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See-through glass

  1. Jan 6, 2004 #1

    jimmy p

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    It may seem like an odd question, but why is glass see-through? What properties of glass (and indeed light) make it possible for us to see through glass?

    Has anyone else ever wondered this?

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  3. Jan 6, 2004 #2
    I do wonder about this from time to time and am not satisfied yet with what I have learned but here's a start:

    The material has one or more natural frequencies which the atoms want to vibrate at. When the electromagnetic energy has a frequency much below the natural frequency it will cause the molecules to vibrate and turn into heat. So glass isn't very transparent to infrared light.

    When the frequency of the em energy matches that of the atoms of the glass, the electrons in the atom vibrate very efficiently in tune with the incident energy. The electrons of neighboring atoms 'bounce' into each other and the energy is turned into heat. Thus glass is not very transparent to ultraviolet light.

    Between the two extremes, the electrons vibrate some but not very efficiently, they only hold the energy a short time and then they re-emit the energy as a little packet of light (photon). This lets the incident light travel through the glass with little loss and a slight delay in travel (about 30% slower).

    Well that's what I know but this model has a few problems that have been discussed here before. For one, do some photons fail to interact with the glass atoms completely and so travel faster than the photons that do interact? That would mean photons are traveling at different speeds through the material.
  4. Jan 7, 2004 #3

    Chi Meson

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    The glass does not have to be very thick in order to have near 100% probability of every photon interacting with the atomic structure of the glass. For example, "half-silvered" mirrors are coated with a layer of silver that is essential a few dozen atomic layers thick. The thickness of this coating can be adjusted to the appropriate level of probabilities of reflection vs transmission.

    I do not recall this specifically, but I assume that if you got a super-thin layer of glass, you would find a percentage of light that has not interacted with the glass as it went through. But for normal thicknesses of glass, there would be near zero probabilities of a photon making it through without interaction.
  5. Jan 7, 2004 #4
    You have to understand the nature of the photon. You turn on a lamp, and a photon is generated from the filament; it then passes through the glass of the bulb and into your ambient (atom-rich) atmosphere, thus diffusing into the room. Photons travel only in a straight line, so it has to be passed from one atom to another when it travels through "clear" matter. Visible light can do this in crystalline structures because of their actual physical layout on the atomic level. The atoms in the latticework of a crystal is very conductive in terms of the (visible light) photons.

    The photon does not, however, pass through the matter that makes up the glass; rather, it's being passed along from atom to atom via "elevated" or "excited" states in the atom's electrons. Basically, it's like, when it comes to visible light, the crystalline lattice structures on an atomic level are very close together and harmonic with those frequencies of vibration, so they pass the energy (not the photon) through the dense atomic structures in the crystalline material with very little opacity or diffusion, refraction, etc.

    Now, when the light leaves the filament, its photon contacts the inside of the glass bulb. If there's a billion atoms in between the inside of the glass and the outside of the glass, there's a billion photons being generated (each atom receiving a photon goes into an excited state until a threshold is reached where in the electron's orbit collapses, releasing a photon) but they're being passed on with great efficiency -- at least for the bandwidth or spectral subset you're dealing with. As in the "Glass being opaque to infrared" example mentioned in this thread, metals can be practically transparent to infrared but extremely opaque visible frequencies of radiation.

    Oddly enough, when some metals are supercooled, they become transparent.
  6. Jan 7, 2004 #5
    I have a friend who works for an optical company. I will ask him about the thicknesses of half-silvered mirror coatings. That will be a good rule of thumb for how thick before near 100% probability of interaction.
    Yes, but the delay for 1 atom/photon interaction must be very small otherwise light would not slow only by 30% in a thick piece of glass. It will take millions of interactions to get the average 30% slowing. Also, nearly all the photons must suffer the same delay in order for there to be a single angle of refraction.

    I still feel like there is a lot here I don't know.
  7. Jan 7, 2004 #6
    Nature of the photon, again

    See, it doesn't ultimately matter how many photons go THROUGH the glass because it's the same effect as the photons daisy-chaining their way through the glass. All that hinders transparency is how many photons are ABSORBED by the glass. Again, in materials science, it's the crystalline structure that assists the photons being passed along.

    Now, yes, you can say that light is traveling slower than C in this example because the medium has an inherent resistance. This has been demonstrated effectively a hundred years ago, and it's been taken to the extreme in the past several years - something to the effect of passing light through an environment of supercooled metallic plasma, dropping the speed of light down to somewhere around 25 miles an hour. It's pretty fascinating stuff.
  8. Jan 8, 2004 #7

    jimmy p

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    OK so it matters on how many photons a material absorbs then. How come we see through it though? surely the atoms in the glass would absorb some of the energy and transmit energy of a different frequency? So wouldnt the image through a window be slightly different to the outside image.

    OK so it matters on how many photons a material absorbs then. A material that absorbs a lot of photons (low energy atoms) will mean it is less likely to make it see through, but glass transmits a lot of the photons so appears clear? am i right?
  9. Jan 8, 2004 #8


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    You do see reflections and if the glass is not uniform you will see distortions. Don't forget the refraction that occurs. These phenomena are all hints that something is going on inside the glass.
  10. Jan 8, 2004 #9
    These phenomena are all hints that something is going on inside the glass.

    Yeah. This always (for me at least) adds to the "mystery" of what is going on inside the glass. Depending on the thickness of glass (and maybe composition) the percentage of light transmitted cycles between a minimum value, usually zero, and a maximum value.

    I'm wondering, can that be thought of as an expression of the transverse vibration of light, and expression of the wavelength?
  11. Jan 8, 2004 #10

    Chi Meson

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    Remember, what we can see with our eyes depends on what the largest numbers of photons are doing. That is, we usually don't notice the reflection off of glass when there is a bright light on the other side because the image that is transmitted swamps the image that is reflected. It would take sensitive instuments to detect the small percentage of photons that do not interact (if any).
    I'd like to know of experments that detect whether or not individual photons make it through a thin layer of glass "faster" than the greater number of photons that get bogged down in the glass. Does anyone have any specifics?
  12. Jan 9, 2004 #11

    jimmy p

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    So how come we see the image so clearly? and with the right colours?
  13. Jan 9, 2004 #12

    The only reason glass is "clear" is because, like diamond, the crystalline structures pass photons with very little diffusion. If you think about it, every other form of gemstone is valued from color, and that's a function of diffraction, producing a shifted spectral effect.

    Metal is opaque but only to our visible spectrum. Metal is preferred when it comes to infrared, etc. What is "clear" depends upon the construction of the "eye" of the beholder.

    Cool, huh?
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