wolfman wrote:
> Sorry,
> 
> I found this in the beginner's newsgroup (hadn't looked there before):

Come on - if you have read Mike's reply you probably have read mine as 
well so i don't need to repeat it...

Note that according to the most common definitions of the index of 
refraction a negative one is not possible (this has nothing to do with 
physics but mere mathematics).  So if you want to know if POV-Ray 
correctly handles what you call negative ior you first have to decide 
what a negative ior actually is.

And just for the record: for refraction (which is not the only place 
where ior is used) POV-Ray uses Heckbert's method, see 'Derivation of 
Refraction Formulas' on:

http://www.cs.cmu.edu/~ph/


Christoph

-- 
POV-Ray tutorials, include files, Landscape of the week:
http://www.tu-bs.de/~y0013390/ (Last updated 24 Jul. 2005)
MegaPOV with mechanics simulation: http://megapov.inetart.net/

Post a reply to this message

Christoph Hormann <chr### [at] gmxde> wrote:

> Note that according to the most common definitions of the index of
> refraction a negative one is not possible (this has nothing to do with
> physics but mere mathematics).  So if you want to know if POV-Ray
> correctly handles what you call negative ior you first have to decide
> what a negative ior actually is.

The sign of the index of refraction is arbitrary, going strictly by the
mathematics. The physical principle that fixes the sign is conservation of
energy. To conserve energy across a material boundary, normal materials
have to have a positive IOR and these metamaterials have to have a negative
IOR.

Post a reply to this message

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Tom York wrote:
> Christoph Hormann <chr### [at] gmxde> wrote:
> 
> 
>>Note that according to the most common definitions of the index of
>>refraction a negative one is not possible (this has nothing to do with
>>physics but mere mathematics).  So if you want to know if POV-Ray
>>correctly handles what you call negative ior you first have to decide
>>what a negative ior actually is.
> 
> 
> The sign of the index of refraction is arbitrary, going strictly by the
> mathematics. The physical principle that fixes the sign is conservation of
> energy. To conserve energy across a material boundary, normal materials
> have to have a positive IOR and these metamaterials have to have a negative
> IOR.
> 
> 
[Semiologic mode on: (I need more explanation)]

The refraction is the propagation of the EM wave inside the material.
The positive IOR (IOR>0) might be used to keep the information that the
EM wave is moving from zone A made of material to zone B made of air (or
whatever). Same for the opposite, moving from B to A.
That's refraction. So far so good.
Now, if I try to imagine your negatif IOR (really, IOR < 0 ? not just
IOR < 1 ?) as an interface which when encountered by the EM wave send
the EM wave back in the zone it is coming from, it is not what I would
call refraction (usually it's called reflection).
There is of course the classical "total reflection angle" due to
refraction, but that's only a side effect.

Please note also that even an IOR < 1 would be quite puzzling: it would
means that light would travel faster than in the void when inside this
material (faster than c). it would be an interesting universe.
(especially if the information can be made to travel with the
corresponding EM wave)... Nahhh, just dreaming.

There is also a possible usage for the sign of the IOR, which could be
an oversimplification of the phase change/polarisation of a signal.
Assuming a clockwise rotation of E toward M, going thru one "negatif
ior" material could end up with an opposite rotation (anticlockwise).

Now, maybe your "metamaterial" is just a mirror with subsurface
scattering/refraction ? A kind of variable angle of reflection ?

[end of questionning]

- --
Eifersucht ist die Leidenschaft, die mit Eifer sucht, was Leiden schafft.
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.0 (MingW32)
Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org

iD8DBQFDDZM/s/YJ43cSjHIRAs/sAKCm8aDA0utdzv5/1t3efFvW6U2+LQCdFjUp
3xxOLC6vEiAuI9emL+KOvIE=
=45bZ
-----END PGP SIGNATURE-----

