When a plane flies faster than the speed of sound, the pressure waves in front don’t have time to flow around the aircraft and the air becomes compressed, eventually producing a shock wave, its energy discharge yielding the sonic ‘boom’ for which supersonic planes are famous.
NASA has managed to capture this image of two such shock waves interfering like ripples on a pond, with the resulting variation of air density causing light to bend differently and so be observed as different colours.
