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What's happening in glass with the electrons, for light to travel through it?

  1. Jan 14, 2016 #1
    ... compared to a opaque materials electrons. Is it the amount of electrons in the atoms shells ?

    If you have a four inch cubic block of glass, and carbon, light passes through the glass no problem, but the carbon will absorb some red, yellow, green, and blue light, but if you look at the carbon absorption for light, not all blue, green, yellow, and red light get absorbed by carbon.
    glass cannot absorb high wavelengths of light.
    So if the two four inch cubic block of glass, and carbon, are placed in a dark room with no light hitting the blocks at all, and you just shined the colors of light at the carbon, that did not excite the electrons to a higher shell energy level, what would happen.
    Question 1. Does the light from the blue, green, yellow, and red pass through the carbon block, but I think you would just see the block of carbon just be black right, even though certain colors of light are passing through it, is this correct.
    All light colors pass through the glass no problem, so for the carbon things are different, its the energy of the electrons in the glass that cannot get exited for the light, so the light gets transmitted through the block of glass.
    So for the carbon, does its electrons either absorb more energy, or because it has 2 electrons in shell 1, and 4 electrons in shell two, silicone which is mostly what glass is made from has two electrons in shell 1,and 8 in shell 2, and 4 in shell 3.
    So it cannot be the amount of electrons I think just the energy of electrons, but can you explain why the energy levels are different for some electrons.
  2. jcsd
  3. Jan 14, 2016 #2


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    Staff: Mentor

    Taken from the here: https://www.physicsforums.com/threads/do-photons-move-slower-in-a-solid-medium.511177/#post-899393 [Broken]

    The post is about why photons travel slower in a medium than in a vacuum, but half of the explanation also explains transparency and opaqueness of bulk materials.

    When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.

    A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.

    On the other hand, if a photon has an energy beyond the phonon spectrum, then while it can still cause a disturbance of the lattice ions, the solid cannot sustain this vibration, because the phonon mode isn't available. This is similar to trying to oscillate something at a different frequency than the resonance frequency. So the lattice does not absorb this photon and it is re-emitted but with a very slight delay. This, naively, is the origin of the apparent slowdown of the light speed in the material. The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay.

    Moral of the story: the properties of a solid that we are familiar with have more to do with the "collective" behavior of a large number of atoms interacting with each other. In most cases, these do not reflect the properties of the individual, isolated atoms.
    Last edited by a moderator: May 7, 2017
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