Fringe shift
In interferometry experiments such as the Michelson–Morley experiment, a fringe shift 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.
The interaction of the waves on a viewing surface alternates between constructive interference and destructive interference causing alternating lines of dark and light. In the example of a Michelson Interferometer, a single fringe represents one wavelength of the source light and is measured from the center of one bright line to the center of the next. The physical width of a fringe is governed by the difference in the angles of incidence of the component beams of light, but regardless of a fringe's physical width, it still represents a single wavelength of light.[1]
In the 1887 Michelson–Morley experiment, the round trip distance that the two beams traveled down the precisely equal arms was expected to be made unequal because of the, now deprecated, idea that light is constrained to travel as a mechanical wave at the speed C only in the rest frame of the luminiferous aether.
The Earth's presumed motion through that frame was believed to cause a local aether "wind" in the moving frame of the interferometer like a car passing through still air creates an apparent wind for those inside. It is crucial to avoid the Historian's fallacy and note that these experimenters did not expect that a mechanical wave would travel varying speeds within a homogenous isotropic medium of aether. Waves have been studied since antiquity and mathematically at least since Jean le Rond d'Alembert in the 1700s. Our modern understanding of the constancy of light, however, grants the additional, new, "non-mechanical," categorization of waves and subsequent new 4D behavior for electromagnetic waves that radically alters the interpretation of what the Michelson-Morley experiment actually measures. It was only since the work of Einstein and Minkowski that waves of a non-mechanical nature were conceived of and electromagnetic waves are no longer considered mechanical, therefore the experiment's interpretation is changed for modern sources. We now believe it to compare the speed of light in different directions instead of the path length difference expected by Michelson[1] and the aether theorists of the time.
If, now, the apparatus be revolved through 90° so that the second pencil is brought into the direction of the earth's motion, its path will have lengthened wave-lengths
