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Transparency of water, glass and plastic materials

  1. Sep 22, 2009 #1
    What causes water and other similar transparent things to become transparent? Visible light can pass through transparent solid object like glass but can't pass through many opaque objects. What is the reason?
    Again when liquid water turns into ice it loses its transparency. I'll be very thankful if anyone explains me which factors control the property of transparency of any material and how.
  2. jcsd
  3. Sep 22, 2009 #2


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    If you freeze pure water carefully without introducing air bubbles, it's very transparent.
  4. Sep 22, 2009 #3
    transparent materials have electrons that do not absorp light at visible wavelength. Water are not transparent with infrared.
  5. Sep 22, 2009 #4


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    I don't know you've noticed, but water isn't entirely 'transparent'. Water is faintly blue.
  6. Sep 22, 2009 #5


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    And window glass is not transparent to ultraviolet.
  7. Sep 22, 2009 #6
    well, then transparency must depend on the interaction between electron and photon. If electrons of the material do not absorb the photons, it becomes transparent. If they absorb photons, it is opaque.
    Now my question is that all matter including transparent and opaque have electrons. Electrons of a transparent material are the same species of the electrons of an opaque material. Then why do they show different behaviors to the photons i.e. electrons (in transparent objects) absorb photons and electrons (in opaque objects) do not absorb photons.
    I want to know WHAT is the factor that compels these electrons to behave differently with the photons. Is it molecular structure, or atomic number, or refractive index bla bla bla.. which one is responsible for the transparency of the object? Please expert physicists help me in this regard. If possible elaborate your explanation and explain it from subatomic viewpoint. I'll be really grateful to you guys.
  8. Sep 22, 2009 #7
    It's the chemical composition of the material. First off energy of electrons is quantized meaning that they can only have certain energy levels depending on the chemical structure.

    So you have photons which hit the material, and if they are energetic enough the photon will raise an electrons energy. When that electron drops back down to a lower energy state, it's quantized so it has to fall to some discrete value, and when the electron does return to a lower energy state it releases a photon with the energy equal to the change in the electron's energy.

    The color of light depends on the energy of the photon. Which ranges over the entire spectrum and not just visible light (the light we can perceive).

    So then a material is transparent/translucent/opaque based on the discrete energy values the electrons can take due to the chemical composition, which effects the energy of the photons, which effects the particular part of the spectrum of light.
  9. Sep 22, 2009 #8
    Okay, I don't understand why no one knows this, it's not a bad question but the answers given were CRAP.

    I will start with a material having color (or black). This is the property of mater to absorb some spectrum of light, but reflect others. The reason this happens is due to the inherent frequency of the electrons in the material. When a material is and black and opaque it is because the materials electrons will pretty freely vibrate (at a broad spectrum of frequencies) so when the photons energy comes in the material just absorbs that energy in to its many electrons.. this is why black materials get hot in light.

    A transparent material on the other hand has generally a very narrow band of electron frequency (but doesn't have to be it could just be a very different wide band of frequencies (ie. transparent to light but opaque to radio waves)), and the light coming in is of a very different frequency and that energy does not get trapped in an electron around an atom. think of this like a resonant frequency with sounds, a material with a very different resonant frequency will allow the sound to pass right through it virtually unchanged (except speed). where as an object with a near identical resonant frequency will begin to vibrate, or even heat up. This property can be more visually demonstrated with kinetic balls, the energy is passed straight through the middle balls to the outside one where the frequency of energy matches the mass for movement. (this is not a perfect analogy... the sound analogy is much better)

    color is produced when a selection of frequencies can't be absorbed (photons at the same frequency as the electrons in the material will be absorbed and easily passed on to the next electron as thermal energy) but energy at a different frequency can not be absorbed in to the matter any further than the first electron it encounters and is thus re-emitted. Because the two frequencies or energy levels are to close together, the photon can't just pass straight through. without being effected by the electrons. (Think of this like a swing, as long as your push at the right frequency the swing will gain energy, push at the wrong frequency and you will encounter resistance from the swing, it will push back (reflection), and it won't change the frequency of the swing) (although you can decrease total energy in the swing which is the same principal as how laser cooling works)

    the quality of the image coming out (mirror versus opaque or transparent versus translucent) has to do with the surface structure of the material. If the incoming light is hitting a very rough surface the parallel lights angle of incidence can be different nanometers apart and this means the angle of reflections is different meaning the image gets completely distorted. if it is a very smooth surface and the all incoming parallel light has very near the same angle of incidence than the light is reflected in a coherent way keeping the image intact.
  10. Sep 22, 2009 #9


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    Twasnow has pretty much covered the area, but maybe I can add some elements.

    As has been pointed out, the key is resonant frequency of electrons in their molecular orbitals.
    A large class of pigments has a molecular structure that involves one or more benzene rings. If memory serves me then 'alanine blue' is such a pigment. In any case, one of the first pigments to be manufactured on industrial scale was a blue pigment, which used to be laboriously extracted from plants, making it very expensive.

    So I think you'll find more information when you look up the molecular structure of synthetic pigments.

  11. Sep 22, 2009 #10
    Hi twasnow.

    Expanding on your explanation... I think it relates somewhat to electron standing wave frequencies that are present at the interface of the material; but I'm not sure if our science attributes electrons as having different frequencies so although I would tend to agree I'm not sure if science does? This is the first time I've heard it expressed your way.

    Given enough photons does the material/electrons begin to not be able to take up the photons and is it by this process that you start to get mass reflection due to overload? Is that how black - and other colours - can be made to look lighter in colour when too much light is shone on them? Is that correct or is there some other scientific explanation?

    I like this but I don't know if science does?

    I think science says things the opposite way round when it comes to which photons are absorbed by the electrons and which photons add vibration (heat) to the material itself. I think that science says that photons that have the right energy to excite a met electron will be absorbed and those that don't will add to the vibration of the material temporarily and will themselves slow down temporarily (or something; I never quite got to the bottom of that) transferring the energy that way. Hence why photons take longer to pass through denser transparent materials than through space. That's what I got from the FAQs. Is my interpretation wrong?

    I think there is always a certain amount of scattering and absorption isn't there even on highly smooth surfaces?
    Is also generally denser self-aligning materials which don't absorb too much in the visible range and which aren't allowed to oxidise (glass coated) that will tend to reflect better for us isn't it eg silvery metals? I mean this agrees with what you are saying but I wonder if the distance between atoms and arrangement doesn't also play some part in reflection or alternate translucence/transparency?
  12. Oct 12, 2009 #11
    If the transparency is dependent on the chemical structure then can u explain why PMMA is transparent while polyethylene (which is also a hydrocarbon polymer) is opaque? is there any other reason other than the electron-photon interaction. I mean can anyone give a better explanation.
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