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Forum: Processes Involving Optical Photons
Re: Question Boundary interaction model (Nicolas Di Vara)
Re: None Re: Boundary interaction model (Sehwook Lee)
Date: 15 Nov, 2012
From: Erik Dietz-Laursonn <Erik Dietz-Laursonn>

Dear Sehwook

If I understand this ( http://en.wikipedia.org/wiki/Refractive_index#Complex_index_of_refraction_and_absorption ) correctly, every material has a complex refractive index:

"When light passes through a medium, some part of it will always be absorbed. This can be conveniently taken into account by defining a complex index of refraction" "Dielectric loss and non-zero DC conductivity in materials cause absorption. Good dielectric materials such as glass have extremely low DC conductivity, and at low frequencies the dielectric loss is also negligible, resulting in almost no absorption (&#954; &#8776; 0). However, at higher frequencies (such as visible light), dielectric loss may increase absorption significantly, reducing the material's transparency to these frequencies."

I under stand this in the way, that a material has a complex refractive index (with maybe a very small imaginary part), as long as light (partly) gets absorbed when passing through. This would mean that only perfect vacuum has a purely real refraction index.

> If dielectric material really has absorption depending on the energy of photon, polarization and incident angle, please let me know. If so, I have known about it wrong. I would like to correct myself about this physics fact.

That the absorption of dielectric materials depends on the energy of the photon is for sure ( http://www.detectors.saint-gobain.com/uploadedFiles/SGdetectors/Documents/Brochures/Scintillating-Optical-Fibers-Brochure.pdf ). That it depends on polarisation and incident angle is something I would doubt, but also for metals. To my understanding, the Fresnel equations only give the reflectivity and transmittance of THE INFINITESIMAL BOUNDARY between two materials (dielectric or metal). If/how much of the transmitted light is absorbed only depends on the attenuation length, which can be calculated from the imaginary part of the refractive index. For metals, the attenuation length is very short (large imaginary part of the refractive index) and for dielectrics this is vice versa.

I might be wrong, too, but this is my understanding of the Fresnel equations. And this brings me to my (still unanswered) physical question: Does the reflectivity of a boundary always depend on the refraction indices of both materials which are forming the boundary? I think we all agree, that for a dielectric-dielectric boundary, both refraction indices have to be considered and can be found in the Fresnel equations. This is also the way how it is implemented in GEANT4 (using only the non-complex refraction indices, this is ok as the imaginary part should be very small). But what about a dielectric-metal boundary? I see no reason, why it should be different in this case. But in GEANT4, the Fresnel equation for the dielectric-metal case does NOT consider the refraction index of the dielectric. And as I wrote before, I found many scripts introducing the Fresnel equations with two complex refraction indices or one complex and one non-complex refraction index (e.g. http://www.astro.phys.ethz.ch/spf/people/hanschmi/astobs/notes/files/astobs3.pdf ). On the other hand, I could not find any measurement comparing the reflectivity of let's say air-aluminum with water-aluminum. If the Fresnel equations consider both refractive indices, this should make a difference, but for the version implemented in GEANT4, the result would be the same, i.e. it is only correct for air/vacuum-metal boundaries, in my opinion.

Best regards, Erik

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1 None: Re: Boundary interaction model   (Sehwook Lee - 15 Nov, 2012)
(_ None: Re: Boundary interaction model   (Erik Dietz-Laursonn - 16 Apr, 2013)
(_ Warning: Re: Boundary interaction model   (Andrea Celentano - 16 Jan, 2015)
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