Snell’s law and SOFAR channels: a particle view

We consider the motion of a “particle” in a medium of variable index of refraction whose motion is governed by Snell’s law. We give a simple derivation of the Newtonian forces that dictate the motion and show that a position-dependent variable mass is necessary. We apply the equations of motion and find the conditions for a SOFAR channel, a phenomenon where rays are trapped in a channel.