Why are there visible coloured bands in a jet engine afterburner flame?
The “bands or rings” you refer to are known as Shock Diamonds or Mach Disks. They occur anytime exhaust exits the nozzle at supersonic speeds and the pressure of the exhaust is lower than atmospheric pressure (known as overexpansion). Because the pressure of the exhaust gas is lower than atmospheric pressure, the exhaust gas is compressed inward. As this happens, the pressure of the exhaust gas begins to increase, and continues to do so until the pressure of the exhaust is greater than atmospheric pressure. At this point, the exhaust gas begins to expand (and the pressure starts to decrease) and continues until atmospheric pressure is once again greater than the pressure of the exhaust gas…and the cycle starts over. Each completed cycle causes a shockwave that creates the rings you see.
