What is the definition of Sagnac effect?

Represents the change in effective distance that light travels in a closed circuit when the circuit undergoes rotation or a change in orientation.

The Sagnac effect is used in optical gyroscopes by sending two beams of light in opposite directions in a fibre optic coil or mirror directed path. As the device rotates, the beam travelling against the direction of rotation experiences a slightly shorter path delay than the other beam. The difference in phase shift (see interferometry) between the two beams is then used to estimate the rate of rotation.

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