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Why is glass transparent?

  1. Feb 26, 2010 #1
    Bit of a silly question I know, but I have never been able to get answer which I'm happy with. I thought it might be due to the energy levels available to the electrons in the glass. Light is able to pass through the material, because the photons in the visible part of the spectrum don't have a high enough energy to excite the electrons in the glass. However if this is the case it should be perfectly transparent. Yet a piece of glass both refracts and reflects light. So light must interacting with the glass by some mechanism. Does anyone have the answer?
  2. jcsd
  3. Feb 26, 2010 #2
    The atoms that come together to produce the molecules of a substance have a lot of electrons. When photons come in contact with these electrons, the following can occur:

    * An electron absorbs the energy of the photon and transforms it (usually into heat)
    * An electron absorbs the energy of the photon and stores it (this can result in luminescence, which is called fluorescence if the electron stores the energy for a short time and phosphorescence if it stores it for long time)
    * An electron absorbs the energy of the photon and sends it back out the way it came in (reflection)
    * An electron cannot absorb the energy of the photon, in which case the photon continues on its path (transmitted)

    Most of the time, it is a some of those that happens to the light as it hits an object. The electrons in different materials vary in the range of energy that they can absorb. A lot of glass, for example, blocks out ultraviolet (UV) light. What happens is the electrons in the glass absorbs the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum. If the electrons absorb the energy of any portion of the visible spectrum, the light that transmits through will appeared colored according to the portion of the spectrum absorbed. In fact, the color of any object direct result of what levels of energy the electrons in the substance will absorb!

    In order for something to be transparent to visible light, the atoms in the material must not interfere with the transmission of that energy. This interference is most often accomplished when the material absorbs the energy from the light and uses it to excite one of it's electrons to a higher energy level.

    The determining factor in if whether or not the material will absorb the light is a property known as the energy gap. It is abbreviated 'Eg' Its the amount of energy needed to excite a valence electron (an electron on the outermost unfilled shell of an atom) to the conduction state. Einstein showed that energy can only be absorbed in discrete packets called quanta, which means there will be some set value of energy at which point the material will start absorbing all energies equal to or greater than that value (again, this is called the optical gap).

    Glass is a non-crystalline material, which means rather than having a regular and repeated alignment of its atoms it has a disordered random atomic structure. This is accomplished by cooling melted glass before the atoms can arrange themselves in a neat way. In essence you must freeze-in the disordered liquid atomic structure. So glass is a non-crystalline solid and because of this, it's bonding is very strong and directional (highly covalent bonding). Since it is electrons that will absorb incoming light energy, there must be electrons available to do this. In glass there are very few free electrons for energy absorption so the optical gap is very high (greater than 3.1 eV). As a result light of energy less than the optical gap (in this case visible light) will pass through. It should be noted this is why glass is also a good electrical insulator.
  4. Feb 27, 2010 #3
    I did little bit of solid state physics years ago, so I suspected it had something to do with the energy gap of the material. However if light is transmitted and not affected by the electrons in the material. How can refraction occur, for the path of a light ray to change (snell's law), the photons must interacting with the material in some manner. Yet your explanation forbids this, the light should be transmitted and it's path unaffected by the glass. It also fails to explain reflection from a glass surface.
  5. Feb 27, 2010 #4


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    Refraction is caused by the photons interacting with bonds in the material.
    A bond absorbs a photon and a finite time later emits a new one, this travels at 'c' to the next bond and is absorbed and re-emitted again. The time taken for the absorb/emit is what gives you the slower speed of light = refractive index.

    To add to the why is glass transparent - you need two factors.
    Nothing to absorb the photons. Metals aren't transparent because the free electrons can absorb most wavelengths. Insulators can generally only absorb a photon if there is an appropriate energy level available, for glass there aren't (m)any in the visible band, there are lots in the UV which is why glass blocks UV.
    Secondly you need a regular arrangement of atoms, so that the re-emitted photons go in the same direction, otherwise the ligth would be scattered until it is eventually absorbed. That's why crystals are transparent, glass is rather a special gas, it isn't ordered in the same way as a crystal but there is a fixed position of the atoms
  6. Feb 27, 2010 #5


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    I know very little about solid state physics, but this sounds very counterintuitive to me.

