An interference effect that occurs when a beam of light is split and sent around a rotating ring in opposite directions, with interference occurring when the beams reunite. As it depends on rotational motion, the Sagnac effect can be used in optical gyroscopes (optical rotational sensors). The effect is named after Georges Sagnac (1869–1928), who performed this type of interference experiment in 1913. It was described by Max von Laue in terms of the special theory of relativity.