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?
[Physics] Why doesn’t infrared radiation pass through objects
electromagnetic-radiationelectromagnetisminfrared-radiationoptical-materialsquantum mechanics
Best Answer
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.