I'm trying to make sense of what actually happens to light when it interacts with an opaque surface.
The fresnel equations give us the proportion of light which is reflected off the surface of a material, and the proportion of the light which is transmitted into the material. For something like glass, this makes sense, as there is a refracted ray which proceeds through the second transparent medium. Its less clear to me what this means with, say a rock or a piece of metal.
In computer graphics, we often represent a BRDF $f(\omega)$ like this:
$$ f(\omega) = \biggl((1 - F)f_{\text{Lambert}}(\omega) + Ff_{\text{C-T}}(\omega) \biggr)\cos\theta $$
where $F$ is the fresnel factor, $K_m$ is either 1 or 0 depending on if the material is a metal or not, $f_{lambert}$ is the lambertian BRDF, and $F_{C-T}$ is the Cook-Torrance BRDF.
It is assumed that all reflections are perfectly specular (and diffuse reflections emerge because, on a microscopic scale, the surface is rough and thus specularly reflects light in odd directions).
The term: $Ff_{\text{C-T}}(\omega)$ makes perfect sense to me. The fresnel factor tells us how much light is reflected, and we evaluate a microfacet-based BRDF (namely the Cook-Torrance model) to calculate those reflections.
The term: $(1 - F)f_{\text{Lambert}}(\omega)$ has me slightly more confused because it tells us what happens to the light that is transmitted into the material, but it isn't immediately obvious to me what it means to be transmitted into an opaque material.
From reading on the topic, it appears that the transmitted light immediately scatters off the internal structure of the material and comes out in all different directions, hence the transmitted portion of the light being represented with a lambertian (diffuse) BRDF. Is this a correct (if not possibly simplified) understanding? How would this differ from subsurface scattering, as it sounds fairly similar?
I've also read that metals only reflect specularly, and that they tint their reflections. What happens to the light transmitted into a metal? Is it absorbed? And why would the reflections off of a metal be tinted with the color of the metal?
