Darren New <dne### [at] sanrrcom> wrote:
> Of course, if the surface isn't smooth to start with, you're unlikely to see
> a mirror-like reflection to start with. But you're going to have a hard time
> polishing untreated wood to a shine no matter how smooth you make it

That's because wood in itself has such an extreme inhomogenity at microscopic
scale.

You can polish stone or plastic though to get mirror-like reflections.
They'll not be as prominent as in metal because they have to compete with
diffuse reflection, but the mirror-like component is still in there.


> and if
> you start getting something like fluorescence, you start seeing that
> reflections aren't really "bouncing" light at all, but light that's absorbed
> and re-emitted..

That's pure nonsense. If you deal with fluorescence you'll see that *all*
re-emitted light is *undirected* (at least with relation to the direction of
the absorbed light).

*Specular* reflections (i.e. the thing we'd normally call reflections - not to
be confused with PoV-ray's shortcut of "specular highlights") are *really*
"bouncing" light; it's the result of the electromagnetic wave (the "probability
wave" of the photon if you want to go Quantum) being unable to fully enter the
medium (speaking of permeability and permittivity), so the light wave (or part
of it) simply reflects, in order to comply with Maxwell's equations.

It can't work with absorption and re-emission - because the "incoming angle =
outgoing angle" law of specular reflection is a result of a light wave's
interference with itself. But as soon as a photon is absorbed by an electron,
the photon's probability wave collapses, so the re-emitted light's probability
wave has no way of interferencing with it, even if the direction of emission
would be in any way related to the absorbed photon.

It also can't work with absorption and re-emission because both are limited to
certain wavelengths for many materials, unless you go high temperature. Yet
specular reflections off most polished materials (except metals) are pure
"white" - even for materials that do show a strong color.

Only *Diffuse* reflections (i.e. the thing we'd normally not call reflections at
all) *may* sometimes be due to absorbed and re-emitted light.

Note however that quite a lot of instances of diffuse reflection are still
"bouncing" light. If you take a heap of fine-grained sugar for example, it the
diffuse white reflection is actually due to a very random sequence of
reflections and refractions at the single sugar crystals. A single "white"
sugar crystal is quite transparent though, so no absorption happening there.
And "brown" sugar gets its color because some of the light traversing the
crystals is absorbed by natural impurities - but there's no re-emission
involved (at least not to any significant degree in the visible spectrum).

You also typically get re-emissions at different (usually longer) wavelengths
than the absorbed photons (if only because of the principle of entropy). UV in,
visible out is a typical thing; or visible in, infrared out, though that's
usually due to thermal emissions. Green in, amber out is a possiblity as well.

In fact, what we'd normally call "diffuse" reflection is typically a combination
of some light "bouncing" at the surface, some light being absorbed by the
medium, some light "bouncing" below the surface ("subsurface scattering"), and
some (though usually not much) light (re-) emitted.

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