Usually several mirrors are used, so that the lightbeams follow a triangular or square trajectory. Fiber optics can also be employed to guide the light. The ring interferometer is located on a platform that can rotate. When the platform is rotating, the point of entry/exit moves during the transit time of the light, so on exit one beam has covered less distance than the other beam. As a consequence, when the platform is rotating the interference pattern will be shifted as compared to the position of the interference fringes when the platform wasn't rotating. The shift is in proportion to the angular velocity of the Sagnac interferometer.