Important Materials Theory - Photographic Rendering with V-Ray for SketchUp - page 248

However, as a surface gets smoother at the microscopic level, the way that light re-
flects from it alters. Instead of continuing to be scattered in a multitude of directions
as in our illustration here, the reflection angles narrow, become fewer in number, and
so introduce blurred reflections.
Eventually, on a completely smooth surface, the light can essentially be described as
bouncing only in a single direction. This, to our eyes, makes a surface look highly, if
not completely, reflective.
To be able to both differentiate between and describe the very different look that
these changes give us, we give this type of light reflection a unique name, referring
to it as specular rather than diffuse light reflection. Specular reflection is what we are
typically referring to when we talk about an object as being shiny or reflective.