While propagating through a homogeneous medium (one in which the refractive index is spatially uniform), a ray simply goes in a straight line at speed c/n, where
c = 299,792,458 m/s is the speed of light in a vacuum and
n (dimensionless) is the absolute refractive index of the medium. In a graded-index medium, the ray can follow a curved path, which is determined by integrating coupled first-order ordinary differential equations over time.
The Geometrical Optics interface is compatible with the Ray Tracing and
Time Dependent study steps. These study types are very similar, except that the
Ray Tracing study step allows you to specify either a list of optical path length intervals or a list of time steps. (Internally, the optical path lengths are converted to the corresponding times, so this is just a matter of convenience.)