# Why is glass 'see-through'?

by n0_3sc
Tags: eethrough, glass
 P: 63 T+R+A=1 where T=transmission coefficient,R=reflection coefficient and A=Absorption coeff. and all depend on material and wavelength of radiation you are talking about.. So for any material, if T is to be high, A and R need to be low... Since metal surface has a lot of free electrons, when the radiation strikes the material, the free surface electrons start oscillating and in the process they emit the radiation which falls on them (oscillating charge will emit radiation). This is how metals are supposed to be good reflectors.. Absorption is something i'm not very sure of but since glass has high T, obviously its A is low for visible radiation.. p.s: i could not read the previous posts due to lack of time so i'm sorry if i'm repeating something.. :)
Mentor
P: 27,641
 Quote by Raze2dust T+R+A=1 where T=transmission coefficient,R=reflection coefficient and A=Absorption coeff. and all depend on material and wavelength of radiation you are talking about.. So for any material, if T is to be high, A and R need to be low... Since metal surface has a lot of free electrons, when the radiation strikes the material, the free surface electrons start oscillating and in the process they emit the radiation which falls on them (oscillating charge will emit radiation). This is how metals are supposed to be good reflectors.. Absorption is something i'm not very sure of but since glass has high T, obviously its A is low for visible radiation.. p.s: i could not read the previous posts due to lack of time so i'm sorry if i'm repeating something.. :)
You really DO need to read the thread before responding with something like that.

Zz.
 P: 63 hmm..k now i read the posts..and i must admit most of them are slightly above my current level of understanding.. but what about this - " materials with free electrons are good reflectors ". Do you think thats right? I mean it works well for most materials i can think of right now but there are bound to be exceptions.. and glass does not have that many free electrons..now if we could find some explanation for why glass does not absorb visible radiation the problem is solved right? or am I missing something here?
Mentor
P: 27,641
 Quote by Raze2dust hmm..k now i read the posts..and i must admit most of them are slightly above my current level of understanding.. but what about this - " materials with free electrons are good reflectors ". Do you think thats right? I mean it works well for most materials i can think of right now but there are bound to be exceptions.. and glass does not have that many free electrons..now if we could find some explanation for why glass does not absorb visible radiation the problem is solved right? or am I missing something here?
Glass is transparent over most of the visible range. So why is this an issue? A typical mirror that you use isn't glass.. it may have a glass COATING on top of it, but the reflective part is usually a metal, possibly aluminum film.

Zz.
P: 265
 Quote by ZapperZ You really DO need to read the thread before responding with something like that.
LOL.

There are so many questions in this thread that I'm losing track...
Emeritus
PF Gold
P: 11,137
 Quote by Raze2dust but what about this - " materials with free electrons are good reflectors ". Do you think thats right? I mean it works well for most materials i can think of right now but there are bound to be exceptions..
This is only partially correct. Most metals have a high reflectivity in the visible range and are highly transparent in the high UV range. The reflectivity in the visible range is, as you point out, due to the excitation of surface plasmons (collective excitations of the free electron gas). However, the electron gas, has a natural frequency (the plasma frequency) above which it is not very good at responding to the EM-field of the incident light. For most metals, the plasma frequency (which is a function of the free electron density and the conduction band effective mass) lies in the UV range.

 and glass does not have that many free electrons..now if we could find some explanation for why glass does not absorb visible radiation the problem is solved right? or am I missing something here?
Yes, that is essentially correct, but it hardly makes the problem any easier to solve. The vast majority of insulators are opaque, and this is true even of amorphous materials. Glasses are kind of special in that they are essentially supercooled liquids.
P: 61
 There are so many questions in this thread that I'm losing track...
Stay focused!

 Would that imply that there exists a delay between the outgoing light and the incoming light - a delay given by the electron's decay lifetime?
Asking that in a different way then would be....why light is slower in mediums then vacuo?
However, though i'd love to say yes, yes, yes (harry and sally moment there), i'd hold reservations on that because i dont believe its only absorption and re-emission that occurs...
Its my wave thingy again....

