  In the field of rare event physics, it is common to have huge masses of
organic liquid scintillator as detection medium. In particular, they are widely
used to study neutrino properties or astrophysical neutrinos. Thanks to its
safety properties (such as low toxicity and high flash point) and easy
scalability, linear alkyl benzene is the most common solvent used to produce
liquid scintillators for large mass experiments. The knowledge of the
refractive index is a pivotal point to understand the detector response, as
this quantity (and its wavelength dependence) affects the Cherenkov radiation
and photon propagation in the medium. In this paper, we report the measurement
of the refractive index of the JUNO liquid scintillator between 260-1064 nm
performed with two different methods (an ellipsometer and a refractometer),
with a sub percent level precision. In addition, we used an interferometer to
measure the group velocity in the JUNO liquid scintillator and verify the
expected value derived from the refractive index measurements.
