On the velocity of light signals in deep underwater neutrino experiments

During the last few years, deep underwater neutrino telescopes of a new generation with volumes in order of 105 m3 and more were taken into operation. For the correct track reconstruction of events and for the interpretation of light pulses from calibration lasers one has to use the group velocity for light signals. The difference between group velocity and the traditionally used phase velocity leads to an additional delay of about 10 ns for a distance of 100 m between light source and photomultiplier.