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Once I asked this question from my teacher and he replied "Because it passes light.". "And why does it pass light?" I asked and he said, "Because it is transparent.".

The same question again, Why glass is transparent? Why does light pass through it, while for opaque objects, it does not?

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Photons pass through glass because they are not absorbed. And they are not absorbed because there is nothing which "absorbs" light in visual frequencies in glass. You may have heard that ultraviolet photons are absorbed by glass, so glass is not transparent for them. Exactly the same happens with X-rays for which our body is nearly transparent whilst a metal plate absorbs it. This is experimental evidence.

Any photon has certain frequency - which for visible light is related to the colour of light, whilst for lower or upper frequencies in the electromagnetic spectrum it is simply a measure of the energy transported by photon. A material's absorption spectrum (which frequencies are absorbed and how much so) depends on the structure of the material at atomic scale. Absorption may be from atoms which absorb photons (remember - electrons go to upper energetic states by absorbing photons), from molecules, or from lattices. There are important differences in these absorption possibilities:

  1. Atoms absorb well-defined discrete frequencies. Usually single atoms absorb only a few frequencies - it depends on the energetic spectrum of its electrons. Regarding atomic absorption, the graph of absorption (plotted as a function of frequency of light) contains well-defined peaks for frequencies when absorption occurs, and no absorption at all between them.
  2. Molecules absorb discrete frequencies but there are many more absorption lines because even a simple molecule has many more energetic levels than any atom. So molecules absorb much more light.
  3. Crystalline lattices may absorb not only discrete frequencies but also continuous bands of frequencies, mainly because of discrepancies in the crystalline structure.

As glass is a non-crystalline, overcooled fluid, consisting of molecules, its absorption occurs in the 1st and 2nd ways, but because of the matter it is composed of, it absorbs outside our visible spectrum.

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    $\begingroup$ Glass does absorb photons - they are absorbed by the inter atomic bonds (phonons) and re-emitted, this is essentialy why the speed of light in glass is slower. It appears transparent because the direction of the light is preserved by the ordered bonds and because little of the energy is lost $\endgroup$
    – Martin Beckett
    Mar 23, 2011 at 15:46
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    $\begingroup$ Thanks, first paragraph was easier to understand and solved enough confusion. Do other type of photons behave like visible light (i.e. light converges with a lens) $\endgroup$
    – SMUsamaShah
    Mar 25, 2011 at 7:51
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    $\begingroup$ @Martin: right, except it is called scattering, not absorption. $\endgroup$
    – Marek
    Mar 25, 2011 at 9:27
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    $\begingroup$ @LifeH2O - picture I describe is far well simplified. When You try to analyse light and matter interactions there is many process to take into account. But from the form of the question I presume You need a basic level answer. Real interaction is quantum one in absorption-emission area, but may be wave dynamic in certain frequencies - for example interference should be analysed etc. It is rather complicated when You would like to take account on all phenomena which may occur. $\endgroup$
    – kakaz
    Mar 28, 2011 at 12:59
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    $\begingroup$ @kakaz Why is band absorption not ruled out in case of glass? In spite of being non-crystalline, glasses too have bands. $\endgroup$
    – SRS
    Mar 30, 2018 at 12:48
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Essentially because of absorption. When photon flies into the material it interacts with its constituents. This interaction can be divided into two contributions. One of them is elastic and is the source of the index of refraction (because effectively it just slows the photon down) while the other one is inelastic. Photon gets absorbed by an atom (say) and later it is emitted as thermal radiation in random direction thereby losing the original information it carried.

When you look at this macroscopically, this process will be described by some parameter like penetration depth and intensity w.r.t. depth will decay exponentially. So if you made opaque objects thin enough, they would still be transparent (although the outgoing light would be weaker depending on thickness). Of course, this discussion completely avoids surface effects (reflection, refraction, scattering, etc.).

Note that all of this depends on frequency of the incident light. Atoms (let's just talk about them for simplicity; in reality there will be contribution also from molecules, lattice, free electrons and whatnot) have something called absorption spectrum. This arises because for certain frequencies electron can catch the photon and get excited to the higher energy state. So, while a material can be transparent in certain range of frequencies (like glass is for visible light) it can be quite opaque in others.

