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A Transparency, colours and heating of materials

  1. Jun 3, 2016 #1
    I know it is many threads here about this topic and I've read most of them. But I have next questions, next step :)

    1) (Summary) Does it true that composition and structure drive an energy levels of electron and it absorbs such wavelengths which correspond to jump to upper (not implicitly nearest) level and such wavelength is than missing in spectrum and results to different colour?

    2) from 1) is for me understandable if and only if it is translucent and we are looking through (photon which come to my eye come through material) but how it works, when material is opaque? How is possible that photon is reflected from material back to my eye? Or why it reflecting only some wavelength (reason of colours)?

    3) form 1) if some photon is absorbed and excite some electron. What happens than with this "system"? I suppose it relaxes in short time and release photon of same energy in random direction. And than? It excite another electron? Or what?

    4) How is connected absorbing of photons (colours) and heating of material? I think that heating means stronger vibration of atoms or molecules (in lattice if it is such material). Is it true? But if photons only excite electrons how is it possible? Black coloured materials are heating more and absorbs more wavelength so I guess it is connected. True?

    Thank you for your answers.
  2. jcsd
  3. Jun 3, 2016 #2


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    If you have been reading many PF threads, you will have read that the Wave description of light is often far better for explaining many things about light than explanations using photons. It is not any 'better' to launch into explanations involving photons. Photons are not to be treated like little bullets and, in fact, can mostly, only be dealt with when EM waves actually interact with matter. Waves (and even Rays) can give a much better idea of what is going on.
    The basic way that EM waves interact with individual and well separated atoms and molecules (in a low pressure gas) is how we first learn about QM. Once we are dealing with 'condensed matter' the interaction is with the whole structure and things can be very different. A single molecule will interact with only a few different EM wave frequencies and it will (may) absorb a photon and re-emit it in a random direction at a random time. The effect is referred to as Scattering. Gases do not absorb strongly because they are so dispersed. Even many km of atmosphere are virtually transparent and only scatter a small fraction of visible light to produce 'red' sunsets and 'blue' sky. (Actually reddish and bluish)
    What happens in solids and liquids is much more complicated and does not involve single molecules. The whole structure is involved and some solids and liquids let light through with very little absorption. They just slow the light down and make it change direction (refraction). Adding some other components (dues and pigments) can cause absorption of all or selected visible wavelengths and that can cause colouration or just attenuation.
    This is best explained using the wave model (as is most optics, actually). When any wave meets an interface between two different media (with different refractive indices) some of the wave energy is reflected and fails to get through the interface. This is what you 'see' because it bounces off but you won't see any light that enters it because it is absorbed.
    This is also very complicated. Take a piece of 'frosted' black glass and it may look white. Polish it to a deep shine and it will look perfectly black as long as you avoid the 'specular' reflections from a light source.
    More extra questions here, than answers, I'm afraid. Just keep reading around the subject - Wiki has loads of worthwhile stuff. But you have to read the same idea in several different sources before you can be sure it's 'pukka'. There is a lot of nonsense in some places!! :wink:
  4. Jun 4, 2016 #3
    I have read probably 3 or 4 here and 2 from other server and unfortunately there was few nonsenses and I was totally confused. Thank you for guiding me to good way of understanding. I don't know why a was still thinking classically.
    So can I have another questions?
    1) Why it slow down? If some part is not absorbed it seems for me it shouldn't be affected but obviously it is. And similar question is how is possible that all beams (of same frequency) change direction in same direction? I would expect some randomness.
    I know I want probably found out deep base of Fermat principle but I like ask and maybe someone will know at least some part of it :)
    2) And what about connection absorption of some wavelength and heating up? Is temperature (and change of it) movement of atoms and molecules (change of kinetic energy)?

    Thank you again ;)
  5. Jun 4, 2016 #4


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    Good question. Imagine a totally different kind of wave - on a long row of masses, joined by springs. one end is moved in and out to produce a wave, travelling along the line of masses. If you ignore friction, the wave energy will not get less along the row. Now imagine replacing the second half of the row of masses with larger masses. The wave will go slower but not lose any of its energy. So speed and energy loss need not be related.
    EM waves do not travel in the same way but, as they pass through a substance, the interaction with the structure can 'load' the progress of the wave and slow it down. Whether or not the substance absorbs the energy is a different matter.
  6. Jun 4, 2016 #5
    I hope I understand well. So if we have aquarium of half water and half oil (suppose some thin elastic barrier as bag) and produce a waves on the side of water the will waves on the side of oil propagate slowly? Is it approximately good example? That energy stored away in wave is conserved but (I hope it IS slower :) ) propagation in denser liquid is slower?
  7. Jun 4, 2016 #6


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    My example is only a partial one (of course). It is not only density that counts. The stiffness of those springs will also have an effect on the mechanical wave speed.
    For an EM wave, the relevant factors are different but often, more dense will mean slower. Lead crystal glass bends light more than plain silicate glass.
  8. Jun 4, 2016 #7
    Ok, thank you for your answers, it was for me very helpful :wink:
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