Why does fire have a shadow??
hm...@yahoo.com
I have a not-very-earth-shattering question, but in my mind a very
mysterious physics phenomena. You out there with broader minds than
myself can make a curious student of physics very happy by answering
this question: Why does fire have a shadow?
In a very dark room, I have seen the flame of a stick candle cast a
shadow of itself on the surface immediately below it.
I understand that fire contains particles (mass), whether they be
plasma or gas. However, these particles themselves emit light if I
understand correctly. Why would a light source cast a shadow of itself?
Thanks,
Chloe H.
Autymn D. C.
> I have a not-very-earth-shattering question, but in my mind a very
> mysterious physics phenomena. You out there with broader minds than
Ye
> myself can make a curious student of physics very happy by answering
> this question: Why does fire have a shadow?
>
> In a very dark room, I have seen the flame of a stick candle cast a
> shadow of itself on the surface immediately below it.
>
> I understand that fire contains particles (mass), whether they be
> plasma or gas. However, these particles themselves emit light if I
> understand correctly. Why would a light source cast a shadow of itself?
It could be the wick, Kloeae.
Ian Parker
Basically because air has a refractive index, and a flame looks
something like a concave lens. (remember the flame has a lower
refractive index than the surrounding air.
- Ian Parker
Dirk Van de moortel
<hm...@yahoo.com> wrote in message news:1167116962.3...@79g2000cws.googlegroups.com...
Yes, the particles of the pasma or glowing gas emit light, but
they also *absorb* some of the wavelengths of infalling light.
That's where the (partial) shadow comes from.
Dirk Vdm
Y.Porat
1
fire is gass
and gass is Atoms
2
fire is not homugenous
there are sections in it that are hotter (giving a brighter lite)
and some aprts of it cooler screening the hotetr parts)
so the hotter parts give shadowes of the cooler parts
ATB
Y.Porat
-------------------------------------
Ian Parker
Dirk Van de moortel wrote:
> Yes, the particles of the pasma or glowing gas emit light, but
> they also *absorb* some of the wavelengths of infalling light.
> That's where the (partial) shadow comes from.
>
casts the shadow however. The shadow is that of a gas lens. If that
were true the shadow would be colored and it isn't. A candle is white
(biased towards the red end). Plasmas tend to be descrete lines. You
can have also solid state "plasmas" in LEDs.
- Ian Parker
Edward Green
hm...@yahoo.com wrote:
> Hi all,
> I have a not-very-earth-shattering question, but in my mind a very
> mysterious physics phenomena. You out there with broader minds than
> myself can make a curious student of physics very happy by answering
> this question: Why does fire have a shadow?
>
> In a very dark room, I have seen the flame of a stick candle cast a
> shadow of itself on the surface immediately below it.
Your subject line could be more formative. I thought the simple answer
was going to be "because the flame may be less bright than the light
source behind it", e.g., the sun, "and the flame is opaque". But
that's not what you're talking about. I'm not sure what you're talking
about! I'll have to try the experiment. How large is the shadow? Are
you sure it's not a shadow of the candle?
> I understand that fire contains particles (mass), whether they be
> plasma or gas. However, these particles themselves emit light if I
> understand correctly. Why would a light source cast a shadow of itself?
That sounds logically contradictory, actually. If the sole light
source is "casting a shadow" in a given direction, that means it is
radiating relatively less light through the apposite solid angle. Why?
mumble...mumble...mumble... flame dynamics, lens... mumble...
I don't know.
Dirk Van de moortel
"Ian Parker" <ianpa...@gmail.com> wrote in message news:1167154156.6...@h40g2000cwb.googlegroups.com...
>
> Dirk Van de moortel wrote:
>> Yes, the particles of the pasma or glowing gas emit light, but
>> they also *absorb* some of the wavelengths of infalling light.
>> That's where the (partial) shadow comes from.
>>
> A candle is not plasma it is incandescent carbon.
ok
> That is not what
> casts the shadow however. The shadow is that of a gas lens.
But that is not an explanation. That is a model.
> If that
> were true the shadow would be colored and it isn't.
How do you know it isn't?
The lamp throws yellowish white light. The shadow is
darker, i.e. slightly less yellowish white. That could still
be colored, couldn't it?
> A candle is white
> (biased towards the red end). Plasmas tend to be descrete lines. You
> can have also solid state "plasmas" in LEDs.
http://www.physics.ncsu.edu/pira/7modern/7B11.html
"A grating spectrometer that resolves the sodium d lines is used to
show emission by a salt flame and absorption of white light by the
flame."
If the spectrometer shows absorption lines, surely that means
that less light penetrates the flame, so a shadow is cast, doesn't
it?
Of course a candle flame does not contain sodium, but why
shouldn't it absorb parts of the lamp's light?
Dirk Vdm
hm...@yahoo.com
No, it's not the wick. I blow gently on the flame. The wick is unmoved,
but shadow goes nuts in mimick of the flame.
..........
<Basically because air has a refractive index, and a flame looks
something like a concave lens. (remember the flame has a lower
refractive index than the surrounding air. >
I still don't have it totally pieced together: the light is coming from
the "lens" itself - the light is not striking the lens (unless
reflected from a surrounding wall). Maybe I'm missing something here.
<there are sections in it that are hotter (giving a brighter lite)
and some aprts of it cooler screening the hotetr parts)
so the hotter parts give shadowes of the cooler parts >
Hmm, makes sense.