    It sounds like a model that may hold up in terms of classical electrodynamics, but does it have a counterpart in terms of quantummechanics?

    I can imagine a wavefront reaching a boundary between two materials with different properties. The wavefront can become re-aligned, corresponding to a beam (of light) changing direction.

    This is completely in terms of wave mechanics, so I think it's awkward to refer to behavior of 'photons' in this context.
  7. Mar 8, 2010 #6
    Whats the difference between diffusive glass and ordinary glass. What i mean is, the glass that you have on your shower door to blur the image obviously scatters the light greatly from its respective incident angle.

    Am I making a correct assumption by saying that the amorphous (random structure or layout) of atoms in glass, on average perturbs the incident light so that the emerging light is in the same direction? And the more structured you glass the more your light will be scattered such that the light does not on average have the same incident angle (such as diffusive glass).

    Basically, if you randomly kick a soccer ball with the same force (changing the direction only), over a huge amount of kicks, the soccer will be kinda near where it started.

    Theres 10^23-25 atoms roughly per piece of matter in your hand, so on average a randomly ordered structure should scatter the light in approximately the same direction as it entered?

    Often times the explanation for being able to read through glass is that the random structure allows a lot of space for the light to move between, orb the atoms absorb in the optical visible light range , I don't agree with this since the light can scatter regardless of energy.

    Any body agree? Comments appreciated
  8. Mar 10, 2010 #7

    Claude Bile

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    I say this with respect, but this entire post is wrong.

    Media (including glasses) exhibit a refractive index because the atoms that comprise them form oscillating dipoles in the presence of an EM wave. Reflection is a result of dipoles emitting their own field, while refraction (the slowing of an EM wave packet) is due to the phase difference between the oscillation of the EM wave and the oscillation of the dipoles.

    We regard media as being transparent if they do not absorb any wavelengths of interest. For example, silica is transparent in the visible, silicon is transparent in the near-IR (but opaque in the visible). The absorption spectrum of a medium depends on what atoms they are made of and how they bond together.

    This applies to both the classical and quantum description of light.

    Last edited: Mar 10, 2010
  9. Mar 10, 2010 #8
    Thanks Claude, that seems very reasonable.

    But you haven't elaborated on my point/question. Glass should still allow for scattering, which would completely blur the image (loose all information about the orientation of the light). Is the reason why glass scatters light generally the same direction due the the random order of glass, and as we start to add more structure, does the glass begin to blue?

    The reason why I'm thinking the random structure eventually scatters the light (of same incident angle) into one direction (same exiting angle) over the whole crystal is due to my previous post on randomly kicking a ball with the same kick (10^23 times). I'm not claiming my explanation is right, however it seems reasonable and no one has yet to specially respond to this.

    Thank you very much.
    Last edited: Mar 10, 2010
  10. Mar 10, 2010 #9


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    Note that an entry in the FAQ on how photons move through a medium might also be relevant to this discussion.

    Ordinary glass is amorphous. It also means that the material can have these "oscillating dipoles" in all the different directions. So a light in a particular polarization will find a dipole in that orientation. If that vibration mode can't be sustained, then that particular light will be transmitted rather than absorbed, and in the same direction. That's why you don't see scattering in that case.

  11. Mar 10, 2010 #10

    Shouldn't that still scatter the light- Regardless of whether the polarization matches the orientation for that particular bond. Do you mean to tell me if I pass photon with a different (non matching polarization) then there will be absolutely no scattering? There should be at least some contribution.

    Shouldn't the ability to scatter be frequency independent and polarization independent?

