[SI-LIST] Re: (no subject)
- From: "Larry Barnes" <larry.barnes@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 11 Dec 2002 09:43:33 -0800
To all,
This has been an interesting discussion from the sidelines. My I =
suggest that a reference that thoroughly discusses the physics behind =
the phenomena is "Fields and Waves in Communications Electronics" by =
Ramo, Whinnery, and Van Duzer? =20
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Lawrence C. Barnes
Principal Signal Integrity Engineer
QLogic Corporation
2660 Laguna Hills Drive
Aliso Viejo, CA 92656
(949) 389-7509
(719) 237-9822 cell
larry.barnes@xxxxxxxxxx
-----Original Message-----
From: Sandor [mailto:sandor@xxxxxxxxxx]
Sent: Tuesday, December 10, 2002 10:09 PM
To: justin.tabatchnick@xxxxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: (no subject)
Hi Justin,
> Hi Sandor;
>=20
> I beg to differ - I don't know who wrote the article but they=20
> are wrong =3D
> - They mention that the dielectric constant varies with=20
> frequency - is =3D
> it a linear or nonlinear variation
In real life, the relationship is probably nonlinear, but does it really =
matter? The important thing for the arguments sake is that it varies=20
(with frequency, given a certain dielectric material). That's probably=20
where our opinions diverge...
> - I don't think they even know =3D
> because they are wrong -are you telling me that the wavelenth=20
> does not =3D
> change in a medium that it is the dielectric constant or are=20
> you saying =3D
> that the frequency changes with the dielectric constant -=20
Is it a trick question? :-)
Everything changes. What we are comparing is the ideal case of Er being=20
constant versus being variable with frequency. For a given frequency,=20
well frequency is constant but the wavelength will change with any=20
difference in Er compared to the ideal case due to a resulting different =
propagation speed.
> think of this =3D
> - if the frequency changes at the medium interface ,
It's the tail wagging the dog again. I did not say, at least I never=20
meant to say that the frequency changes at the medium interface. What=20
did I say that implied this?
> it goes=20
> from higher =3D
> to lower or reverse then at the interface there is a=20
> discontinuity that =3D
> can not be imagined. I have better analogy let's discount any=20
> losses, =3D
> energy equals plank's constant times the frequency . Energy=20
> conservation =3D
> dictates that the energy crossing a boundry will be the same on both=20
=3D
> sides in a lossless medium which means the frequency has to remain =3D
> constant.
>=20
> Justin
I can not possibly argue against the conservation of energy but I'm not=20
quite sure how that ties back to the original argument about dispersion. =
If the system is lossless, you can pump in a certain amount of energy=20
in and the same will come out in the end, but nothing says it all has to =
come out with the same timing profile.
Perhaps what we had been saying were simply on two different planes=20
which intersect somewhere...
I see Er as a complex number, a bit like <insert feverish internet=20
search here> what is described in=20
http://www.sbu.ac.uk/water/microwave.html , and well, that article=20
explains everything better then I could. I couldn't quite work out at=20
the time, but perhaps it's something related to this that you were=20
referring to at the end of your original contribution to this thread,=20
when you mentioned the loss tangent.
It's just when you said...
> - you can =3D3D
> explain light dispersion not by a variation of dielectic with=3D20
> frequency =3D3D
> but a variation of speed in the medium with the relative=20
> dielectric =3D =3D3D
> constant . Velocity =3D3D3D frequency times wavelength ,=3D20
> wavelength is equal =3D3D
> to the wavelength divided by the square root of the relative=3D20
> dielectric =3D3D
> constant- white light is made up of a spectrum of varying=3D20
> wavelengths =3D3D
> all traveling at the same speed in air however in a medium=3D20
> their speeds =3D3D
> differ and that is why you see a seperation.
..I could not work out how different frequency components of light can=20
travel at different speeds unless the Er they are "seeing" is different. =
The speed is different because the Er is different. This may be a=20
question of viewpoint, and I'm sure first the variability of speed was=20
measured and then Er was "invented" to model the phenomenon with a nice=20
equation. I guess now we are arguing chicken and egg.
The point is, I don't see how you can have different speeds in a medium=20
(and I regard air as a medium) unless the Er is different.
> the only way you will see a =3D3D
> variation in the dielectric constant is in a non-homogenous=3D20
> medium.
Let's take a perfectly _homogeneous_ piece of glass in a shape of a=20
prism. It would still bend different frequency light differently. We=20
seem to agree that different spectral parts of the white light will=20
travel at different speed. The actual speed of the light is=20
1/SQRT(Eo*Er*Mu). How can we have different speeds for the different=20
spectral components without a difference in relative permettivity?
Regards,
Sandor
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