Ray tracing fails except on very abrupt or very gradual impedance changes.

Evidence

Ray tracing is only an approximation. It misses diffraction altogether and is accurate and practicable only when the changes in the impedance (and thus the wave speed) in the medium are either very abrupt, in which case there is ray splitting involving reflection and refraction at the interface, or quite slow on the scale of a wavelength, in which case the wavefronts and associated rays curve gracefully. — Eric J. Heller, Why You Hear What You Hear- An…, 208

Categorization

• DEFINE: Precise conditions for when geometrical acoustics/optics is valid — the domain of ray tracing.