A device for determining a refractive index (Rl) in a large number of points of a physical medium, comprising: a light laser for performing a series of p pulsed laser rays (PLRs) each proceeding from a starting point Ap and arriving in an arrival point Bp following the shortest optical path between the two points Ap and Bp; a timer to record a time Tp when each PLR reaches the arrival point Bp; and a computer programmed for finding the shortest optical path among all the optical paths starting from the starting point Ap and all reaching the arrival point Bp, and from p linear equations linking elementary optical paths (L) and the refraction indices (N) to the detected travel time Tp, for solving the matrix equation [N]*[L]=[T] relative to a vector of the refractive indices [N] and repeating until [L] and [N] converge.