Sagnac type gedanken experiment with linear motion (“Linear Sagnac Effect” (LSE)) was probably first proposed by Ghosal et al. Interestingly, it was found that both the segments contribute to the generalized Sagnac effect which implies that rotational motion is not essential for the Sagnac effect. In the early part of the present century a couple of experimental studies have demonstrated that a Sagnac like effect, which the authors called the generalized Sagnac effect, occurs in a light waveguide loop consisting of both linearly and circularly moving segments. Most of such approaches yield the correct magnitude of the Sagnac effect despite their vast differences in the physical basis of the effect. Since then numerous studies have been conducted on the interpretation of the effect, particularly from the viewpoint of the (co-)rotating observer which include standard special relativistic description, general relativistic description, synchronization issues in rotating frame, violation of relativity in rotating frame etc. Sagnac, however, proposed the experiment in support of his ether model and explained the effect without using relativity theory. The Sagnac effect, which is generally considered as one of the basic experimental effects of STR, is a first-order kinematic effect in v/ c. The Sagnac interferometer is nowadays used as a tool in civil aviation, to test gravitation theories, to examine quantum properties. The effect also has applications in geodesy and seismology. which are essentially Sagnac interferometers. The Sagnac effect receives a lot of interest owing to its practical use in Global Positioning System, fiber optic gyroscope, ring laser gyroscope, etc.
The effect has been observed experimentally in a wide range of wavelength bands, from radio to x-rays as well as using matter waves. The effect was first discovered by Sagnac. Sagnac effect is the difference in phase (or time of arrivals) of two coherent light beams (originated from a single light beam) propagating along a rotating closed loop in opposite directions.