What is the distinction between a "ray" and a "wave" in optics? What is the distinction between a ray and a wave in optics? From what I can gather, the only discernible difference is in nomenclature, where a ray simply refers to an EM wave with short wavelengths. Is this valid, or is there something else hidden that I may be missing?
 A: Diffraction occurs when the wavelength and dimension of aperture or slit through which it is passing  becomes comparable. When the dimension of  the respective system is much larger than the wave length we can neglect the wave properties and consider it to be a ray.Depending on this two different cases we use the terms "geometrical optics" or "physical optics".
"geometrical optics" is where light can be considered to be a ray and it is geometric because the path that it follows can be obtained by using geometric diagram.Where as "physical optics" is where the diffraction and interference becomes important and to describe these phenomenon wave nature of light must be considered.
"Eikonal equation" gives a link between ray(geometric) optics and wave(physical) optics.For more information refer to  $\textit{Born and Wolf }$(chapters 3-5).
A: You're more or less correct. The ray is the limit of a wave where interference effects go to zero, and we can then treat light classically using ray diagrams and such. 
A: The ray is associated with being normal to the wavefront.  However, the ray concept, energy flow, and wavefront somewhat breakdown when you're in birefringement media.  
A: A ray is a simplified (asymptotic) description of a wave (which is a field) for the case when neither diffraction or scattering have to be considered. Ray optics requires that the curvature of wavefronts be large compared to the wavelength. JQK is correct that birefringent media require a slight modification of the simple ray algorithms for reflection and refraction.
