Why doesn't infrared radiation pass through objects?

Question

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?

Answer 1

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.

Answer 2

Infrared passes through (undoped) silicon. Many glasses also transmit infrared. The reason materials do not pass infrared may be absorption by internal degrees of freedom, such as vibrations, or reflection/absorption by electronic excitations.

A good place to get information on infrared properties of materials is refractiveindex.info.

Answer 3

Whether electromagentic radiation pass through the object depends on how it interacts with this object: it can be reflected, absorbed, it may not interact with an object or it may bypass it completely, if the wavelength is larger than the object size (as it is often the case with radiowaves).

Assuming the wave length is comparable to the object size, whether the radiation is absorbed or reflected depends on the electronic properties of the material and whether it is ordered or not, as well as on the radiation frequency. For example:

• semiconductors absorb at frequencies larger than the bandgap and weakly reflect (via Rayleigh scattering) any light frequencies lower than the bandgap. Metals, on the other hand, reflect most of the incident visible light. Both materials allow X-rays pass uninhibited, due to very high frequency of this radiation (although some of it is absorbed, causing ionisation).
• disordered materials, composed of the pieces of metallic or semiconducting crystals, reflect the rest in all possible directions, which makes them completely non-transparent - "white-ish"
• small molecules absorb or effectively scatter only certain frequencies, which is why liquids are often transparent. On the other hand, mixtures of complex organic molecules absorb quite a lot.

Infrared and microwave radiation is efficiently absorbed by many materials, which is why they are not transparent in this range. As an anecdotal evidence, it is for a good reason that this radiation is used in microwave and infrared ovens. Night-vision devices similarly detect infrared radiation, interpreted as heat.

Answer 4

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?

Answer 5

The penetrating capacity of light waves is higher when their wavelength is smaller. This is the basic principle which we need, and this follows from Planck’s equation. So, visible light rays have more penetrating capacity than infra red rays, ultra violet rays have more penetrating capacity than visible light rays, X-rays have more penetrating capacity than ultra violet rays, and gamma rays have more penetrating capacity than X-rays.

Answer 6

Higher energy X-rays and gamma-rays have such short wavelengths that they can pass through the space between the atoms of solid matter without much interference. This means that X-rays and gamma-rays will not reflect much of the information about matter they happen to pass through. This is why it makes more sense to use an infrared telescope rather than an x-ray telescope for viewing information on stellar material out in space. This may seem counter intuitive since infrared images show information inside clouds of gas, but remember that space is very big with astronomical distances between objects that usually surpass the distance between infrared wavelengths.