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Questions on UV and IR light

  1. Mar 7, 2013 #1
    Hi There

    Am training to be science teacher at secondary school. A few issues have arisen for me lately and would like clarification:

    Issue 1: have been studying electromagnetic spectrum with my year 10 class. We have discussed Herschel's experiment in which he discovered IR radiation by passing sunlight through prism and placing thermometer at the point beyond red light. According to the class' textbook, the thermometer rose more in temperature when placed in the IR area than in any part of the visible spectrum.
    My issue with this: The sun is approximately a blackbody with T_eff roughly 6000K. This means that peak energy emission is in green light. Surely then the thermometer would have the highest increase in energy when placed in the green part of the spectrum? Is this a case of the school textbook oversimplifying or am I missing something?

    Issue 2: relates to UV light. I bought UV lights (LED I think) for my class. When we use them, there is a significant amount of blue light emitted. Why is there so much blue light? Is it because the glass covering over the LED is slightly fluorescent and so some UV gets degraded to blue light as it passes through glass? Or is the bandgap in the LED such that it covers a large enough range of wavelengths so that it goes from the bottom end of blue into ultraviolet... ? Note, the package says the wavelength range of the pen is something like 380-360nm... (These figures are from memory so might be a little off). Assuming figures are correct though, it is fair to assume that the blue light that can be seen is roughly 380nm (although the standard figure for vision cutoff is quoted as 400nm!).
    Also, if you look at UV lights used for disinfecting water and sewage, it can be seen that there is quite a bit of visible light coming off these as well! Again my question is: why isn't it possible to have lights that emit solely in UV - why is there this fluorescence even when the purpose is not for illumination? Is it simply a safety thing - a way to let people know when the lights are on or off? Or is it a technical difficulty with producing lights that emit solely in UV?

    Apologies if these points are overly verbose!
  2. jcsd
  3. Mar 7, 2013 #2


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    What you are missing is the whole point- while any em wave will impart some energy (i.e. heat) it is infra-red that best "excites" the molecules that will cause heat. The actual intensity of the different wave lengths is less important.

    No, light cannot change wavelength like that.

    Yes, no light, or other source of em waves, will produce exactly one wavelength. The "colors" in the visible spectrum that are closed to "ultra-violet" are, of course, violet and blue so there is some "spill over" to those colors.

  4. Mar 7, 2013 #3


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    Also remember that the lower frequency of the IR light will disperse less than the visible light. So you can have a wider range of frequencies packed into the same area, increasing the energy deposited into the thermometer, perhaps enough to beat out green light.
  5. Mar 7, 2013 #4


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    Those were my thoughts exactly.
    Are you looking directly at the LED (which probably is not a good idea!), or are you seeing the light reflected off a sheet of paper or other object? These near-uv wavelengths will make paper fluoresce and give off blue light. I've seen it with uv lasers that really emitted only a single uv wavelength (about 380 nm).
    There is no sharp cutoff wavelength for visible light. I have seen 780-790 nm beams from bright lasers, even though 700 nm is often given as the upper limit for visible wavelengths.

    Google human eye sensitivity table to see how the human eye really responds to different wavelengths. It does extend down to 380 nm or so, and in the near uv range drops a factor of 10 for every 20 nm into the uv.

    Again, it sounds like that is fluorescence of the material being lit with the uv, not something emitted by the uv source.
    Last edited: Mar 10, 2013
  6. Mar 7, 2013 #5

    Andy Resnick

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    Possibly- while it is true that the solar spectrum peaks at 550nm (or so), the absorption of 'the thermometer' (whatever that means- glass, fluid, etc) is greater in the IR, and as HallsofIvy points out, it's the rate of energy absorption that drives the change in temperature.

    If the source is an LED (I see here are some 395nm ones out there), the output is fairly broad spectrum and does indeed reach into the blue. Hg arcs, on the other hand- there are penlights that have them as UV wavelength standards- have UV spectral peaks that are eye hazards. I urge you to check the specs and be careful about looking directly at any source that you suspect has appreciable UV output.
  7. Mar 9, 2013 #6
    Thanks for the replies... I'm satisfied with the explanations for my IR query! but the UV one is still a little off I feel!
    For example just to address the comments of hallsofivy:

    and Redbelly98

    I am not seeing it off a sheet of paper.. I am seeing it coming off the bulb area (although I am looking at it from an oblique angle so I doubt there is risk to vision (also I do have not spent much time doing this!)).
    I have attached a few images of what I mean... so my question remains - why is there this level of visible light coming off of these uv lights when their purpose is for disinfection NOT illumination. Is it a technical difficulty with generating pure UV? is it perhaps that the water or glass is slightly flourescent?

    One final point about the images I have attached: I am aware there is a chance that the camera sensor used might be sensitive to these UV wavelengths (filter band dependent) so that this UV might appear on photograph but would not necessarily be seen if looking at these lights directly - yet this doesn't explain the blue light I can see when looking directly at my own UV lgihts!

    My own feeling at this point is that it is to do with the fact that we do have limited sensitivity going into the <400nm region as stated by redbelly.

    Please give any further comments that you think are relevant.

    Attached Files:

  8. Mar 10, 2013 #7


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    Okay, the images help -- but at this point I'm just taking an educated guess as to why the light appears blue in them.

    In the 3rd image, it looks like the actual UV source is either (A) inside the opaque housing somewhere, and they are using a glass or transparent plastic extension as a "light guide" of some kind; or (B) a mercury lamp, and the glass extension forms the discharge path. My best guess is that either the transparent part is exhibiting fluorescence, or the light source might include some blue light as a safety feature, to allow the user to see clearly when the thing is turned on. Just my best guess here.

    Hard to tell with the water treatment photos without knowing what the actual uv source is. For example, halogen lamps can emit significant uv and visible (they're incandescent, with a broad spectrum), if they use a glass that transmits the uv. Or there might be fluorescence going on with material in the water (green in your photos) or the concrete walls (that appear blue).

    I'll just add from my own experience that near uv does appear violet. I worked for a couple of years with 380 nm uv lasers. Normally I could not see the beam at all, due to my glasses not transmitting the uv. But if I took my glasses off, I could see a faint, blurry, violet blob on the mirrors where the beam was striking. I assume it was blurry due to chromatic aberration in my eyes' lenses -- it was much blurrier than normal objects viewed without my glasses on. But it was definitely violet in appearance.
  9. Mar 11, 2013 #8
    The UV LED is probably not very monochromatic. It's output spectrum may spill into the visible spectrum. Check the manufacturer's data sheet to see if they have a spectral graph for the device. That may answer your question.
  10. Mar 11, 2013 #9


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    This question:
    Got this response:
    which could be confusing. Flourescence is a well known phenomenon and is used to produce optical light from UV in 'flourescent' tubes. However, you do need special rare earth phosphors to achieve this and I don't think it happens at all in ordinary glass. Indeed, the manufacturers would deliberately choose another form of glass if it was actually happening in their UV diodes.
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