Post a reply to this message

Le Forgeron <jgr### [at] freefr> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
>
> The refraction is the propagation of the EM wave inside the material.
> The positive IOR (IOR>0) might be used to keep the information that the
> EM wave is moving from zone A made of material to zone B made of air (or
> whatever). Same for the opposite, moving from B to A.
> That's refraction. So far so good.
> Now, if I try to imagine your negatif IOR (really, IOR < 0 ? not just
> IOR < 1 ?) as an interface which when encountered by the EM wave send
> the EM wave back in the zone it is coming from, it is not what I would
> call refraction (usually it's called reflection).
> There is of course the classical "total reflection angle" due to
> refraction, but that's only a side effect.
>
> Please note also that even an IOR < 1 would be quite puzzling: it would
> means that light would travel faster than in the void when inside this
> material (faster than c). it would be an interesting universe.
> (especially if the information can be made to travel with the
> corresponding EM wave)... Nahhh, just dreaming.
> There is also a possible usage for the sign of the IOR, which could be
> an oversimplification of the phase change/polarisation of a signal.
> Assuming a clockwise rotation of E toward M, going thru one "negatif
> ior" material could end up with an opposite rotation (anticlockwise).
>
> Now, maybe your "metamaterial" is just a mirror with subsurface
> scattering/refraction ? A kind of variable angle of reflection ?
>
> [end of questionning]

The refractive index n is defined as n^2=permittivity*permeability, so n=+/-
sqrt(permittivity*permeability) (in reality, it's even more complicated).
Hence the negative sign in some cases.
Let's consider an interface between two "normal" materials. An incident beam
coming from material 1 which has a smaller n than material 2 is refracted
towards the surface normal in material 2 according to Snell's law
sin(theta1)/sin(theta2)=n2/n1. The refracted beam propagates "on the other
side"  of the surface normal than the incident beam.
Now, if you replace material 2 by a "metamaterial" with n2<0, the refracted
beam propagates "on the same side" of the surface normal as the incident
beam. That's what would be weird to look at!
Another strange thing in a metamaterial is that the group velocity of a wave
is antiparallel to its propagation direction, the wave vector k is
antiparallel to the Poynting vector S. S determines the overall energy flow
as well as the propagation direction, so that's consistent with the
conservation of energy, thanks Tom York!

If you don't believe me, google "metamaterials", there is lot's of stuff.

So, back to my problem, has anybody any idea of where the IOR is implemented
in POV-ray? Or what can I do to be able to have a negative sign for the
IOR?

Post a reply to this message

wolfman wrote:
> 
> The refractive index n is defined as n^2=permittivity*permeability, so n=+/-
> sqrt(permittivity*permeability) (in reality, it's even more complicated).

Just to make it clear: n^2=permittivity*permeability is not a 
definition, it's just a furmula n is supposed to comply with (and there 
is no unique solution for n).  n = sqrt(permittivity*permeability) would 
be a definition but it would not allow negative n (sqrt(x) is positive 
or imaginary for any non-complex x).

> If you don't believe me, google "metamaterials", there is lot's of stuff.

No one said this - you just did not say what negative ior means to you 
previously.

> So, back to my problem, has anybody any idea of where the IOR is implemented
> in POV-ray? 

Using

grep -r -I -E "[[:space:][:punct:]]ior[[:space:][:punct:]]" *

on the POV-Ray code finds pretty much every use of IOR.

> Or what can I do to be able to have a negative sign for the
> IOR?

Again: why do you assume you can't use a negative IOR.

Christoph

-- 
POV-Ray tutorials, include files, Landscape of the week:
http://www.tu-bs.de/~y0013390/ (Last updated 24 Jul. 2005)
MegaPOV with mechanics simulation: http://megapov.inetart.net/

Post a reply to this message

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Thank you.

wolfman wrote:
> Now, if you replace material 2 by a "metamaterial" with n2<0, the refracted
> beam propagates "on the same side" of the surface normal as the incident
> beam. That's what would be weird to look at!

With a little more google, I start to see... the ray go thru the
surface, but instead of crossing the normal, it reflect on it.

> 
> If you don't believe me, google "metamaterials", there is lot's of stuff.

Best so far: http://www.answers.com/topic/metamaterial

> 
> So, back to my problem, has anybody any idea of where the IOR is implemented
> in POV-ray? Or what can I do to be able to have a negative sign for the
> IOR?

Yep! you want to play with the computation of refracted rays.
That should be a small patch... I leave you tweaking the parser to
accept the negative ior (but notice that ABS(IOR) should still =>1, even
with metamaterial, no revolution of physics)

Then a few explanation for povray: a ray keep its current ior value, and
it get updated by the ratio of the leaving/entering thru an interface.
So far, so good, you only have to be able to handle the computation of
the direction of the refracted ray.

