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Change EM wavelength

  1. Dec 16, 2008 #1
    Alright first time poster here with a question that has been eating at me for a while. I am curious to know if there is a substance that when one part of the EM spectrum is passed through it, it would come out as another. I know that if you are to shine a lightbulb on some black cloth you would get infra-red radiation, but could you turn that infra-red into red or back into all the visible light spectrum?
  2. jcsd
  3. Dec 16, 2008 #2
    The emission radiation always has lower energy than the excitation, so the answer is NO.
  4. Dec 17, 2008 #3
    With your example of a cloth it will be difficult to go "backwards", but in certain situations you can get visible light "back" out of black objects. For example, if you heat an iron stove to a certain temperature it will start to glow "red" and if you heat it much hotter it will start to glow "white", etc. The key idea here is you would have to raise the energy of the black object high enough so that as the atoms inside the material relax, they emit visible light. Only materials that can withstand large temperature increases can do this...
  5. Dec 17, 2008 #4
    Hi Renge Ishyo,

    I wonder how you can heat the iron stove up? Can you use only IR radiation to heat it to an extent that it glows?
  6. Dec 17, 2008 #5

    Andy Resnick

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    The principle of conservation of energy implies that a passive material (i.e. no energy supplied to it) can only keep the wavelength the same, or re-emit the light at a longer wavelength- fluorescence and phosphorescence, for example. Your example of heating up a cloth with a lightbulb stretches the analogy becasue that is an incoherent process, but is basically correct.

    Certain non-linear materials are able to upconvert light to shorter wavelengths, but this is done by (essentially) absorbing 2 photons at low energy and emitting 1 photon at a shorter wavelength (harmonic generation), so the principle of conservation of energy is still obeyed.
  7. Dec 17, 2008 #6
    You can plug the stove into a wall socket for one. The emission spectrum is a function only of the temperature in the ideal case of a blackbody, and you can raise the temperature of a system by doing work on it or transferring heat to it. IR radiation can transfer heat to the extent that it glows providing that the system is confined and well insulated to prevent heat loss so that the temperature of the system doesn't decrease faster than you can transfer heat to it. You aren't going to get out the same amount of "visible" radiation that you put in, but when the temperature rises you can get it to glow eventually.

    The original poster was thinking along these lines, but the problem is the light bulb he is shining on the cloth is transferring energy at such a slow rate that it is dissipating to the surroundings as heat long before it can concentrate and raise the temperature (which is needed to get out the visible light).
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