K. Scharnhorst: On propagation of light in the vacuum between plates. Physics Letters B 236(1990)354-359 (DOI: 10.1016/0370-2693(90)90997-K).
Abstract: QED is considered in the presence of two parallel plates (Casimir effect type configuration) imposing boundary conditions on the photon vacuum fluctuations. Two-loop corrections arising from the boundary conditions for the photon vacuum fluctuations to the QED effective action are calculated in a physically reasonable approximation. From this effective action we find that for light propagating perpendicular to the plates in the vacuum between them, their impact phrased in the simplest terms consists in causing a change in the velocity of light.
The article has been covered by various media in the following popular
Also see: Tim Bierman: Speed of light. New Scientist 126, No. 1714 (1990)93, 28 April 1990.
Also see: Philippe Jetzer, Norbert Straumann: Kausalität und Lichtgeschwindigkeit [Causality and the speed of light]. Sterne und Weltraum 30:7 (1991)422-423. [in German]