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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On Fri, 16 May 2003 08:57:32 -0400, Warp wrote:

> 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. :)

	Assume wise expression and far away look. 

	Can you see the wind, Grasshopper?

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simian wrote:
> 	Assume wise expression and far away look. 
> 	Can you see the wind, Grasshopper?

Trees moving back and forth makes the wind blow.

-- 
Tim Cook
http://home.bellsouth.net/p/PWP-empyrean

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N++ o? K- w(+) O? M-(--) V? PS+(+++) PE(--) Y(--)
PGP-(--) t* 5++>+++++ X+ R* tv+ b++(+++) DI
D++(---) G(++) e*>++ h+ !r--- !y--
------END GEEK CODE BLOCK------

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In article <9l1acvo9rnd6u8ci0pehb6qv6pvguudkla@4ax.com>,
 Peter Popov <pet### [at] vipbg> wrote:

> 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.)

Well, dust can be of any colored substance, but particles of the size 
that will stay in the atmosphere for any significant amount of time are 
small enough that they scatter blue wavelengths more. Similar to the 
reasons shorter wavelengths are used for higher magnification microscopy.

Try pouring a little milk into a glass of water and shine a flashlight 
through it...depending on the angle you observe it at, the scattered 
beam will appear blue or orange.

-- 
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 21:39:44 -0400, Tim Cook wrote:

> simian wrote:
>> 	Assume wise expression and far away look. Can you see the wind,
>> 	Grasshopper?
> 
> Trees moving back and forth makes the wind blow.
 
	Is the rippling grass to be ignored because we trample it underfoot? 

	Are the clouds to be forgotten because we cannot reach them? 

	Is the man off stage with the big fan to be given no credit? 

	You have only answers, Grasshopper, should you not be asking questions?

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On 16 May 2003 08:57:32 -0400, Warp <war### [at] tagpovrayorg> wrote:

>
>  It can't be invisible if it's observable. :)
>
You can observe something without seeing it. The meaning of  "observe" is not
the same as the meaning of visible.


Regards
        Stephen

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"sascha" <sas### [at] userssourceforgenet> wrote in message
news:3eba479a@news.povray.org...
> At school a teacher told us that reflection is caused if the surface is
> electroconductive

Interesting anyway. Seems this discussion is including refraction as being a
part of reflection, then I happened to think about how shallow angles cause
more reflection (Fresnel) and so could easily be thought of as a refractive
effect instead of purely a reflective one. That being to do with air or an
atmospheric effect anyhow. I don't know if this holds true for solid objects
and absolute voids (vacuum of space) or not. Never learned enough from
physics, myself, to know.

But going by what you said there, glancing lightwaves could be electrically
different than ones bouncing directly off of a surface.

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