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Can visible violet light be harmful?

  1. Dec 17, 2011 #1
    If UV-A radation can cause sunburns, could this also be said of visible violet light of the highest frequencies (at least theoretically)? I can't imagine the "cut-off" point between harmful EMR vs non-harmful EMR falls right on the border between visible violet light and UV radiation, which means either the least energetic UV rays are not harmful or the most energetic visible violet rays are. Which is it?
     
  2. jcsd
  3. Dec 17, 2011 #2
    I am not sure if there are agreed upon standards for every color and type of light. I don't think so;In any case there is no sudden line of demarcation, no precise border, between one type of electromagnetic radiation and another unless we happen to define it via some standard.

    If you look here
    http://en.wikipedia.org/wiki/Color_spectrum#Spectral_colors

    you'll see see violet light described as in this wavelength :


    and ultraviolt here

    http://en.wikipedia.org/wiki/UV-A
    described in this wavelength

    So these author(s) seem to have some overlap.
     
  4. Dec 17, 2011 #3
    Harmful to what? Harmful at what intensity levels? If we presume human beings for the former, then to assess the potential for harm we would need to consider a range of factors. To create harm the radiation has to interact in some way with the material it is harming. Potential interactions of EM radiation which may contribute to harm could include scattering or absorption of the radiation, primarily the latter since it results in deposition of energy.

    To be absorbed by some component of an organism, the atomic/molecular structure of the organism has to be such that electrons exist in bound states which have an energy difference from a free electron which is equal to the energy of the photon ( wavelength/frequency are equivalent terms to the energy). The absorption of an EM photon results in the ejection of an electron from these bound states. Harm to the organism generally occurs when the disruption to chemical bonds caused by this process causes some irreversible damage to the biological functioning of the organism (or physical function for inanimate objects) which cannot be repaired by mechanisms within that organism.

    The rate at which such damage may occur depends upon the intensity of the radiation. I.e. the number of photons/square-metre/sec of the radiation which impinge on the organism. At high intensity levels, cellular repair mechanisms can become overwhelmed and radiation which is not harmful at low intensity can become harmful (e.g. exposure to midday sun on a high UV index day compared to exposure in late afternoon when the atmosphere does absorb much of the UV radiation - cloudy).

    The (bio)chemical structure of the organism determines what range of frequencies/wavelengths/energies of radiation a particular organism may be harmed by. Our skin varies between pink and brown or black depending primarily on the amount of the polymer melanin present in our skin which affects the absorption and scattering of energy in specific energy ranges - a protection mechanism since energy is adsorbed more in the outer layers of the skin rather than in the deeper layers where cell division takes place and the potential for damage via changes (mutations) in the DNA is greater. We have evolved with the distribution of EM radiation frequencies at the surface of the earth at the intensity they occur at hence we have evolved protection mechanisms to minimize/prevent harm to us.

    Changes in the distribution of the intensity of radiation with frequency/wavelength/energy can increase the likelihood that harm might result if it is outside the ranges we have evolved with. (an intense white light source can cause pain (a protection mechanism) in our eyes that at a lower intensity is tolerable).

    Lower energy photons generally have a lower likelihood of ionising electrons out of chemical bonds so we can expect as we move from the UV towards the visible, the potential decreases, however when we reach the infra-red - radio wave region the absorption of EM radiation is governed more by the absorption of energy by rotational states of molecules ( principally, but by no means exclusively, water molecules) rather than the ejection of electrons) and the potential for harm increases (sunburn on a cloudy day when the UV content is lower but the IR content is increased by inelastic scattering of electromagnetic radiation).

    There is no simple answer to your general question.
     
  5. Dec 17, 2011 #4

    mathman

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    Short answer - don't look at it if is very high intensity.
     
  6. Dec 17, 2011 #5

    sophiecentaur

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    This link is worth looking at because it shows where the colours sit in the CIE chart of colour space. The only way to define a colour reliably is to give it the appropriate tristimulus values. That brings colour in step with the way we measure everything else, where possible. Using the 'colour words' is just like using clothes sizes XXl,XL,L,M,S,SS; it depends which shop you go to.

    Interestingly, on the lower one of the two CIE diagrams in that link, 'Violet' is defined as a non-spectral colour which can be simulated with a lot of B plus a bit of both R and G. There you go: the Munsell colour system.
     
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