The similarity between the differential equations governing the motion of a particle and the rays in a wave field is recalled. The two become identical if the particle has unit mass and moves under the action of a potential energy V = - 1/c2, where c is the phase velocity of the waves....
Eckart, Carl. 1950. "The ray-particle analogy." Journal of Marine Research 9, (2). https://elischolar.library.yale.edu/journal_of_marine_research/727