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    $\begingroup$ All this is fine, but there is something special about glass in that (visible) radiation absorbed by its atoms (or fibers ...) is emitted coherently in the same direction. That is why glass is not only transparent but also preserves images for eg. Why is this the case? $\endgroup$
    – user346
    Mar 23, 2011 at 7:52
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    $\begingroup$ I am not sure what you mean. Image is preserved in every transparent material unless it possesses some additional symmetry (e.g. crystals can have different propagation and polarization properties along different axes -> anisotropy). But for disordered materials there is no such thing. Glass is no different from e.g. air or water: photons just scatter off individual constituents but the only surviving contribution to path integral is propagation in straight line (albeit slower, resulting in bigger index of refraction). $\endgroup$
    – Marek
    Mar 23, 2011 at 11:23
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    $\begingroup$ @Deepak: it's interference between the different atoms that a photon is absorbed by then re-emitted that makes photons keep going the same direction in glass. It's not that an individual atom absorbs a photon and them re-emits it in the same direction; you can see this because only the interference between different atoms would lead to Snell's law of refraction. I believe Feynman has an excellent explanation of this in his book QED, which I highly recommend as an elementary presentation of quantum mechanics. $\endgroup$
    – Peter Shor
    Mar 23, 2011 at 14:08
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    $\begingroup$ @Peter, @Marek I understand that interference leads to these effects. My question was (is) why do light waves interfere constructively, rather than destructively, in the case of glass. Feynman also has a very nice chapter (in Vol I of his lectures I believe) on the "origin of the index of refraction". But the model he uses is that of a lattice of dipoles. I don't see why for a substance such as glass - with no obvious underlying order - such a description should work. I hope I've made myself clear(er). $\endgroup$
    – user346
    Mar 23, 2011 at 16:42
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    $\begingroup$ @Deepak: What I believe you need is that each absorption-reradiation event does approximately the same thing to the light (and doesn't change the phase radically). $\endgroup$
    – Peter Shor
    Mar 23, 2011 at 16:49
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There is a lot of nonsense around about this. It is NOT a very thick stiff or cold liquid, nor is it due to how ordered the structure is. In simple terms it is all about the electrons in the substance. When photon of light enters a substance, it will interact with an electron changing its energy state.

KEY POINT — Electrons can only exist in fixed (banded) energy levels.

In common opaque material, it takes a small amount of energy to move the electron from its resting energy state to a higher energy state, so the low energy photon of visible light is absorbed, transferring its energy to the electron which in turn moves to a slightly higher energy state.

In much rarer transparent material the distance between the electrons rest energy level and the next higher state is much much greater. So because the electron can only be "resting" or at a high energy level NEVER in between, the little photon of visible light doesn't carry enough energy to transfer to the electron to make it jump to the higher state. So it keeps its power, doesn't get absorbed and passes through the material. Hey presto, see through.

Some glass only lets through light of a certain colour. This is because different colour light haas different energy levels and so depending on the energy needed to 'move' electrons to a higher energy state some colours will be strong enough and absorbed and others wont and will pass through

NOTE, I am simplifying a bit, for example it doesn't literally 'pass through' as such, the the way the electron and photon interact are quantum mechanical and none of this includes polarization, reflection etc. all of which works on a quantum level. That said on a macro scale, without the quantum detail (which would take years to teach) this is what is happening.

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This may be a little technical, but I always thought it was cool: one of my professors once pointed out that transparency only happens because the material is (approximately) a linear dialectic over the frequency range that you care about. Turns out water is a linear dielectric over precisely the range of frequencies our eyes can detect. coincidence?

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    $\begingroup$ Intresting comment. Not the best answer but intresting. Are you suggesting this is caused by evolution ? $\endgroup$
    – mick
    Jan 21, 2013 at 21:13
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    $\begingroup$ @mick it might be one factor in the evolution but there are others. The visible range is also in the peak of the Sun radiation (which is clearly an independent phenomenon from the water light absorption but may have also impacted evolution) $\endgroup$
    – akhmed
    Jan 26, 2017 at 22:17
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There is no absorption and re-emission process when light travels in a transparent medium. Medium does absorb some portion of the light, but no re-emission happens, or re-emission is so small that it can be neglected.

The absorption doesn't make electrons to transit to higher energy levels, but to increase their kinetic random movement, including atom's random vibration. This results in increased temperature of the glass.

Some fluorescent substances absorb visible light and re-emit it. When this happens, after the visible light is turned off, the matter has its own light emitted for a while. This involves electron energy levels transition.

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This answer is a little circular and like Burley's. Transparent materials have uniform electromagnetic coupling between its molecules. Think of glass as a uniform array of tiny capacitors.

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