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The Sagnac effect is a type of interference that can be induced light beams from sources moving relative to each other. If a pair of light beams are arranged so they travel in opposite directions around an optical path circuit (using reflectors, such as a square with angled reflectors at each corner), then rotation of the entire set-up (e.g., around its center) affects interference patterns produced if the beams are combined. Such rotation lengthens the path of light traveling in one direction and shortens that traveling in the other (since the light takes time to travel around the circuit) affecting the phase that each beam has at the point where they are combined.
This effect is used in current technology, such as a device termed a ring laser (a laser with mirrors as described above, so its internal light is encircling in both directions), which is a component of a laser gyroscope, an optical device that fulfills the function of a gyroscope, sensing changes in attitude. The Sagnac effect has also been proposed as a possible mechanism for a future gravitational-wave detector.