However, a good example of your thought would be Photoluminescence....you know those things you expose to light and they re-emit in the dark....and the time differences in these are very significant....
With normal transmission, i'd like to think that the IOR could be a good indicator of flight time for visible transmission through media....we do know that a wave packet of white light will spread due to the different wavelengths down a fiber, i take it this is due to the IOR of the fibre acting on the different wavelengths and hence you get the different arrival times. Am i right on this?

From my point of view, any wave interferance is bound to produce a delay and further to the arguments added today about glass and metal electronic structures....i dug up a couple of things that slightly relate.....with my usual bias on waves of course...

One of those things is Anderson localization, which according to wiki....
 Anderson localization is a general wave phenomenon that applies to the transport of electromagnetic wave, acoustic wave, quantum wave and spin wave, etc. This phenomenon is to be distinguished from weak localization, which is the precursor effect of Anderson localization. This phenomenon finds its origin in the wave interference between multiple-scattering paths. In strong scattering limit, the severe interferences can completely halt the waves inside the random medium
http://en.wikipedia.org/wiki/Anderson_localization

and the other was an article on slow light by physorg....

http://www.physorg.com/news128268191.html

which talked about slow light taking place alongside anderson's localisation....
but the part that peaked my interests was....
 “Light localization enables us to control photons and the various aspects of their propagation and interaction with matter,” said Bandaru, who works in the electrical properties of nanometer scale structures.
which ties in with your question....about delays....and for these modern ic progress, its a very important field of study....

i'd like to say that their succes is more likely to come out of treating their problem as one of waves than just discrete electron states alone....in fact, if you havent fallen as yet...i'd like to make one more connection....though i haven't been able to look more in depth at this...the abstract looks good with respect to my thoughts...and drags up what was discussed before....remember?
 LOCALIZATION OF LIGHT has been achieved by an Amsterdam- Florence collaboration (contact Ad Lagendijk, adlag@phys.uva.nl). Consider the movement of light through a diffuse medium such as milk, fog, or sugar. The light waves scatter repeatedly, and the transmission of light decreases as the light gets reflected. In the Amsterdam-Florence experiment something different happens. By using a gallium-arsenide powder with a very high index of refraction but with very low absorption at near infrared (wavelength of 1064 nm), the researchers were, in a sense, able to get the light to stand still. That is, the light waves get into the medium and bounce around in a standing wave pattern, without being absorbed. This is the first example of "Anderson localization" for near-visible light. This medium is not what would be called a "photonic bandgap" material (analogous to a semiconductor for electrons) but more like a "photonic insulator."
http://www.aip.org/pnu/1998/split/pnu356-1.htm

Just imagine if we could build models that could cater for the wave (material field) behaviour in terms of response alongside the electrons own response....we could characterise new materials based on their structure alone.....
I'd also wish that overall simplistic equations (for general speculative behaviour) could come into play without having to root down to the myriad of small actions taking place...them thar hamiltonians are scary spooks in disguise!
and while i'm being simplistic, i'll add another speculative thought i had....which would be to have a tie in on some quantum variable based on kT above zero.....my reason for this is that there must be a level at which a material comes into its own without outside energy....along the lines of the Bose-Einstein condesate level.....and that there must be discrete jumps of the system above that.....the higher up we get, the more random the system looks because of all the variables....but applying the kT constants would give us more information as to their (material) response....
hey...i agree...this is way beyond me....just speculating here....

food for thought?
P: 61
 Quote by Gokul43201 This is only partially correct. Most metals have a high reflectivity in the visible range and are highly transparent in the high UV range. The reflectivity in the visible range is, as you point out, due to the excitation of surface plasmons (collective excitations of the free electron gas). However, the electron gas, has a natural frequency (the plasma frequency) above which it is not very good at responding to the EM-field of the incident light. For most metals, the plasma frequency (which is a function of the free electron density and the conduction band effective mass) lies in the UV range. Yes, that is essentially correct, but it hardly makes the problem any easier to solve. The vast majority of insulators are opaque, and this is true even of amorphous materials. Glasses are kind of special in that they are essentially supercooled liquids.
its funny...i had a thought....but its more of a question really...

would metals be more reflective because of their layered crystalline structure as compared to insulators which less so?
I say this because each crystal layer within the metal could be acting like a barrier and would aid the reflection rather than the transmission.

