Sagnac effect
The Sagnac effect named after French physicist Georges Sagnac
Georges Sagnac
Georges Sagnac was a French physicist who lent his name to the Sagnac effect, a phenomenon which is at the basis of interferometers and ring laser gyroscopes developed since the 1970s. Sagnac died at Meudon-Bellevue....

, is a phenomenon encountered in interferometry
Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

 that is elicited by rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...

. The Sagnac effect manifests itself in a setup called ring interferometry. A beam of light is split and the two beams are made to follow a trajectory in opposite directions. To act as a ring the trajectory must enclose an area. On return to the point of entry the light is allowed to exit the apparatus in such a way that an interference pattern is obtained.