It cannot be due to Raman scattering because $1$ in $10^7$ photons are raman scattered. It cannot be rayleigh scattering because rayleigh scattering of infrared radiation is very low due to the inverse dependence of scattered intensity to $\lambda^4$. It cannot be due to molecular absorption of the photons since a single type of molecule cannot absorb every frequency of infrared radiation because the vibrational transitions can happen from one vibrational mode to the next vibrational mode, also the vibrational modes are unique. What could the answer be?

  • $\begingroup$ do you consider your object to be at room temperature? If so, every transition will get significant line broadening due to the Doppler effect. $\endgroup$ – wcc Sep 13 '18 at 15:13
  • $\begingroup$ yes,the objects are at room temperature $\endgroup$ – Iamme Sep 14 '18 at 5:40

In the case of conducting objects, it is because free charge redistributes to diminish the electric field, up to the so-called plasma frequency of the material (metal reflects light and infrared, but not X-rays). In the case of nonconducting objects, it is because each atom can become slightly polarized (the electron cloud can change shape), and this makes every surface (every discontinuity in material) a potential scatterer.

Scattering, as of visible light by (transparent) water droplets in a cloud, can block the direct light path very effectively.

Solid materials (and even compressed gasses) do not have sharp spectral lines, but fattened ones, due to "pressure broadening". It is important to note that heterogeneous materials have many surfaces that are not pure substances, which blurs the spectral absorption lines even more. Absorption can be very likely over broad spectral ranges.

The only light-blocking mechanism that is NOT likely in the case of IR illumination, at standard temperature and pressure, is photoelectric effect.

  • $\begingroup$ Is there any theory behind this process,because it is so confusing for me.is there a way to change this?. $\endgroup$ – Iamme Sep 13 '18 at 12:30
  • $\begingroup$ The quantum theory of light interacting with matter is well-studied, but usually only applies to ideal (pure) materials in detail. Glassy materials that can be formed without grain boundaries, and with high purity, which have no intrinsic spectral features in the IR, include quartz (SiO2), which is the dominant material in IR applications like fiber optics. CaF is another durable IR-transparent material. $\endgroup$ – Whit3rd Sep 13 '18 at 20:34

I would express it differently, because it is not only infrared that does not pass through solid objects, but all frequencies of the electromagnetic spectrum from ultraviolet down, with the exception of objects made up of organized lattices.

X-rays and gamma rays see mostly spaces between atoms and molecules, because the wavelength is short and they see mostly space and can go through, the higher the frequency the more penetrating. At the photon level making up light, Xrays and gamma rays have small probability of interacting due to the enormous space between atoms .

So the question should be "why are some materials transparent to electromagnetic radiation" . The answer to that is that the interactions photon+lattice allow for the incoming photons to scatter elastically with the whole lattice and exit while retaining phases and energy (color in the optical range). This transparency depends on the way the atoms and molecules build up the lattice.

This holds also for specific lattices which are transparent to the infrared photons and the light they build up, as with silicon. In the article opaqueness is attributed to the breaking of bonds in the lattice.

Have you not been heated by glass doors of modern fireplaces?

  • $\begingroup$ Is there any theory behind this process,because it is so confusing for me.is there a way to change this?. $\endgroup$ – Iamme Sep 13 '18 at 12:29
  • $\begingroup$ The theory generally is quantum mechanics and particularly solid state physics en.wikipedia.org/wiki/Solid-state_physics . unfortunately it is not a simple one sentence or paragraph or article that can explain . a course in solid state? ocw.mit.edu/courses/physics/8-231-physics-of-solids-i-fall-2006/… $\endgroup$ – anna v Sep 13 '18 at 12:42
  • $\begingroup$ glass fibers are excellent, essentially lossless, light conductors without being organized into lattices $\endgroup$ – hyportnex Sep 13 '18 at 12:50
  • $\begingroup$ @hyportnex all solids can be modeled by lattices at the quantum mechanical level, imo. glass fibers should work as waveguides in classical electrodynamics, $\endgroup$ – anna v Sep 13 '18 at 12:59

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