Deconstruction of light speeds in moving refractive media

The independence of the Sagnac phase shift from refractive index can appear puzzling, particularly when compared with the Fizeau experiment where refractive index plays a central role. This article provides an intuitive basis for this difference by decomposing the light-transit time in a moving dielectric into: vacuum transit, refractive delay, and motion-induced differential distance. This decomposition reveals why the refractive contributions are negated in a Sagnac setup (hence the n-independence), yet contribute unequally in a Fizeau arrangement. The magnitudes of the effects reveal that light signals are 100% dragged at the speed of a moving refractive medium during refractive delay periods whose time fraction is in proportion to the refractive index n minus 1.