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> On desert you can see images in the air. You know how ?
> http://www.islandnet.com/~see/weather/graphics/photos/supmirag.gif
On hot roads too, you know...
It's an ior gradient.
You can easily reproduce this phnomenon in pov... and get max_trace_level
very high. (you put boxes with progressive ior and check the result).
selsek
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On Wed, 07 May 2003 15:26:48 -0400, Andrew Coppin wrote:
> Here's a simple enough question...
>
> Is it possible for a transparent substance to be non-reflective? Or are
> all transparent substances inherantly reflective to a certain degree?
If you work at it hard enough ... If you look at it as an impedance
mismatch causing the reflection then you can layer materials on a surface
which "match" the impedance of air to glass. That gives a number of
smaller reflections which sum to less than the single reflection without
them. That is all the anti-reflection coatings do on lenses.
If you could apply a single layer that smoothly transitioned the IOR from
air to glass you would theoretically have no reflection as there would not be a
point of different IORs -- unless a low pressure front moves in or the
humidity changes.
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On Thu, 08 May 2003 06:17:03 -0400, Warp wrote:
> Andrew <ast### [at] hotmail com> wrote:
>> If something was truly non-reflective, it would be perfectly invisible
>
> A transparent object which refracts light is never invisible.
Observable and invisible are different things. A gravitational lens is
invisible but observable. The hot air over a road is invisible but
observable by the turbulent distortion.
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On Wed, 07 May 2003 15:26:48 -0400, Andrew Coppin wrote:
> Here's a simple enough question...
>
> Is it possible for a transparent substance to be non-reflective? Or are
> all transparent substances inherantly reflective to a certain degree?
>
Also the turbulent hot air over a road in summer is an example of
smoothly varying IOR that does not in itself reflect. We cannot see the
turbulence whorls themselves as they do not reflect because of the
smoothly varying IOR.
The shallow angle mirage is somewhat different phenomenon.
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On Thu, 08 May 2003 03:48:09 -0400, Andrew Coppin wrote:
> OK, well, a lump of shashed up ice doesn't look reflective at all... but
> take a bit and polish it to give it a nice smooth surface and you find
> that it actually IS reflective. Was thinking, if you have a layer of
> gas, it's surface probably isn't smooth, which might explain why it
> doesn't look reflective.
>
> If that makes sense.
>
Smoothed up it is specular reflection which is a special case of
reflection. The gas has a smoothly varying IOR so it does not have a
reflective surface.
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On Tue, 13 May 2003 17:02:50 -0400, selsek wrote:
>> On desert you can see images in the air. You know how ?
>> http://www.islandnet.com/~see/weather/graphics/photos/supmirag.gif
>
> On hot roads too, you know...
> It's an ior gradient.
> You can easily reproduce this phnomenon in pov... and get
> max_trace_level very high. (you put boxes with progressive ior and check
> the result).
>
Shallow angle reflection is a different issue. A sheet of glass is
totally reflective at a shallow angle.
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simian <sim### [at] localhostlocaldomain> wrote:
> Observable and invisible are different things.
Right. They are opposite things.
> The hot air over a road is invisible but
> observable by the turbulent distortion.
It can't be invisible if it's observable. :)
--
plane{-x+y,-1pigment{bozo color_map{[0rgb x][1rgb x+y]}turbulence 1}}
sphere{0,2pigment{rgbt 1}interior{media{emission 1density{spherical
density_map{[0rgb 0][.5rgb<1,.5>][1rgb 1]}turbulence.9}}}scale
<1,1,3>hollow}text{ttf"timrom""Warp".1,0translate<-1,-.1,2>}// - Warp -
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3eba0869@news.povray.org...
> Hmm... gases don't *appear* reflective... but does a gas have a surface???
The point is IOR variation : on a hot tarmac you can observe air reflecting
the sky because the temperature of the air film is higher than the ambient
air.
If you have density variation between 2 medias or inside a media, you get
refraction (and reflection if the ior gradient is high enough).
Marc
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In article <3ec4ecee@news.povray.org>,
"Marc Jacquier" <jac### [at] wanadoofr> wrote:
> 3eba0869@news.povray.org...
> > Hmm... gases don't *appear* reflective... but does a gas have a surface???
> The point is IOR variation : on a hot tarmac you can observe air reflecting
> the sky because the temperature of the air film is higher than the ambient
> air.
> If you have density variation between 2 medias or inside a media, you get
> refraction (and reflection if the ior gradient is high enough).
Also, I've read somewhere about the possibility of using high-density
plasma as a mirror. This was in the context of a nuclear-pumped X-ray
laser, where the mirror didn't have to exist for more than a few
microseconds.
I'm not sure about the exact principles behind that one though...since
light interacts through electromagnetic effects, I'm guessing a
highly-ionized plasma will have quite different optical properties
compared to the neutral gas, maybe acting more metallic. One place I
know of this kind of difference is in atmospheric nitrogen lasers...they
lase in the ultraviolet range, but produce very short pulses because the
lasing medium (atmospheric pressure air with a corona discharge through
it) becomes highly opaque to the wavelength being emitted.
And this is all about specular reflection...gases do reflect diffusely,
scattering in other words. For example, our atmosphere scatters blue
wavelengths more than red ones, making the sky appear blue except for at
sunrise and sunset, where you're seeing the wavelengths that weren't
scattered as highly. This effect is enhanced by dust in the air, but I
don't think it depends on it.
--
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: chr### [at] tagpovrayorg
http://tag.povray.org/
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On Fri, 16 May 2003 11:16:15 -0400, Christopher James Huff
<cja### [at] earthlinknet> wrote:
>For example, our atmosphere scatters blue
>wavelengths more than red ones, making the sky appear blue except for at
>sunrise and sunset, where you're seeing the wavelengths that weren't
>scattered as highly. This effect is enhanced by dust in the air, but I
>don't think it depends on it.
It's just that the nitrogen molecule scatters blue. Dust can be of any
color, making up for some gorgeous sunsets after a sandstorm (we had
some last month when a heat wave from North Sahara hit us.)
Peter Popov ICQ : 15002700
Personal e-mail : pet### [at] vipbg
TAG e-mail : pet### [at] tagpovrayorg
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