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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