I'd say "yes".

I mentioned it in a different thread here, wondering the same thing myself.

Section 7 in
http://www.islandnet.com/~see/weather/eyes/wxwatch2.htm


And then here's a nice paper with lots of math and such
http://www.ncas.org/condon/s6chap04.htm

And then a page on rendering mirages:
http://www.cs.man.ac.uk/aig/mash/MSc_stuff/render.html


"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?
>
> Samuel benge
>
> Simon de Vet wrote:
>
> > Mike wrote:
> >
> > > The effect is real.  It's more noticable if you are on higher ground
> > > looking down on the water.  I put an example in binaries.images.
> >
> > Better yet, look at a puddle.
> >
> > When it's in the distance, it is very shiny, like a mirror. You cannot
> > see the ground beneath the water. As you approach it, and view it at
> > steeper and steeper angles, the reflection seems to go down, and you can
> > see through it more easily. When you are directly above the puddle, it
> > seems to be almost completely transparant, and hardly reflective at all.

--
"My new computer's got the clocks, it rocks
But it was obsolete before I opened the box" - W.A.Y.

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I believe so.  Air of different tempuratures have slighter different IORs.
That's why hot air rising casts shadows like ripples on the ground and
distorts the view.  Hot air has the lower density, so the viewing ray is going
from a higher IOR to a lower one on the ground.  At a shallow enough angle,
TIR should occur.

Mostly speculation on my part. :)

-Mike

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?
>

Post a reply to this message

"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.

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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 meant to bring this up with a couple of the other comments made to this
thread and that is that it's not a reflection action that occurs rather
it is a refraction through the differing IOR's that makes the phenomenon
happen. Reflection and refraction are two different physical occurrences
and they are in most cases mutually exclusive.

-- 
Ken Tyler

See my 700+ Povray and 3D Rendering and Raytracing Links at:
http://home.pacbell.net/tylereng/index.html

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Ken wrote:
> 
> 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 meant to bring this up with a couple of the other comments made to this
> thread and that is that it's not a reflection action that occurs rather
> it is a refraction through the differing IOR's that makes the phenomenon
> happen. Reflection and refraction are two different physical occurrences
> and they are in most cases mutually exclusive.
> 
	??? What do you mean "mutually exclusive"? In most cases, both occur
simultaneously (each time you change medium: air/water, air/glass, ...).
The only cases I can think of where you can have one without the other
is the mirage (or anytime the IOR changes continuously) where you only
have refraction and the metallic surface where you have no transmitted
light (and therefore no refraction...)

		Jerome

-- 
*******************************

* they'll tell you what can't * mailto:ber### [at] inamecom
* be done and why...          * http://www.enst.fr/~jberger
* Then do it.                 *
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I believe Ken means light only enters the medium and is refracted if it is not
reflected. Light can be partially reflected or refracted, but only one of the
two can happen on a photon by photon basis.

Margus

"Jerome M. BERGER" wrote:
> 
>         ??? What do you mean "mutually exclusive"? In most cases, both occur
> simultaneously (each time you change medium: air/water, air/glass, ...).
> The only cases I can think of where you can have one without the other
> is the mirage (or anytime the IOR changes continuously) where you only
> have refraction and the metallic surface where you have no transmitted
> light (and therefore no refraction...)
> 
>                 Jerome
> 
> --
> *******************************

> * they'll tell you what can't * mailto:ber### [at] inamecom
> * be done and why...          * http://www.enst.fr/~jberger
> * Then do it.                 *
> *******************************

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Margus Ramst wrote:
> 
> I believe Ken means light only enters the medium and is refracted if it is not
> reflected. Light can be partially reflected or refracted, but only one of the
> two can happen on a photon by photon basis.
> 
> Margus

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.

  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.


-- 
Ken Tyler

See my 700+ Povray and 3D Rendering and Raytracing Links at:
http://home.pacbell.net/tylereng/index.html

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Margus Ramst wrote in message <37B0DFCB.4BB5598C@peak.edu.ee>...
>I believe Ken means light only enters the medium and is refracted if it is
not
>reflected. Light can be partially reflected or refracted, but only one of
the
>two can happen on a photon by photon basis.


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

Mark

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>Ior is a property of a material. Reflection is a property of a
surface.<
Hmm, actually, they can both be considered properties of the boundaries
between two materials(and the same substance at a different temperature
could be considered a different material.) And gaseous materials do
reflect light, although usually in small amounts.(although clouds, fog,
etc are not examples of this because they are droplets of liquid
suspended in the air.)
Of course, this has nothing to do with the way POV considers it...

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Ken wrote:
> 
> Margus Ramst wrote:
> >
> > I believe Ken means light only enters the medium and is refracted if it is not
> > reflected. Light can be partially reflected or refracted, but only one of the
> > two can happen on a photon by photon basis.
> >
> > Margus
> 
> 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.
> 
>   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.
> 
> --
> Ken Tyler
> 
> See my 700+ Povray and 3D Rendering and Raytracing Links at:
> http://home.pacbell.net/tylereng/index.html

	Well, actually I think that reflection depends on the ior (or rather on
the difference of iors between two materials) which in turn depends on
its electrical conductivity (is that the right word?). At least, that's
what happens when an electromagnetic wave hits a metal (please don't
think in photons as they are governed by quantic laws, which I'm not
familiar with): the electric field in the wave agitates the metal's
surface electrons creating an eletric current (since the metal is a
conductor) which in turn generates a new electromagnetic wave perfectly
in sync with the incoming one but of opposite sign. This wave is of
course emitted in all directions which means it *cancels* the incoming
wave inside the metal and creates what we call "reflection" outside.
When the material isn't as good an electrical conductor, the new wave
isn't as strong as the incoming one and that's what causes partial
reflection and refraction. Moreover the exact effect depends on the
angle between the *eletrical field* (not the direction of travel) and
the surface, therefore: angle dependant effects and polarization
dependant effects. This same effect causes absorption and scattering: as
the wave travels through a slightly conductive material, it will agitate
more and more electrons along the way and each time a small fraction of
the wave gets cancelled out and reemitted in other directions.

	Okay, there's just one problem with this model: I don't remember how it
accounts for materials like wood that neither reflect nor transmit any
light...

	Just my 2c,
		Jerome

-- 
*******************************

* they'll tell you what can't * mailto:ber### [at] inamecom
* be done and why...          * http://www.enst.fr/~jberger
* Then do it.                 *
*******************************

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