text: what is fringe in optics?   a fringe shift is most often referred to in interferometry experiments such as the michelson-morley. it is the behavior of a pattern of "fringes" when the phase relationship between the component sources change.  a fringe pattern can be created in a number of ways but the stable fringe pattern found in the michelson type interferometers is caused by the separation of the original source into two separate beams and then recombining them at differing angles of incidence on a viewing surface.