What factors determine the ability of a substance to transmit light?

[kcodon]

Transmission of Light...How?

Hi there all,

I'm not sure whether this is in the quantum mechanics area but here goes...

I was just wondering how light is actually transmitted through a substance like glass. I'm assuming there aren't "holes" that the photons/waves pass through, so I'm assuming the energy is absorbed by atoms and then re-emmitted...however wouldn't this imply something along the lines of 50% being reflected at the first surface, so by time light has passed through a few layers of atoms, it is extremely diminished in intensity? Hmmm I'm flummoxed.

Ok so I'm preempting another question of mine now then...what makes glass able to transmit light and not other substances? It can't just be because it's crystallic structure, because many crystals are opaque.

Any help here would be much appreciated,

Kcodon

 

[Claude Bile]

A photon will be transmitted through a substance if the energy of the photon does not correspond to any absorption bands. Note that absorption in a solid is not only dependent on atomic band structure, but phonon band structure as well - this is important because phonon band structure is dependent on crystalline structure, and is why Diamond has a different absorption spectrum than Graphite, even though both substances comprise of Carbon atoms only.

This touches on another point - transmission through a solid is not absorption and re-emission, so you have every right to be flummoxed by that approach. A transmitted wave through a solid acts to polarise the atoms that it passes through momentarily.

Claude.

 

[kcodon]

Thanks Claude,

You lost me a bit there with the talk of phonons...would you mind explaining? And also the idea that a transmitted wave polarises the atoms temporarily? Sorry I'm keen to learn this, its just i haven't ever heard of these ideas before.

I do however understand the bit about absorbtion bands, and that makes sense...I also assume its what makes every substance coloured too?

Thanks anyways,

Kcodon

 

[f95toli]

I would suggest you read Feynmanns excellent book on QED if you want the "real " answer (it is a good book even if you already understand the physics). There are simpler answers (which work fine 99% of the time) but for a deeper understanding you need QED.

However, glass is by no means an "unique" substance. The electromagnetic properties of all materials are strongly frequency dependent. Some, like glass, are transparent in the visible part of the spectrum but then they are opaque in other (e.g. UV radiation; which is why you won't get a tan if the sun is shining through a window). Materials such as teflon and silicon are opaque at hight frequencies but can be used to make lenses for far-infrared radiation (just as glas can be used to make lenses in the visible spectum) etc.

 

[kcodon]

Thanks f95toli,

Thats interesting about the frequency dependence, I never considered that but its quite nifty. Yes I managed to procure QED off my teacher for the holidays, and would love to read it, though at the moment have so much work to complete its unbelievable! However i will set to it definitely, throughout Christmas time.

Thanks for the reference,

Kcodon

 

[Claude Bile]

Thanks Claude,

You lost me a bit there with the talk of phonons...would you mind explaining? And also the idea that a transmitted wave polarises the atoms temporarily? Sorry I'm keen to learn this, its just i haven't ever heard of these ideas before.

Sure, phonons are the quanta of vibration wave, much like photons are the quanta of an EM wave. The concept of phonons is important because their properties in a solid depend on how all the atoms are connected, and thus explains why substances made of the same atoms, but different crystal structures (allotropes) have such different optical properties.

The idea that atoms are polarised temporarily as a wave in transmitted is a classical slant on the whole picture (QED), as f95toli alluded to. Nonetheless, the classical description conveys some important concepts such as the reduced group velocity of a wave in a medium, and the idea of transmission being a non-resonant phenomenon. It also ties in well with the concept of non-linear optics.

I do however understand the bit about absorbtion bands, and that makes sense...I also assume its what makes every substance coloured too?

It's not the whole story, but it plays a huge part. The concept of an object being a "colour" is a more complex idea that many people realize, since you need to take into account reflection and emittance spectra (i.e. we refer to a light as being "red" if it emits red light, not because it happens to absorb other wavelengths and reflect red for example).

But yes, to a first approximation the absorption bands will determine the colour of an object.

Claude.

 

[kcodon]

Thanks Claude,

I'm pretty sure I get the basic idea of the phonon's...I did some reading up on them. I suppose it's just similar to how light can be quantised into photon's, that vibrations can be thought of as phonons. As for why only non resonant waves travel through medium I'm still a little lost. Here is my thinking so far...

Is it because when the photon was at a resonant frequency with the phonon, all the energy of the wave is used in moving atoms around i.e. heat. Whereas if they are non resonant, you'll get some of the energy absorbed, but not all, and the excess is the transmitted light wave. However this doesn't seem to work in my head, as how is the wave actually emitted out the other side of medium...how is it absorbed? Because if it was a non resonant incident photon, then the resultant motion of atoms would not be perfectly sinusoidal, it would be an intereference pattern kind of moevement. I suppose this would result in a ray of light, simply an intereference of phonon's frequency and photon frequency...? And a flaw with my previous argument is that if the movement of atoms results in energy loss, then yes the resonant photons would not pass through, but also no other light would pass through unaltered, as some energy would be lost in atomic movement. I'll leave it at that because I think I'm just confusing myself here, and if anyone could explain why only non resonant waves travel through medium, I'd greatly appreciate it.

And just quickly how does the phonon have various frequencies at which resonance can occur? Similar to a standing wave I'm assuming...? And this means you would have like an absorption spectra, missing certain frequencies (at regular intervals?) that are resonant.

Phew,

Thanks anyway,

Kcodon

 

[AbedeuS]

Just adding, this is a pretty interesting read and prompted me to read into phonons more also.

-Abe.

 

[meopemuk]

Note that absorption in a solid is not only dependent on atomic band structure, but phonon band structure as well - this is important because phonon band structure is dependent on crystalline structure, and is why Diamond has a different absorption spectrum than Graphite, even though both substances comprise of Carbon atoms only.

You are right that diamond and graphite have different phonon bands. However, I don't think this is the main reason for different optical properties in the visible and ultraviolet spectrum. These properties are determined mostly by the electronic bandgap, which is about 5eV (if I remember correctly) in diamond and about 0eV in graphite. So, diamond is transparent for the visible light, while graphite absorbs it (black). This difference in electronic configuration can be explained by the different chemical bond structure (3D vs. 2D) in the two compounds. The phonon bands have effect on the infrared (vibrational) spectrum.

Eugene.