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UV Light Penetration

  1. May 12, 2015 #1
    For a photopolymer, the heuristic goes that longer wavelengths of UV light penetrate deeper than shorter wavelengths of UV light. For example, the UVA spectrum penetrates more deeply into the photopolymer than the UVC spectrum. What is the best way for this to be explained?
  2. jcsd
  3. May 14, 2015 #2
    This question has already been addressed here:

    The answers given are very technical, however, and I am not as clever as many here, so I am very prepared to be corrected if I am wrong. Some years ago, I read Richard Feynman's book QED. If I understood correctly, as photons pass through a medium, such as glass, some will strike a particle within the material and be reflected, and some will pass through without striking anything. This is why we can see a reflection in glass as well as seeing through it.

    Now, when its wavelength is longer, a photon has a greater probability of passing through without striking anything. This could be illustrated by a skier on a slalom course. If he is skiing with a "long wavelength", for example, if he only has to turn at every fourth flag, he would be less likely to collide with one within a given distance than he would if he was skiing with a "short wavelength", as when he must make a turn around every flag.

    The case with a polymer would be different to glass, in that it is not as transparent to photons. Nevertheless, there is still a greater probability of the photon avoiding a collision for longer if its wavelength is longer.
  4. May 25, 2015 #3


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    UV light's "penetration" by wavelength is very material specific and without clear details about the photo-polymers chemical structure (polymers) and composition (additives), no more can be said.
  5. May 25, 2015 #4


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    What is needed here is the phonon spectrum of the material.

    The physics of light transport in a material is not trivial, and it is dealt with in solid state/condensed matter physics. In fact, the use of optical spectra in the study of material is a well-established area of study within this field of physics. Many optical diagnostics such as Raman, UV-VIS, FTIR, etc. use the properties of light transport in solids to extra information such as the phonon structure of the material. The optical mode of the phonon structure, for example, greatly influences light transport in that material, often producing significant characteristics such as transparency, penetration depth, absorption bandwidth, etc.

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