Jerome M. BERGER <jbe### [at] enstfr> wrote:
: 	Yes, what I meant was that you can't see any reflections on a wood
: surface (unless it is higly polished and varnished). The effect that
: causes wood to become visible is probably different from the one I
: described since it is independant on the viewing direction...

  I think it is angle dependant. That's why we see specular highlights.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Matt Giwer wrote:
> 
> "SamuelT." wrote:
> 
> > When I read this statement of yours I immediately thought of mirages. I
> > wonder if mirages are caused in part to this natural effect?
> 
>         Yes. It is a total reflection angle that is dependent upon the
> IOR of what is reflecting it. Air near the ground is hotter,
> decreasing its IOR. 

	I forgot about this discussion. 

	At some critical (shallow) angle there is "total" reflection.
Debating total is left to the purists. There is also refraction
agreed. 

	Now as to the mirage case ...

	The total reflection plane is the closest to the ground. That is
why it look blue because it is reflecting the sky. Above that is
the refraction area. As it is rising heat, there is turbulence.
That is what makes the reflected sky appear to ripple and thus
appear to be water -- a fashion only a person desperate for water
would imagine. Also note the critical angle surface is also
rising heat so it is also turbulent and will contribute to the
water-like appearance. 

	Generally we are familiar with a critical angle for water and
glass. The critical angle is dependent upon the difference in
IORs for the media. The less the difference, the shallower the
angle and the further away the mirage water appears to be. 

	In the mirage case we are not dealing with our usual experience
with water and glass but with a material with a turbulent
boundary. We have both refraction and reflection at work. What we
can say is that refraction is in play higher than the reflection
height but we can not see refraction effects below that
reflection height. 

	This is the sort of thing that makes physics fun. It is
separating two phenomena and they seeing how they interact.
Newton's first law of motion (stay in motion) would have been
known to cavemen were it not for friction. Once motion and
friction were split there was progress. 

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Ken wrote:

> Ior is a property of a material. Reflection is a property of a surface. At least
> this was the rational for moving ior into it's own property wrapper i.e. interior.
> I know that surfaces can be considered to have an ior value but this is generaly
> a very minor property where roughness and color have a much greater impact than
> does a surfaces ior value will.

	Yes but ... a surface can be defined as a constant IOR from any
cause. In the case of air the isosurfaces for temperature and IOR
would be the same. You can go through any perturbed volume and
define a surface as all points with the same perturbation value,
be it temperature, IOR, electric or magnetic potential. 

>   To really clarify my point air does not generaly reflect light. It will transmit
> and refract light in varying amounts however. Shiny steel will reflect light redily
> but has extremely poor light transmittance.

	However, when the IOR varies there is an angle at which incident
light is reflected. The smaller that difference, the shallower
that angle. 

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Nieminen Mika wrote:

>   Actually wood reflects light. If it didn't, you would see it as black.
> And a white object in a room with wooden walls will certainly be colored
> differently than in a room with white walls.
>   I think that the color of the wood (or any material) depends on the
> frequency of the electrons the light affects.

	I think here we are getting into coherent (mirror, waxed wood)
and incoherent (regular wood) reflection. If we could smooth
regular wood to a fraction of a wavelength of light it would be a
mirror also. To the mirage case, were there no turbulence people
would clearly see any clouds in the sky. If there were more
turbulence such as a wind the it would not be smooth enough to
reflect an image. 

	As to color, yes it is a matter of what is reflected and
absorbed. Due to the wavelength of light it is most always only
chemical bond lengths that affect the color rather than
monatomic. (I think it is always rather than most always but I'd
have to think about it.) 

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Mark Wagner wrote:

> Take this one up with the quantum physicists -- they will tell you that a
> photon can be simultaneously reflected and refracted. :-)

	Yes but that is just the way we have to make it fit the macro
world models we have. We could always solve the quantum equations
instead after we break 4096 bit PGP in a year to test the
processor's speed. 

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