Typically three or more mirrors are used, so that counter-propagating light beams follow a closed path such as a triangle or square (Fig. 1). Alternatively fiber optics can be employed to guide the light through a closed path (Fig. 2). If the platform on which the ring interferometer is mounted is rotating, the interference fringes are displaced compared to their position when the platform is not rotating. The amount of displacement is proportional to the angular velocity of the rotating platform. The axis of rotation does not have to be inside the enclosed area. The phase shift of the interference fringes is proportional to the platform's angular frequency ω ω and is given by a formula originally derived by Sagnac: Δ Δ ϕ ϕ ≈ ≈ 8 π π λ λ c ω ω ⋅ ⋅ A where A is the oriented area of the loop and λ λ the wavelength of light.