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Infrared light and Heat

  1. Nov 22, 2007 #1
    I was reading an article about lasers, in specific about CO2 lasers, and it had a sentence like this:
    "The reason that the CO2 laser is so dangerous is because it emits laser light in the infrared and microwave region of the spectrum. Infrared radiation is heat, and this laser basically melts through whatever it is focused upon."

    what confused me was that "infrared radiation is heat"... i don't exactly understand how you could say that... I mean, IR light is just an EM wave, and heat is basically a measure of kinetic energy. Why wouldn't any other laser frequency work for the melting? what is so special about IR? I heard the two connected many times now, but I'm unclear about the connection.

    could anyone try to explain?
    thank you,
  2. jcsd
  3. Nov 22, 2007 #2
    You're basically right, nothing special about IR (perhaps some materials absorb it especially well, not sure), and it isn't technically correct to say the radiation from a IR laser is heat.

    But the thermal radiation from everyday objects is mostly IR, which explains the statement you saw (especially since someone along the line may not have been completely on top of the physics, or explaining for a lay-audience). When you warm yourself in front of a radiator, that's IR.

    Generally, CO2 lasers would be dangerous because they are both extremely powerful and invisible.
    Last edited: Nov 22, 2007
  4. Nov 22, 2007 #3


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    I think that is the key point that they were trying to say about it melting through stuff. If all of the energy is absorbed and none reflected then a laser can heat an object really fast. That said, the article is really wrong. The spectrum produced by a laser is a very sharp peak and would look nothing like a blackbody spectrum at any temperature.
  5. Nov 22, 2007 #4


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    Technically, the mechanism of heat transfer used by radiators is convection, and not radiation.
  6. Nov 22, 2007 #5


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    That is a poor choice of wording. However, there is a reason for all this.

    There is a connection that I address below.

    Actually there is!

    This is the key point. Vibrational and rotational modes of most molecular and/or organic matter are in the IR (and for lighter molecules, the microwave) regime. As a result, the absorption coefficient of such materials is much higher in the IR wavelengths than in others. To pick a somewhat poor example, the absorption coefficient of water in the far-IR is about 103 times higher than the mean absorption coefficient in the visible range. Its peak absorptivity is in the microwave range (hence the microwave oven), where it is over 105 times greater than in the visible range.

    PS: Mephisto, when quoting from an article, do include a complete citation of the source.
    Last edited: Nov 22, 2007
  7. Nov 22, 2007 #6

    Claude Bile

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    This is a really misleading statement. The danger of CO2 lasers is in their sheer power output, not the fact that the wavelength of a CO2 laser happens to coincide with some significant thermal property of something (other than that the wavelength is invisible). Even a "low" power CO2 laser can belt out about 20 Watts plugged into a standard electrical wall socket, so yes it does melt through a lot of things (it mostly burns from experience), but then a 20W laser of any wavelength will melt through (or burn) just about anything.

    CO2 lasers are often described as "heat" sources because generally, that is all they are useful for - heating things up in a controlled fashion, and are frequently used in welding and cutting applications. CO2 lasers are used instead of other wavelengths because they have excellent power efficiency, around 30%. Power efficiency in lasers drops roughly as 1/f^3, for visible wavelengths, efficiency is less than 1%.

  8. Nov 24, 2007 #7
    Thank you for all replies! The quote, by the way, is from www.howstuffworks.com, from an article called "How Lasers Work", found here: website:http://science.howstuffworks.com/laser8.htm

    I found a graph of the absorption coefficient as a function of frequency for water on britannica as well:

    I was expecting to see a huge peak at microwave part because microwaves are used to heat up our food.
    I was also expecting to see a peak at IR region because we are mostly made up of water, and heat lamps (IR) warm us up, so apparently we absorb it well too.

    Instead though, there is a bigger peak at IR(~10^14Hz) than at microwave (~10^11Hz), which is interesting - why aren't we using IR in microwaves instead of microwaves to heat our food? Water absorbs IR better!
    Also, why is there even a bigger peak right after visible light? That's in UV range. Why aren't we using that for heating our food as well? Wouldn't that be Much better and faster? Also, UV has shorter wavelengths, so we wouldn't have any "heat spots" that are created in microwaves. The only reason I can think of is safety concerns because UV is ionizing?

    Also, how come that visible light is EXACTLY in that huge gap of absorption for water? Is this a coincidence? That's very weird!
    Last edited: Nov 24, 2007
  9. Nov 24, 2007 #8
    "Visible" Light and Water Absorbtion Spectra

    "Also, how come that visible light is EXACTLY in that huge gap of absorption for water? Is this a coincidence? That's very weird!"

    It is widely thought that our eye's structures evolved to their present basic functionality while vertebrate life still existed solely in the oceans. To maximize visibility, the eye's cones evolved to be sensitive to the light which passed most easily through the ocean water above, hence "visible light" is indeed by biological definition, "light which is less easily absorbed by water". Good insight.
  10. Nov 24, 2007 #9


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    UV is so strongly absorbed that you would flash burn the outer thin layer of the food before putting any heat into the middle.
    A similair argument also applies for microwave over IR but it is also easy to efficently generate microwaves in an oven.
  11. Nov 24, 2007 #10
    I'd like to point out that we do use IR spectrum heat to warm our food - in every oven that isn't a microwave :)
  12. Nov 25, 2007 #11

    Claude Bile

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    CO2 lasers have great power efficiency, but they still need a high-voltage head and water cooling to work, two engineering factors that largely prohibit the use of these lasers in consumer goods.

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