  The Sagnac effect is usually considered as being a relativistic effect
produced in an interferometer when the device is rotating. General relativistic
explanations are known and already widely explained in many papers. Such
general relativistic approaches are founded on Einstein's equivalence principle
(EEP), which states the equivalence between the gravitational "force" and the
pseudo-force experienced by an observer in a non-inertial frame of reference,
included a rotating observer. Typically, the authors consider the so-called
Langevin-Landau-Lifschitz metric and the path of light is determined by null
geodesics. This approach partially hides the physical meaning of the effect. It
seems indeed that the light speed varies by c\pm\omega r in one or the other
direction around the disk. In this paper, a slightly different general
relativistic approach will be used. The different "gravitational field" acting
on the beam splitter and on the two rays of light is analyzed. This different
approach permits a better understanding of the physical meaning of the Sagnac
effect.
