Simple forms for equations of rays in gradient-index lenses
American Journal of Physics
The calculation of the shape of an optical ray in a gradient?index lens is often greatly simplified by the use of formalism in which the equation governing the ray assumes the form of Newton’s law of motion. A new justification of this method is presented, and the method is applied to a gradient?index lens with cylindrical symmetry.
Evans, James. "Simple Forms for Equations of Rays in Gradient-Index Lenses." American Journal of Physics. 58.8 (1990): 773-778. Print.