0.7, with increasing lengths of absorbing column, for our observing frequencies in the observed frame. We assume an electron density n_e = 50cm⁻³ and temperature T = 10⁴K. Right: as Fig. 7, but with the effect of free–free absorption on a simple power law spectrum plotted in orange, with increasing lengths of absorbing column from zero (left) to 300 pc (right).