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How light rays from a fireplace wood fire on Earth compare to the Sun's fire?

  1. Jan 5, 2018 #1
    While lying in front of a 500 degree woodfire, I wonder if besides light and heat that we take as ordinary does it also have ultraviolet rays of light like the sun?? What are the types from light from an ordinary on earth fire?? Would they be beneficial to plants as main light source?, would plants grow by a large firelight if far enough away from the heat?
  2. jcsd
  3. Jan 5, 2018 #2
    Fire in general produces red light and infrared. which we perceive as heat.
    Plants do make use of red light and also blue, but most plants don't use the intermediate wavelengths. it is reflected;
    That is why we see plants as green,
  4. Jan 5, 2018 #3


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    Unless you're burning some strange stuff, typical fireplace or candle flame emits predominantly in the red end of the visible spectrum. This graphs compares spectra of the Sun and candle light (and an incadesent lightbulb):
    (source: https://osa.magnet.fsu.edu/teachersparents/articles/sourcesoflight.html)
    As you can see, the peak intensity is somewhere farther down the infrared regions.

    Plants absorb light across the entire visible spectrum, with a slight efficiency dip in the green region.
    This means that plants benefit very little from flame light, as it's only providing the red part of the spectrum, and even that at much lower intensity than sunlight.
    One could try and increase intensity of the flame light (by putting a plant closer to the fire) so as to provide more energy for the plant to absorb, but due to the excess radiation in the infrared region, it'd also mean cooking the plant.

    The extracted pigment does not absorb well in the green region, but when inside the overall structure of a leaf, the absorption is quite good:
    More on this here:
    https://www.heliospectra.com/sites/default/files/general/What light do plants need_5.pdf
  5. Jan 5, 2018 #4
  6. Jan 6, 2018 #5


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    If it is an ornamental plant, it may be maintained because it is human selected. There are ltos of examples of human selected plants that are not optimized for the natural world, such as plants that are variegated for having chloroplasts. Some parts of leaves can be white because they don't have chloroplasts. In the wild, this would be a dis-advantage due to its reduced use of light/leaf area.

    What it may be gaining is the human husbandry that keeps it going rather than some metabolic function.
  7. Jan 6, 2018 #6


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    Ohhh and the sun ISNT on fire :wink:

    and there is a huge temperature difference
    your wood fire ~ 500 C
    the surface of the Sun ~ 6000 C

    this results in the very different wavelengths of Electromagnetic radiation
  8. Jan 6, 2018 #7
    Thank you so much -all of you who have replied to my question with much information about plant absorption of light and also the chart comparing sun light to a candle and tungsten lamp and their wave lengths ! That is very helpful :)
    But no one said anything about ultraviolet?? I do not know what wavelength it is and if a huge stoveful of red glowing coals emits any? (It is at 500 and 600 F not C) and I get very deeply comfortably hot but don't seem to get a sunburn so I guess no ultraviolet. Is it only the ultraviolet that causes sunburn on people or sunscald on plants or is it the heat also...??
    Of course the sun has all kinds of electric magnetic radiation which I dont quite understand except it is like a big nuclear power plant that has caught fire..but with alot more materials for its use. But it sure looks like its burning when I see pictures of solar flares :)
  9. Jan 6, 2018 #8
    The process going on within the Sun is nuclear fusion;
    It produces heat and light with far more energy than a wood fire.
    Start with this.

    Some of the Sun's output is in the ultraviolet range but Earth's atmosphere blocks a lot of it.
    Ultraviolet light in general is something that is not good for the health of living things, though not deadly.
    Last edited: Jan 6, 2018
  10. Jan 7, 2018 #9


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    Ultraviolet has even shorter wavelengths than blue light. The amount of UV from a fire (or glowing coals) is completely negligible.

    Classical sunburn is a reaction to UV. You can get actual burns from very intense visible light or infrared, but for that you would have to focus the sunlight with a lens on your skin or do similarly stupid things.
  11. Jan 7, 2018 #10


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    UV starts roughly below 400 nm, so just to the left of where the graphs end. As you can see, the amount of light a candle emits at 400 nm is tiny as compared to red light (around 700 nm). Further down to the left, in the UV region, the graph keeps going down, while further right (into the infrared - which heats you up) it goes up for a good while more.
    Candlelight and coal burn in comparable manner.
  12. Jan 7, 2018 #11
    Thank you all again for taking the time to further and more completely answering my question. I dont think I could have found the answer anywhere else but this forum!! :)
  13. Jan 11, 2018 #12
    Do 'rare earth' gas-lamp mantles emit non-trivial UV ? I'm fairly sure basic 'lime light' does not, but my google-fu has failed me, I cannot find their spectra...
  14. Jan 15, 2018 #13
    The rare earth metallic salts in gas-lamp mantles have a very low emissivity and therefore do not emit very much infrared radiation. Most of their light is focused in the visible spectrum. However there is some evidence that candoluminescence aids in it's production of light which has a higher intensity at certain wave lengths than expected by incandescence at the same temperature.
    Last edited: Jan 15, 2018
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