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Can fire produce shadow?

  1. Sep 14, 2008 #1
    Hi. So my friend thinks he's smart, and one time, while we were talking, he asked me whether fire can produce shadow. Of course he does not know the answer, but knowing him, he asked me this question to show that I am not smarter than him. Nevertheless, his question started bothering me, because generally everyone knows that in order to have a shadow you need a source of light and an opaque object. I know how broad of a term a fire is because of its variation, considering different gases and light it produces that may either be visible or non visible. I know that there may not be any specific answer to it, so treat my question as some kind of a challenge. I know that a lot of people who visit this forum are much more knowledgeable than me when it come to physics and chemistry, and this is why I am posting this question. Feel free to discuss different kinds of flames, gases, spectrum, and all that stuff. Feel free to give me your honest opinion as well - as long as it has some physics behind it.
    So tell me: DOES FIRE PRODUCE SHADOW?
     
    Last edited by a moderator: Sep 24, 2015
  2. jcsd
  3. Sep 14, 2008 #2
  4. Sep 14, 2008 #3

    Borek

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    Do the simplest experiment - light a match in a dark room. Are you casting shadow or not?

    Edit: I just realized I probably misunderstood your question.

    No problem. Light a match in a dark room and take a picture with a flash. Does fire cast a shadow, or not?
     
    Last edited by a moderator: Sep 24, 2015
  5. Sep 14, 2008 #4

    Redbelly98

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    Or light a match in a dark room and shine a flashlight across the flame. Look for a shadow.
     
  6. Sep 14, 2008 #5
    If the light source other than the flame is brighter, then yes. Fire isn't transparent.
     
  7. Sep 14, 2008 #6
    Fire is mostly transparent so there is only a very weak or no shadow.
    You can see how transparent it is if you have a gas stove. Wait for a bright sunny day and when the sun is shining through the window onto the stove, turn it on and look at the flame. The flame is nearly invisible but it will cast a weak shadow.
     
  8. Sep 14, 2008 #7
    probably not for the same reason that other things produce a shadow. the heat bends the path of the light passing through.

    smoke of course does produce a normal shadow but I'm sure thats not what your friend was asking about.
     
  9. Sep 14, 2008 #8
    The shadow is more easily seen with the proper band-pass filter. Isn't this what dark band spectroscopy is about?
     
  10. Sep 14, 2008 #9
    I think you really need to better define as what you classify as "fire". What is giving you the shadow is not the radiation or light produced but the gasses that are emitting it. Also any smoke or other products from the reaction would cause a shadow as well. In order for a shadow to be cast, light from an external source must be absorbed.
     
  11. Sep 15, 2008 #10
    You're right. I did not clearly classify fire and this makes the whole thing a more complex. I guess it is necessary to make a distinction among different kinds of substances (our fuel) that undergo combustion reaction that causes visible flame to occur.
    Let's take it to another level. Is it possible to produce a clear flame? I do not mean visibly clear, but physically clear, pure heat. And would that produce shadow also? Someone said that heat bends the path of light as it passes through a flame, and I am not saying that this person is wrong but this "bending of light" obviously does not give me the shadow I want.
    From my own experience I can tell that during the days of extreme heat, you can notice little shade looking areas on surfaces like sand for example, produced by "waves of heat" or simply hot air. I have to admit that I am getting into some complex stuff involving whole lot of chemistry and physics and know that there is no precise answer to my question, but why not discuss it a little.
    SO TRY TO TELL ME HOW CAN I PRODUCE THE CLEAREST FLAME POSSIBLE AND HOW TRANSPARENT WOULD THAT FLAME BE? MAYBE THERE WILL BE AN INVISIBLE SHADOW... MAYBE THERE WILL BE NO SHADOW.

    For now, thanks a lot for contributing to the yet small discussion. Ask your friends, teachers, anyone who may know something or would be willing to do some experimenting.
     
    Last edited by a moderator: Sep 24, 2015
  12. Sep 15, 2008 #11

    Borek

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    Hydrogen flame is practically invisible, sooty flames (like burning benzene) contain a lot of carbon particles that should cast a shadow. Bending occurs due to density changes of the air.

    [​IMG]
     
  13. Sep 15, 2008 #12
    So, will hydrogen flame cast a shadow? And ok, hydrogen flame may be practically invisible, but I suppose it changes the gaseous structure of the air - meaning gases in hydrogen flame (their amounts and behavior) are different than gases in the air at room temperature. Even though it may look invisible and transparent, is it possible that some particles present in hydrogen flame will absorb light or somehow block the light path. I know that it will bend it, but will it actually absorb any light?
     
  14. Sep 15, 2008 #13

    Borek

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    There are no particles in hydrogen flame, that's why it is invisible. Most flames are visible because they contain hot soot which behaves like a black body - and emits light.
     
  15. Sep 16, 2008 #14
    I've seen the shadow of a candle flame in direct sunlight... It was mentioned in a 1950's book. Some of it might have been refraction of light in the much hotter flame as compared to the surrounding atmosphere.
     
  16. Jan 9, 2009 #15
    no,a fire doesn't cast a shadow!
    first,for a shadow,you don't need a light source and an opac object...you simply need not to have photons in an area(or at least very very phew of them,or if you do have a source of light,block it with an object...if you turn of the lights at night,you will have one big shadow(except maybe a phew LEDs)
    and as long as fire does produce light(or at least most of the time) FIRE its self can't produce shadows...but:
    take a sheet of paper,maybe it's like a magician's hat,and burn it...you will notice that the fire lifts small particles of burned paper,witch produces shadows...in the same way,when you burn the candle in the image,the shadow is produced by the debris that was lifted,witch is so small,that it doesn't even block the light properly...
    also,I remember a while ago I was playing with some candles,and found out something very curious:when you stop the fire,this strange "gas" starts to come out for a couple of seconds...if you ignite that smoke,it will rapidly "follow the line" and reignite the candle...why?
     
    Last edited by a moderator: Sep 24, 2015
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