An optical waveguide comprises a body (13), the body including an entrance window (9) and an exit window (11) defining an optical path (13) through a cavity. The cavity contains a first fluid (A) and a second fluid (B), with an interface between the first fluid and the second fluid defined by a meniscus. The meniscus lies longitudinally along the optical path. Means for adjusting the meniscus are provided, for example a voltage source and at least two electrodes. Electrowetting can be used for influencing the fluids.