Comments on: The Photon Push-Me Pull-You

By: Akira

Akira — Sat, 10 Jan 2009 21:39:43 +0000

Poynting predicted exactly the opposite: when light enters or leaves dielectric, it pulls the dielectric outwards. (Philosophical Magazine, vol. 9, p. 393, 1905). Was Poynting wrong? Hard to believe.

By: Uncle Al

Uncle Al — Thu, 19 Jun 2008 22:49:04 +0000

Fused silca, n_D = 1.45840. Commercial optical glass K-PSFn214 n_D = 2.14352,
(2.14352/1.45840)^2 = 2.16 signal advantage “8^>)

http://techon.nikkeibp.co.jp/article/HONSHI/20070424/131621/

By: Uncle Al

Uncle Al — Thu, 19 Jun 2008 22:27:53 +0000

Fused silica refractive index,

Fused silca, n_D = 1.45840; Fused germania, n_D = 1.6075
( 1.6075/1.45840)^2 = 1.215

For commercial optical glass, SF66 n_D = 1.9228 and K-PSFn214 n_D = 2.14297 (Sumita Optical Glass),

By: Matt

Matt — Thu, 19 Jun 2008 21:54:25 +0000

That’s a different effect. I’m taking about absorption with the material, like x-rays in lead. The light that makes it past the interface will heat the material – possibly enough to screw up the experiment.

By: Uncle Al

Uncle Al — Thu, 19 Jun 2008 15:04:59 +0000

Given two media of refractive index n0 and n1, reflectance R at normal incidence is

R = [(n0 − n1)/(n0 + n1)]^2

Given n0 = 1. for air and n1 = 4.1 for Ge, then R = 36.9% for the interface. That makes Ge internal transmission at least 52.3% plus 36.9% or 89.2%. Not so bad, especially if pulsed. Long runs of Ge fiberoptic are not advertised, perhaps from polycrystalline scattering. Does single crystal germanium fiber grow by edge-defined growth? Does cinnabar form a glass phase like quartz? An important divergence between two theories should be decided by more than one observation.

By: Matt

Matt — Thu, 19 Jun 2008 01:59:43 +0000

Ah, that was a tough one! I spent a few hours that day wondering if I was just a terrible physicist for not being able to quickly figure out such an “easy” problem.

Al: I believe transmittance will be the problem there. The laser has to be strong enough to produce an observable effect, but not so strong as to wreck the transmitting medium. Looks like the transmittance of Ge is not better than .5 anywhere in the spectrum.

By: Uncle Al

Uncle Al — Wed, 18 Jun 2008 23:02:27 +0000

The two models diverge as n^2. What fiber has the biggest n? Germanium hits n = 4.10 at 2060 nm, cinnabar hits n_D = 3.256 (for n_e). Fused silica, though convenient, is a wuss with n_D = 1.4584.

http://www.almazoptics.com/Ge.htm
http://www.translume.com/m_optical.htm