I’m interested in why matter, in the form of gases, plasmas, and solids, is transparent or opaque to EM radiation. The Earth’s atmosphere is mostly transparent to visible light, but soft x-rays do not travel far before they are absorbed. Some materials, like germanium, are transparent to infrared but opaque to visible light. Common glass is opaque to infrared and ultraviolet light. What determines whether a given frequency of EM radiation is transmitted or absorbed?
Whether there is a molecular transition that matches that particular frequency. Every transition has a resonant frequency - just as pumping your legs on a swing in the right way will cause you to swing higher and higher, so will alternating an EM field cause certain transitions to occur.
At very low frequencies (radio waves) in a magnetic field, what you are doing is exciting atomic nuclei from having their spins aligned with the magnetic field to having their spins anti-aligned. The energy of the transition is very, very small and the phenomenon is known as magnetic resonance (the basis of MRI). These pass through just about everything.
At low frequencies (microwaves), what you are doing is spinning the molecule, exiciting the rotational bands. This is the method by which a microwave oven heats food up (it causes water molecules to spin very rapidly).
At medium frequencies (infrared), what you are doing is exciting the vibrations in the molecule. Most organic molecules will absorb IR. This is still not destructive to molecules, and because different molecules vibrate at different frequencies, the IR signature of a molecule can be used to determine the identity of the molecule.
At higher frequencies (ultraviolet), what you are doing is exciting electrons from one molecular orbital to another. Absorbing in the UV region is very common, especially organic molecules (such as DNA and proteins), which is why an excess of UV will start to damage your DNA.
At high frequencies (X-ray), what you are doing is knocking very low-lying electrons out of the molecules completely. This is very destructive to chemical structures. Just about everything absorbs X-rays.
At very high frequencies (gamma rays), what you are doing is transforming nucleons in the atom. For example (I’m not a physicist, so don’t jump all over me if this is a little off), a neutron can absorb a gamma ray and then break into a proton and an electron. Gamma rays generally do destructive damage to living matter because it’s changing the chemical composition of important elements such as carbon and nitrogen and oxygen.
Knowing why certain frequencies are absorbed, you can now make educated guesses as to which frequencies will be absorbed by which states. For example, in a plasma, none of the electrons are in molecular orbitals – hence, plasma won’t absorb in the UV or X-rays, but will absorb gamma rays. Depending on whether there are chemical bonds in a gas (O[sub]2[/sub], N[sub]2[/sub]) or not (Ar, O, N), it may absorb in the UV or in IR. Metallic solids are difficult to classify because in a metal, every atom is sharing electrons with every other atom. It’s like metal cations floating in electron soup. But since everything is one big block, you don’t get very much IR (vibrational) or microwave (rotational) absorbance.
I hope that helps. There are some other factors (such as wavelength-dependent scattering by particulate matter) which contribute, but I tried to stick with things that are very general. If you have a more specific question, I can try and provide a more specific answer.
SDMB threads:
I’m taking this very class for my EE degree as we speak…
Whether or not the material is a conductor or EM propagator also plays a huge role, especially in the radio/microwave realm. Internal resonances, reflections, material transitions and standing waves can create bizarre unintended results.
My prof used to work with defense radar systems, and related once how changing one of the bonding glues in the material on a fighter radome (nose cone) made the difference between near total internal reflection, and near perfect transparency.