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Can a material become transparent with enough light?

  1. Aug 31, 2015 #1
    I don't actually find this probable, it was merely the most concise way of phrasing my concern.

    In the linked description of the mechanism of transparency (), it is stated that opaque objects do not allow light to pass because the incident light is absorbed (as photons, naturally) in the process of promoting electrons to excited states. Great. But if this is the whole story, isn't it conceivable then that so many photons / "so much light" might be continually absorbed such that all available electrons are "maintained" in their excited states? And thus, if an excess of this amount of light is applied, might some photons "slip through" the material without being absorbed?

    I realize an electron cannot spend a large amount of time in an excited state, but will return to the ground state emitting a photon. Is it conceivable that some of these photons might seep through to the other side of the material? Is it possible to "saturate" a material's electronic structure with light, and if not why not?

    Thank you.
  2. jcsd
  3. Aug 31, 2015 #2


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    There may be a more sophisticated answer available, but here's my simple one.
    If you hit an object with enough light - which is electromagnetic radiation, and carries energy - it will heat up and the heat will gradually spread through the object to the other side. Once the other side gets hot enough, it may - depending on the material - start to radiate photons.

    This is easiest to imagine with an iron sheet. Imagine taking an iron sheet quite close to the sun, so it receives an enormous amount of incident light. At first it will block the sun's visible light rays (but not x-rays or higher energy rays) but soon it will start to glow red, then yellow then white. Those photons of red, yellow and white light are in a sense the photons from the sun 'seeping through' the iron sheet.
  4. Sep 1, 2015 #3


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    It is indeed possible to make a material "transparent" by saturating it. This process is sometimes known as "hole burning". In the simplest case the material can be thought of as being made up by a collection to two level systems that can either in the ground or excited state. The basic idea then is to imply to put so much energy into the material that all these two-level systems spend 50% or their time in the excited state (this is the absolute maximum you can get); any "extra" energy beyond this can not be absorbed.

    However, the process is quite subtle and usually restricted to a narrow set up frequencies and it also only work for some absorption mechanisms. Hence, you can't just irradiate any material and expect it to become transparent to a visible light (it will just heat up).
  5. Sep 1, 2015 #4


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    I think you pose a very interesting question PiaB. I'm not a physicist and I can't pretend to understand either of the replies so far, but allowing that they might be correct, could we modify the question to sidestep them?

    To avoid the confusion of photons being absorbed, heating the material and the material emitting new photons to give the illusion of transparency, we could illuminate with monochromatic light. Thermal radiation seems generally to have a broad spectrum, so the emitted photons would not be confused with transmitted photons. (That is if thermal radiation is photons? Perhaps these are waves?) If the illumination were towards the blue end (short waves or high energy photons), then at first the emitted photons would be very different as the material would gradually warm up starting with most of its radiation towards the red end (long waves or low energy photons.)

    I wonder if a very thin layer of opaque material on a thick transparent substrate might be able to dissipate the heat via the substrate, so that you could saturate it without its getting too hot?

    I'm not sure what f95toli is saying - whether opaque materials can or can't become transparent? But if the 50% excitation is a problem, perhaps you could look at some material that is used to make solid state lasers. Presumably that can have more than 50% excitation of its electrons, unless I misunderstand (as I probably do) the concept of population inversion. But then again, I guess these materials are transparent even though they can absorb some of the photons - so not really opaque materials.
  6. Sep 2, 2015 #5


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    To clarify: there are materials (mainly dielectrics) that can become less absorbant (but in most cases not completely transparent) in a narrow frequency range if you "pump" them so hard that they become saturated. This is the best you can do, mainly because there are some mechanisms of absorption that can't be saturated (the material would just heat up). This would for example not work for a metal where there are lots of free electrons.

    Also, if you have a system with two levels (ground and excited state) it will never spend more than 50% of the time in the excited state. This is because any incoming electromagnetic radiation (or thermal energy) that can excite a system (causing the transition ground->excited state) can also de-excite the system (excited-> ground). Hence. if you work through the math you will find that 50%is the maximum (and the system can not absorb any more energy after that). This is why lasers use three levels, this is the only way to cause population inversion where the system spend more than 50% of the time in an excited state.
    Last edited: Sep 2, 2015
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