in news:3bf8eeef@news.povray.org Warp wrote:

> Why does it go slower inside the
> object?
> 

From Maxwells third and fourth

c = 1/(E0*U0)^0.5

E0 = electric permittivity for free space
U0 = magnetic permittivity for free space

E0 = 8.85 x 10^-12 C^2 N^-1 m^-2
U0 = 4pi x 10^-7 Wb A^-1 m^-1

c = 2.999 x 10^8 m s^-1

In matter E0 and U0 become E and U
For most optical materials is nearnly equal to unity so that

n = c/v = E^0.5

From some very old notes from optics class.

Ingo

-- 
Photography: http://members.home.nl/ingoogni/
Pov-Ray    : http://members.home.nl/seed7/

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In article <3bf8ca76@news.povray.org>, war### [at] tagpovrayorg says...

> extremely far apart from each other, which means that there's quite a lot of
> space between them; how come every and each one of the incoming photons are
> absorbed and none goes through the space between the atoms without never
> colliding with them?

You are thinking of photons as a kind of simple particle traveling along 
a well-defined route from point A to point B. 
But this is completely wrong when the photon is "near" some kind of 
obstacle, thinking of it as a wave is a far better approximation in this 
case -> since a wave can, at the same time, hit and miss ALL obstacles in 
its path, it should be clear they all photons behave equally.

Lutz-Peter

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Timothy R. Cook scribis news:3BF8DB9D.E7FA7611@scifi-fantasy.com:

> Because it DOES go slower through the object.  The line is
> 'nothing travels faster than the speed of light *in a vaccuum*'.
> If you pass the light through an object, it isn't going as
> fast as in vaccuum.

I had read of ceasium(sp?) gass heated to a certian high temprature that 
has an IOR of less than 1. i.e. light traveles faster than the speed of
light in a vacuum. I havent seen this article disputed yet either. So maybe 
it is true.

---
G^is poste,
Arto

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Having no mass, photons always travel at speed of light, even when they
go through transparent objects. They are absorbed and emitted several
times before they escape the material. The collective effect of billions
of them in a beam is as they travel slower than c. This collective
behavior is correctly described by Maxwell's equations. Individual
photons are better described by quantum mechanics.  This
collective behavior is what is simulated in povray, the behavior of
beams of light
 best described by Maxell's eqs.

Alberto

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> I had read of ceasium(sp?) gass heated to a certian high temprature
that
> has an IOR of less than 1. i.e. light traveles faster than the speed
of
> light in a vacuum. I havent seen this article disputed yet either. So
maybe
> it is true.

I've had a look, but there are not too many clear explanations on this
matter.  It seems that the Caesium was used to create a region of
"anomalous dispersion", which means that the wavelength dependence of
the IOR is reversed.  Shining a laser pulse through this caused a pulse
to be emitted from the other side of the chamber BEFORE the original
pulse had even reached it.  Sounds amazing, and sounds like
faster-than-light, but apparently it isn't.

What had happened was that the group velocity of the light had been
increased above the normal speed of light.  Normally, the group velocity
of light is the same as the signal velocity, ie information is
transmitted at the group velocity.  However, in the case of anomalous
dispersion, the group velocity is no longer the signal velocity, and no
information is carried at this speed.  The peak of the pulse travelled
at the group velocity, but energy did not.

More than this I can't tell you, I'm afraid.  However, many people note
that this kind of thing is very prone to media misinterpretation, mostly
because the "speed of light" NORMALLY refers to the speed of energy
transfer, but in fact a wave has many different speeds of propagation,
depending on exactly what property of the wave you are looking at.

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>   I wasn't referring to electric current. I was referring to what happens
> in nuclear reactors (I think that it's called Tserenkov's phenomenon or
> something similar): Electrons travel in water faster than light, which
> causes a greenish glow (a kind of "sonic boom" but with light instead of
> sound).

Cerenkov Radiation.

AFAIR, subatomic particles travelling faster than the speed of light in 
water (or air) hit the air, come to a (relatively) screeching halt and 
dump their excess energy, some of which makes it into the blue end of the 
visible spectrum.

If you want more info, I'll ask my friend who works on Nuclear systems.

Bye for now,
     Jamie.

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Andrew scribis news:3bf96612@news.povray.org:
> What had happened was that the group velocity of the light had been
> increased above the normal speed of light.  Normally, the group velocity
> of light is the same as the signal velocity, ie information is
> transmitted at the group velocity.  However, in the case of anomalous
> dispersion, the group velocity is no longer the signal velocity, and no
> information is carried at this speed.  The peak of the pulse travelled
> at the group velocity, but energy did not.
> 
> More than this I can't tell you, I'm afraid.  However, many people note
> that this kind of thing is very prone to media misinterpretation, mostly
> because the "speed of light" NORMALLY refers to the speed of energy
> transfer, but in fact a wave has many different speeds of propagation,
> depending on exactly what property of the wave you are looking at.
 
That's the best explanation I have seen in a long while of what happened.



-- 
Gis poste, Arto.

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> That's the best explanation I have seen in a long while of what
happened.

Glad to help, but I can't really say I understand it all that well ;-)

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Alberto Mendoza <jac### [at] usbve> wrote:
: Having no mass, photons always travel at speed of light, even when they
: go through transparent objects. They are absorbed and emitted several
: times before they escape the material.

  I can grasp this. However, what I still don't understand is the refraction
phenomenon.

-- 
#macro N(D,I)#if(I<6)cylinder{M()#local D[I]=div(D[I],104);M().5,2pigment{
rgb M()}}N(D,(D[I]>99?I:I+1))#end#end#macro M()<mod(D[I],13)-6,mod(div(D[I
],13),8)-3,10>#end blob{N(array[6]{11117333955,
7382340,3358,3900569407,970,4254934330},0)}//                     - Warp -

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On 20 Nov 2001 06:38:08 -0500, Warp wrote:
> Alberto Mendoza <jac### [at] usbve> wrote:
>: Having no mass, photons always travel at speed of light, even when they
>: go through transparent objects. They are absorbed and emitted several
>: times before they escape the material.
> 
>   I can grasp this. However, what I still don't understand is the refraction
> phenomenon.

Understanding refraction (and diffraction, too) requires one to see the 
energy as waves instead of as particles.  It's impossible to understand as 
long as you keep thinking of them as strictly photons.

--
#macro R(L P)sphere{L __}cylinder{L P __}#end#macro P(_1)union{R(z+_ z)R(-z _-z)
R(_-z*3_+z)torus{1__ clipped_by{plane{_ 0}}}translate z+_1}#end#macro S(_)9-(_1-
_)*(_1-_)#end#macro Z(_1 _ __)union{P(_)P(-_)R(y-z-1_)translate.1*_1-y*8pigment{
rgb<S(7)S(5)S(3)>}}#if(_1)Z(_1-__,_,__)#end#end Z(10x*-2,.2)camera{rotate x*90}

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