Rays are surrogates traveling perpendicular to wavefronts.

Evidence

Optical devices are designed by ray tracing because following the wave motion is far too expensive. A simple rule is used: Rays travel in straight lines unless interrupted by walls or lenses, following the course of the energy emitted from the source. More precisely, the rays travel perpendicular to the wavefronts, for which they are the surrogates. — Eric J. Heller, Why You Hear What You Hear- An…, 204

Categorization

• DEFINE: Core operational definition of a ray: the energy path, perpendicular to wavefronts, used when full wave computation is too costly.