Demonstrating the Sagnac Effect Using Tabletop Optics on a Rotary Platform

A Sagnac interferometer splits an incident beam of light into two components which travel in opposite directions of the same path. Consequently, each beam travels an equivalent distance. However, by rotating the entire apparatus at a sufficient speed, a noticeable change in the beams’ interference pattern is observed. This pattern results from one beam travelling against rotation and the other travelling with rotation, resulting in an increase or decrease in apparent path length, respectively. This is known as the Sagnac Effect. By using a traditional mirror-and-laser interferometer setup and a large turntable, we demonstrate the Sagnac Effect by showing that a given angular velocity results in a phase shift which matches what is predicted.