Why do insulators appear transparent?

[Wminus]

So normally we explain transparency by saying that the band gap in insulators is too large for mere visible light photons to be absorbed by the electrons. But what about the classical EM explanation? I made an attempt at an explanation below. Feel free to correct mistakes.

Since Insulators lack free electrons, only elastic scattering by the electron clouds surrounding each atom will occur and hence light is simply refracted. In a conductor, on the other hand, there are lots of free electrons that absorb and re-emit the light in such a manner that it gets converted into heat.

Hmm, but then why do metals reflect light so well instead of just absorbing everything? Can this stuff only be explained by mathematics and use of maxwell's equations?

 

[ZapperZ]

So normally we explain transparency by saying that the band gap in insulators is too large for mere visible light photons to be absorbed by the electrons. But what about the classical EM explanation? I made an attempt at an explanation below. Feel free to correct mistakes.

Since Insulators lack free electrons, only elastic scattering by the electron clouds surrounding each atom will occur and hence light is simply refracted. In a conductor, on the other hand, there are lots of free electrons that absorb and re-emit the light in such a manner that it gets converted into heat.

ALWAYS do this: Take your explanation, and continue with "... if this is true... then..."

So if your explanation is true, then ALL insulators are transparent to ALL light! Yet, we all know that ordinary glass is opaque to UV. In fact, most, if not all, "transparent" material is transparent only over a limited range of wavelength!

So now your explanation is incompatible with what we have observed.

Zz.

 

[Wminus]

Yeah you're right, but is my explanation simply all-out nonsense, or is there some truth to it? For very high frequencies I would guess the motion of the electron cloud becomes more unpredictable and chaotic, particularly if the wavelength of incoming light is so small that the electrical field varies significantly across the space containing the electron cloud! So perhaps my explanation holds at lower frequencies?
Hmm, but if this was true then what about semiconductors which have a relatively narrow band gap? Their electron clouds are the same size, yet these materials are opaque to visible light.

All right I guess I have nowhere near enough knowledge on solid state physics (particularly on the nature of band-gaps) and electromagnetism to make an explanation. Probably the real explanation is all maths anyways

 

[ZapperZ]

First of all, read this:

https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/#post-899393

Secondly, you seem to have ignored the entire function of the lattice ions and molecular vibrations.

Zz.

 

[DrDu]

Perfect conductors aren't absorbing, as they reflect all of the light. Absorption is due to scattering on, e.g., impurities which limits conductivity. On the other hand, in an insulator, there is no scattering, as the band is full. Absorption results only from transitions to other bands.
That there is no current in an insulator can be viewed from another point: While the electrons near the bottom of a band behave more or less like free electrons, electrons near the top of a band have a negative effective mass. This has the consequence that the current carried by these electrons due to an electric field points in the other direction as the current carried by the electrons near the bottom of the band. When the band is full, the net current vanishes exactly.

 

[ugenetic]

While the electrons near the bottom of a band behave more or less like free electrons, electrons near the top of a band have a negative effective mass. This has the consequence that the current carried by these electrons due to an electric field points in the other direction as the current carried by the electrons near the bottom of the band. When the band is full, the net current vanishes exactly.

You just single handed-ly destroyed everything I know about insulator, What bottom band? shouldn't insulator be about "fully filled bands is too far away from conduction, and filled up full so can't accept any electrons either"?

 

[DrDu]

The bottom of the filled valence band.

 

[ugenetic]

The bottom of the filled valence band.

still have not cleared my mind... Are you saying electrons in the bottom of the valence band have free states to jump to? Does this have something to do with dielectric effect? I guess there should be a transient current when all dipoles anti-align with the external field.

 

[DrDu]

No, I am not talking about dielectric effects.
One has also to be carefull because there are insulators, namely those of the Mott type, which cannot be described in terms of a band picture. It is these where hopping is hindered by the electrons on the other atoms.

Band states are already delocalized over the whole crystal, so in some sense there is always hopping.
There are several different ways to describe conductivity. That (non-Mott) insulators aren't conducting is due to the fact that in an insulator there are always electrons undergoing Bragg reflection from the lattice which inverts the current they are carrying. So there is no net current.

 

[ugenetic]

oh... I see, never heard of those concepts, but sounds very intuitive. I need to look into those.

so you have not destroyed what I knew about insulators, you expanded my horizon about insulators =)