An archetypical jet visible across the electromagnetic spectrum is that in 3C273. Comparison of MERLIN 18cm and HST optical images shows that the optical jet is narrower than its radio counterpart and that knots in the optical jet in the main correspond to narrow oblique features within the radio outline. Assuming the optical emission is incoherent synchrotron the lifetime of the optically emitting electrons (typically ~1000 yr) is 1000 times shorter than at radio wavelengths. The optical jet therefore traces the present location of the active jet whilst the radio emission provides information on its history for the past 1 Myr.
Bahcall (Princeton) et al., 1995, ApJ, 452, L91