    Or Am I mistaken by combining classical/quantum pictures

    Can someone suggest a text, that explains all of this well
    Last edited: Mar 10, 2010
  12. Mar 10, 2010 #11


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    But you'll notice that one could ask the same thing about "reflection" of metallic surface, i.e. when you look in the mirror. Why would the reflection, which is basically a emission off the conduction electrons, would be in a particular direction? I would say that without going into details, that the particular plane of polarization and momentum are conserved.

    The ability to scatter IS frequency dependent. It depends on the nature of the material. If the phonon mode exists for that particular frequency, then that frequency can be absorbed by the material and that photon is absorbed and converted into heat. This is where you have to consider that the material/medium plays a major role on what gets transmitted.

  13. Mar 10, 2010 #12
    But note that, take away the conduction electrons, and you won't have your mirror anymore.

    The thing you need to understand is that when atoms form a solid, their "individuality" is often no longer relevant for many phenomena. These atoms have form a collective behavior that dominates the physical properties and characteristics of the solid. The phonon modes that I talked about earlier doesn't exist when you break down the behavior up to the atomic scale.

    In a crystaline material, one can, in fact, have different index of refraction in different crystallographic orientation. How light travels in graphite along the ab plane is different than how it travels along the c-axis of the crystal. This clearly indicates that it isn't just the atom that is at play here, but rather how these atoms are arranged in the lattice. The fact that when you have such symmetry, and that light also would have transport properties that are also affected by such symmetry, means that there is a non-isotropic behavior for an unpolarized light when the medium has a preferred symmetry. Now turn this around and consider if the medium has no preferred symmetry, such as an amorphous medium. Then, if light can go in a particular direction and get be transmitted, chances are that direction will be conserved since the medium is isotropic in all directions.

    Last edited by a moderator: Mar 10, 2010
  14. Mar 10, 2010 #13


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    Er... CRAP! Instead of replying, I accidentally EDITED your post. I apologize! I can't find your original post in my buffer to restore it.

    I should never do this without coffee...

  15. Mar 10, 2010 #14

    Your clearing things up for me and I'm very thankful.

    #1 By Isotropic do you mean the randomness of the atoms / molecules creates a uniformity in all directions?

    #2 So.. is it correct to say that because of the isotropic nature of an amorphous solid. The individual scatterings combine in such a way that the original orientation of the light is preserved (almost completely) over the whole route traversed by light through it?

    #3 I understand the concept of collective behavior (metallic bonding) and phonon modes. I have taken solid state, but with an incredibly moronic young professor, I feel that I have learned more conceptually through physics forums.

    #4 Maybe you can clear up the part about (taking away conduction electrons means no mirror). Is this because the free electrons mobilize in response the incoming radiation (reflection occurring in the first 10^4 10^5 planes of atoms).

    But then how does reflection occur in glass if there are no free conduction electrons? Obviously glass doesn't reflect light as well as a mirror, but it still does.

    Thank you very much

    Last edited: Mar 10, 2010
  16. Mar 12, 2010 #15
    This if for JerryClower.

    Your post in response to this question is fascinating. However, there is one thing I don't understand.

    You said: An electron absorbs the energy of the photon and sends it back out the way it came in (reflection)

    If an electron absorbs the energy of the photon, how could there still be a photon? I ask because I am curioius about the nature of reflection. In high school we are told it is a "bouncing" phenomenon. This can't be right. But if a photon interacts with an electron and the photon thereby loses its energy, how can it be "sent on its way?" Is reflection not really an absorption/re-emission event? That is: electron absorbs energy from photon, photon is gone, then electron instaneously re-emits energy, new photon appears and goes on its way.

    Thanks JerryClower

  17. Mar 12, 2010 #16

    Claude Bile

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    Yes, but often only a fraction of light passing through a transparent solid will undergo scattering. You can still see clearly through glass because most of the light has not scattered at all.

    To address your latest post;

    1. Yes.

    2. No. See above.

    4. Conduction electrons are not essential for obtaining reflection; bound electrons can also contribute to a reflected wave. Reflection is fundamentally a result of a refractive index (or impedance) mismatch between media, for example, water and air.

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