Sketches of the van der Laan (1966) model. As the plasmoid expands (left panel), its initial self-absorbed synchrotron spectrum is shifted toward lower frequencies (middle panel). If the source is observed at optically thin frequencies from the beginning, then it remains optically thin and the flux keeps decreasing as time moves on (right panel, NIR curve). Alternatively, if the source is initially optically thick, then the flux rises until the source turns transparent and subsequently declines in the thin regime (right panel, sub-mm and radio curves).