It wouldn't be difficult to see that higher eV energy (UV) would penetrate as they fall under the higher exiton polaritons, whose energy levels are considerable for the fields available in the metals for stopping or diverging such photons...
P: 265
 Quote by deakie With normal transmission, i'd like to think that the IOR could be a good indicator of flight time for visible transmission through media....we do know that a wave packet of white light will spread due to the different wavelengths down a fiber, i take it this is due to the IOR of the fibre acting on the different wavelengths and hence you get the different arrival times. Am i right on this?
IOR = Index of Refraction? Then yes your partly right for Chromatic Dispersion. It also depends on Material+Waveguide dispersion, the profile of the refractive index and the length of the fiber - after all, you can have the profile of the refractive index such that a white pulse will not separate after so many meters of fibers. (Used in Supercontinuum Generation).

I also agree that the Anderson-Localization is something different, but however interesting it is, it still avoids why defined lattice structures appear more opaque than undefined structures (glass).

 Quote by deakie would metals be more reflective because of their layered crystalline structure as compared to insulators which less so? I say this because each crystal layer within the metal could be acting like a barrier and would aid the reflection rather than the transmission. It wouldn't be difficult to see that higher eV energy (UV) would penetrate as they fall under the higher exiton polaritons, whose energy levels are considerable for the fields available in the metals for stopping or diverging such photons...
Yes! Thats exactly what I want too know too. Do you think it acts as a barrier because it is easier for the light to be absorbed by phonons? Whereas the phonons are harder to be absorbed for undefined lattice structures.
 Mentor P: 27,641 "Layered crystalline" is very misleading. I have graphite that is a "layered crystalline". In fact, it is so layered that it is soft because the crystal along the c-axis has very weak bonding. Yet, is it as reflective as metals? Going to the other end, I can make thin films of metals that is polycrystalline, and you won't know the difference in terms of reflectivity. The crystalline nature doesn't play that big of a role in visible-range reflectivity as compared to the presence of the conduction electrons. I thought this has already been explained in this thread? So why are we insisting on an alternative explanation? Are you not satisfied with the explanation given? Why? Zz.
P: 61
 Are you not satisfied with the explanation given? Why?
ok....i'm guilty of it because surface plasmons are all about the electron/particle role....

Gokul43201's explanation is solid for that role and rightfully so, also, his explanation of the plasma frequency having limits for the electron in participating in photon interaction....I cannot argue with either.

however, that said, what i find curious, is that UV is transmitted through the material at all.....
Especially if all the interactions are done with electrons, shouldn't they be, at that frequency and above, ionised?

 Yes! Thats exactly what I want too know too. Do you think it acts as a barrier because it is easier for the light to be absorbed by phonons? Whereas the phonons are harder to be absorbed for undefined lattice structures.
I'll look more into this surface plasmons thingy first mate! We have to first deal with the accepted models before moving on...and we need a way to experiment with what we are thinking....
P: 125
 Quote by deakie ...food for thought?

Here's an appetizer:
http://135.196.210.195/chemistryworl...r/10120701.asp

:>
 P: 63 well if free electrons are the fundamental cause of reflection, how do you explain total internal reflection from glass?
P: 125
 Quote by Raze2dust well if free electrons are the fundamental cause of reflection, how do you explain total internal reflection from glass?
Once you increase the angle of incidence above the critical angle, total internal reflection will be the result.

http://en.wikipedia.org/wiki/Total_i...Critical_angle
 P: 265 light is NEVER totally internally reflected. Even after the critical angle, there still exists a beam propagating along the surface (eg. the evanescent wave - which is another philosophical issue...)
 P: 63 but thats applicable to metals too.. leave it what i meant was just that free electron theory alone is not sufficient to explain reflection from all surfaces
P: 265
 Quote by Raze2dust but thats applicable to metals too.. leave it what i meant was just that free electron theory alone is not sufficient to explain reflection from all surfaces
I agree.
 P: 880 To me, asking, "Why is glass 'see-through'? " is the same as asking, "Why is 'air' 'see-through'? ".

 Related Discussions General Physics 9 General Physics 1 General Physics 11 Biology 1