You want to change BacktraceRefract in lighting.cpp,
in particular the test of line 3056 (reference of 3.6.1)  (testing that
change is nearly irrelevant, may be that test is ok)
and mainly the relevant ELSE block

Looking at it, it seems (but YOU check!) that a change in Refract_Guts
(same file) could be enough. (you need to trap the ior<0, because it use
the square root a loot..., that's only a small function to update...)

Good luck.

I wonder if such strange things could make it so late in 3.7 ?





- --
Eifersucht ist die Leidenschaft, die mit Eifer sucht, was Leiden schafft.
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.0 (MingW32)
Comment: Using GnuPG with Thunderbird - http://enigmail.mozdev.org

iD8DBQFDDcTgs/YJ43cSjHIRAjKuAJ94g17tBHfxTQdUoyEZitVm0O1joQCghre4
jeJUh3QT7KPUgEAixHZgSgM=
=geg0
-----END PGP SIGNATURE-----

Post a reply to this message

Le Forgeron wrote:
> Best so far: http://www.answers.com/topic/metamaterial

The doppler shift is reversed!?   <boggle!>

-- 
   Darren New / San Diego, CA, USA (PST)
    "Yes, someone maintains a big web site
     about that. I think her name is Wiki."

Post a reply to this message

Christoph Hormann <chr### [at] gmxde> wrote:
> Just to make it clear: n^2=permittivity*permeability is not a
> definition, it's just a furmula n is supposed to comply with (and there
> is no unique solution for n).  n = sqrt(permittivity*permeability) would
> be a definition but it would not allow negative n (sqrt(x) is positive
> or imaginary for any non-complex x).

n*n = x, but also (-n)*(-n) = x. The positive n is the principal value of
the root, not the only valid value.

Post a reply to this message

Le Forgeron wrote:

> Best so far: http://www.answers.com/topic/metamaterial

The only thing I have seen so far that even remotely fits the 
meta-materials description (with negative refraction) is something on 
the scale of microwaves (which looks like a grid of printed circuit 
boards), unless opal does. Opal seems to show irridescent-like 
properties (gee, opalescense?), but none of the weirder properties that 
the described metamaterial has. I don't know if microwaves refract in 
the same fashion as light does, though I'm sure there are theoretical 
materials that refract microwaves.

---
~Mike

Things! Billions of them!

Post a reply to this message

Mike Raiford <mra### [at] hotmailcom> wrote:

> The only thing I have seen so far that even remotely fits the
> meta-materials description (with negative refraction) is something on
> the scale of microwaves (which looks like a grid of printed circuit
> boards), unless opal does. Opal seems to show irridescent-like
> properties (gee, opalescense?), but none of the weirder properties that
> the described metamaterial has. I don't know if microwaves refract in
> the same fashion as light does, though I'm sure there are theoretical
> materials that refract microwaves.

I think opal is mentioned because it fits the definition of a metamaterial,
which according to wikipedia etc is "an object that gains its
(electromagnetic) material properties from its structure rather than
inheriting them directly from the materials it is composed of." So probably
all materials with negative refractive indices are metamaterials, but not
all metamaterials have negative refractive indices.

The effect was first demonstrated with microwaves, because to create these
bizarre materials you need to alter their electromagnetic properties on
distances that are roughly the same size as the wavelength of the radiation
involved. If you use microwaves, you can take advantage of the wavelengths
being on the order of millimetres to centimetres, and fabricate your
metamaterial on a circuit board in civilised surroundings and with easy
ways to check it's all working.

There's no theoretical reason why this shouldn't work with light, despite
the wavelengths being 100,000 times smaller, but in practice it's obviously
many times more difficult to do this on such small scales. Recently a group
got it to work, though:

http://www.eurekalert.org/pub_releases/2005-04/uoc--nso041805.php

It also seems that one group got it to work with ultrasound!

http://www.eetimes.com/showArticle.jhtml?articleID=47902254

Plenty of common materials refract microwaves. It works the same way as for
light (except that diffraction and wave effects are much more obvious). You
can make a microwave lens out of quite ordinary stuff, most dielectrics will
work I imagine.

Post a reply to this message