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wiki says that Terahertz radiation penetrates through non metallic materials.

Do you know how deep they get in metals, and how deep IR radiation (10^13 - 10^14 Hz) can get in all sorts of materials and how does heat propagate inside?

user157860
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  • This link discusses heat http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heat.html . related question https://physics.stackexchange.com/questions/80983/how-does-infrared-relate-to-heat – anna v Jun 16 '17 at 05:54
  • I'm voting to close this question as off-topic because it lacks prior elementary research in any intermediate level E&M textbook. – ZeroTheHero Jun 16 '17 at 13:11

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Heat is not the same as IR. Heat is often in the form of random kinetic energy of particles, transport is diffusive by random collisions.

Metals reflect infrared. The field cannot propagate inside, the attenuation length is very short, micrometer maybe. IR with a frequency of $3 \cdot 10^{13}$ Hz has a wavelength of 10 $\mu$m. That is the thermal infrared, seen by thermal imagers. Almost every non-metal is black in that range. But the camera lenses are transparent of course - often made of semiconductors like germanium or zinc selenide.

  • @user157860 The answer by Whit3rd already mentioned skin depth of metals, losses are by resistance, mechanisms are optical conductivity with Drude behavior. –  Jun 16 '17 at 07:51
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For metals, there is a 'plasma frequency'. X-rays are higher than the plasma frequency. At lower frequency than this, the electrons are free to move (and most metals have higher plasma frequecy than IR). The IR penetration then is just the metal's so-called 'skin depth' for the frequency of interest.

There are many 'skin depth calculators' online; aluminum at 10^13 Hz has a skin depth of 26 nm (nanometers; that's VERY thin).

Heat propogates in metals largely by kinetic energy transport through the conduction electrons (which wander freely), and partly by sound waves jostling atoms.

The conduction contribution due to gross movement of the charges is high because there are a lot of free charges in a metal; even if the metal were a black body (it isn't), IR photon generation and absorption are likely to be rare events, acting at short range, and not a large contribution to heat conduction.

In nonmetals, especially partly transparent ones, the IR can be significant.

Whit3rd
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