Is transparency of material has something to do with inter- or intra-molecular bonding? E.g. both graphite and diamond are carbon, but graphite is opaque and diamond transparent.
[Physics] Transparency of materials
crystalsmaterial-sciencevisible-light
Related Solutions
I believe your puzzlement comes from confusing two frameworks: the quantum mechanical (photons) and the classical mechanics one, waves.
When one is calculating in terms of classical electromagnetic waves there are classical considerations : refraction, absorption, reflection with their corresponding constants .
When one is zooming in the microcosm and talking of photons, a wave is composed of zillions of photons which go through, each at the velocity of light.
The bulk of the target material is in effect the electric and magnetic fields holding the atoms together to form it: the nuclei are tiny targets and the electrons are small zooming targets. The probability of a single photon to scatter on a nucleus or an electron is miniscule. It interacts/scatters out of its optical ray path with the electric/magnetic fields that are holding the glass or crystal together. The scattering angles are very small in transparent materials thus preserving the optical path, enormous in opaque ones . It is those fields that one has to worry about, not the individual atoms and their excitations.
The photons scatter mostly elastically with the fields holding the solids together with tiny or high cross sections depending on the frequency of light and spacing of the materials. In crystals and glasses the optical frequencies have small probability of interaction.
x rays find most materials transparent because the photons' energy is much larger than the energies available by the fields holding the crystals together, and the scattering angles with the fields are very small, except when they hit the atoms, and then we get x ray crystallography.
Edit after comment
Here is the sequence as I see it:
A classical electromagnetic wave is made up of photons in phase according to the wave description.
There is an enormous number of photons in the wave per second making it up. Here is a useful article which explains how a classical wave is built up from a quantum substrate.
Each photon does not change the atomic or crystalline energy levels going through a transparent material in the quantum mechanical way by emitting a softer photon. It scatters quantum mechanically elastically, through the medium, changing the direction infinitesimally so that it keeps the quantum mechanical phase with its companions and displays transparency. Since a medium has a composite collective electric and magnetic field it is not a simple "electron photon going to electron photon" QED diagram. In the case of a crystal one could have a model of "photon crystal photon crystal" scattering amplitude for example.
The higher the sequential probability of scattering going through a medium the larger the final deflection through it will be, and the higher the over all probability of losing the phase with its companions in the wave.( the thicker the glass the less transparency and image coherence).
The transparency of the medium depends on the ordering of the atoms and molecules composing it so that it allows to keep the coherence between individual photons of the beam. The lower the density the better chance to keep the transparency, viz water and air.
hope this helps conceptually.
Beginner's guide to band structure follows. I've taken considerable liberties with the details to simplify this so don't take it too literally!
This is going to seem an odd place to start, but consider filling up the atomic orbitals in an atom with electrons. If you take a noble gas, e.g. Xenon, you'll find each orbital is filled completely with two electrons and this is why Xenon is inert. If you take Potassium instead you find all the lower orbitals are filled with two electrons, but the outmost orbital contains only one electron so the orbital is only half full. This is why Potassium is very reactive.
When you clump atoms together into a solid, the interactions between the atoms spread the sharp atomic orbitals into energy bands. Suppose our solid contains $n$ Xenon atoms, then each band can contain 2$n$ electrons. But each Xenon atom contributes 2 electrons to each band, so the energy bands in solid Xenon are all full. That's why solid Xenon is an insulator. In the case of Potassium all the lower energy bands are full up with 2$n$ electrons, but the top band contains only $n$ electrons, i.e. it is only half full, because each Potassium atom has only 1 electron left over to put into this band. That's why solid Potassium is a conductor.
The position of an electron in an energy band doesn't just determine its energy, it also determines its momentum. If you want to make an electron move so it can conduct electricity you need to change it's momentum, and therefore you need to change its position in the energy band. But when bands are full you cannot change an electron's energy/momentum because there are no free spaces in the band for the electron to move into. That's why filled bands are insulating and part filled bands are conducting.
Now, if you imagine taking your solid and filling up the energy bands with electrons there is going to be a highest occupied band and a lowest unoccupied band. Now the nomenclature can be a little confusing. If the highest occupied band is full (like solid Xenon) we tend to refer to it as the valence band, and the lowest unoccupied band as the conduction band. The energy difference between the bands is the band gap. The reason why we call the lowest unoccupied band the conduction band is because any electrons that get excited into it will conduct; electrons in the valence band won't conduct (because the valence band is full).
But, if the highest occupied band is only part full (like solid Potassium) we call this band the conduction band because the electrons in it can conduct. Strictly speaking the highest band is both the valence band and conduction band, but convention dictates we call it the conduction band. In metals we're usually not fussed about the lowest unoccupied band and the band gap because they aren't involved in conduction of electricity.
Now, on to transparency. When a photon interacts with an electron it transfers it's momentum to the electron i.e. it changes the momentum of the electron. But if you recall from above, you can't change the momentum of an electron in a full band. The only way to change the electron momentum is to hit it hard enough, i.e. with enough energy, to make it jump over the band gap into the lowest unoccupied energy band. So if you measure the optical absorption as a function of energy you find there's little absorption until the photon energy matches the band gap, and the absorption suddenly rises. For many materials the band gap energy corresponds to ultra-violet light, so the solid doesn't absorb visible light i.e. it's transparent. As you say, these solids are also insulators because the same mechanism (change of electron momentum) determines both conductivity and optical absorption.
In metals the lowest occupied band (the conduction band) is only partially full so electron momentum can be changed by any amount you want. That's why metals absorb light (and radio waves etc) very strongly and are opaque.
Incidentally you do get borderline cases. Pure silicon is an insulator, but the band gap is only about 1.12 eV and this is less than the wavelength of red light. So silicon absorbs light even though it's an insulator. Well, it's an insulator in the dark. As soon as you shine light on it the electrons you excite over the band gap conduct electricity, so silicon conducts when you shine light on it.
I hope all this helps. If you want to clarify any of the above please comment.
Best Answer
Neither nor. Inter-, intra-molecular bonding types (van der waals, ionic, covalent, metallic bonding) are all playing minor or major role in a solid depending on its material (element mixture, inter-atomic distance, crystal lattice type...).
In the end the dielectric function will theoretically define the reflectivity, transparency, absorption of a solid for a specific wavelength of light for these types of bonding (ionic/electronic polarisation). Look at the picture and formula here.
Transparency is mainly defined by free electrons, position of Fermi Energy & type of bandstructure in a solid. For instance, metals (metallic "intermolecular" bonding) are often reflective and non transparent due to free electrons, their Fermi Energy is outside of eventual band gaps. Many isolators (ionic "intermolecular" bonding) have Fermi Energy within an energy band gap of >4 eV , but optical photon energy is around 1 to 3 eV. So electrons in valence band beneath the band gap cannot absorb these optical photons, the material is transparent. Many semiconductors have a band gap around only 1 eV, so their valence electrons can absorb photons.
Of course the main specific bonding types in an semiconductor will co-define Fermi Energy, band structure. But they are necessary, not sufficient factors for optical properties of solids. You can roughly classify solid types by their dominant bonding types (isolators-ionic/covalent, metals-metallic,...).