<Yes, the particles of the pasma or glowing gas emit light, but
they also *absorb* some of the wavelengths of infalling light.
That's where the (partial) shadow comes from. >
Do you mean the light that is reflected back from say, the wall? That
makes some sense. So a flame wouldn't have a shadow in infinite space,
because the light would never return? And its shadow would be more
distinct in a hard-walled room than a room made of pillows?
Is that decent reasoning?
Thanks,
Chloe H.
Autymn D. C.
> No, it's not the wick. I blow gently on the flame. The wick is unmoved,
> but shadow goes nuts in mimick of the flame.
But the shadow must start with a surface of occlusion. The wiggly
flame would make a wiggly diffraction off the wick; diffraction
patterns fan out shadows easily.
> <Yes, the particles of the pasma or glowing gas emit light, but
> they also *absorb* some of the wavelengths of infalling light.
> That's where the (partial) shadow comes from. >
>
> Do you mean the light that is reflected back from say, the wall? That
> makes some sense. So a flame wouldn't have a shadow in infinite space,
> because the light would never return? And its shadow would be more
> distinct in a hard-walled room than a room made of pillows?
>
> Is that decent reasoning?
no, inside the flame
Edward Green
> "Ian Parker" <ianpa...@gmail.com> wrote in message news:1167154156.6...@h40g2000cwb.googlegroups.com...
> >
> > Dirk Van de moortel wrote:
> >> Yes, the particles of the pasma or glowing gas emit light, but
> >> they also *absorb* some of the wavelengths of infalling light.
> >> That's where the (partial) shadow comes from.
> >>
> > A candle is not plasma it is incandescent carbon.
>
> ok
>
> > That is not what
> > casts the shadow however. The shadow is that of a gas lens.
>
> But that is not an explanation. That is a model.
Huh? I guess you mean that is a piece of an explanation, left to the
reader. That's a good tactic for implying vast knowledge and
intimidating inquiry. ;-)
Anyway, he's not talking about a "shadow", except in appearence. He
says this effect is seen in an otherwise darkened room, so this has to
do with the distribution of the light emitted by the flame in angle,
not a shadow. On one level no further "explanation" is necessary
beyond the observation that the flame is not spherically symmetric;
there is no reason to expect it to emit light evenly through a
spherical angle. On a more detailed level, why would emission fall off
noticeably underneath the flame, assuming this is so? Flame structure:
underneath the flame what we mainly see is the less luminous blue core,
rather than the glowing mantle: a difference in illumination or color
may appear as a "shadow". The lensing thing has to do with a much
larger shadow, including non-flame regions and the effect of heating on
the refractive index of air. An opaque candle flame itself is not much
of a lens.
Explicating the blooming obvious since 1977.
Edward Green
> fire is not homugenous
>
> there are sections in it that are hotter (giving a brighter lite)
> and some aprts of it cooler screening the hotetr parts)
> so the hotter parts give shadowes of the cooler parts
Hey, that's about right! The first time I read it I couldn't get past
your non-standard spelling (no offense intended).
Quibble: the less luminous parts may actually be hotter.
Quibble for me: since the less luminous parts are blocking light from
the more luminous parts, I guess this _could_ be properly described as
"a flame casting a shadow of itself", after all.
andy everett
I got some candles out and lit them but could not see this effect. What
was your setup? I did however see some interesting diffraction effects
by shining a flashlight at a candle flame in a darkened room and
observing the "shadow" on a white wall.
andy everett
Like this but better:
http://www.physics.lsa.umich.edu/demolab/graphics2/6a40_61b.jpg
Sorcerer
"andy everett" <vze2...@verizon.net> wrote in message news:d2ukh.8437$6Z5.239@trndny01...
I shone a laser pointer right through a flame and noticed nothing.
Why does he still kick his mother in the stomach?
Autymn D. C.
> Anyway, he's not talking about a "shadow", except in appearence. He
illiterate
Ian Parker
Dirk Van de moortel wrote:
> Of course a candle flame does not contain sodium, but why
> shouldn't it absorb parts of the lamp's light?
>
when you have a point source of light and open an oven. You can "see"
the air rising. I don't know what Sourcerer means when he says he
cannot see any shadow with a laser. It all depends on the
configuration. Lasers are used to measure wind shear in Clear Air
Turbulence, they can easily measure the temperature of air surrounding
a rock bathed in sunlight.
In fact the Physics of the effect is much the same as stars
"twinkling".
- Ian Parker
Edward Green
I misspelled "appearance". Thank you for this valuable and substantive
correction.
Y.Porat
ok
yet we got the mainidea:
to common fire is not homogenous
and that is the key tounderstand the problem
btw
it is alike with the light form the sun
we have those 'spots' there
ATB
Y.Porat
------------------------
Autymn D. C.
You misspelld she. Whether you misspelld the collective progressiv
depends on whether you know what a thèmatic vowel is.
-Aut
jmfb...@aol.com
Jewish Cowboy
So you're still wasting time in sci.physics? How can you stand it?
I posted something last week in sci.physics.electromag it's called
"The Case Against the Photon". You should check it out. We used
to argue back and forth over these things a few years ago.
Marty Green
(no relation)
Winnipeg, Canada
Autymn D. C.
Timo A. Nieminen
>
> hm...@yahoo.com wrote:
>> Hi all,
>> I have a not-very-earth-shattering question, but in my mind a very
>> mysterious physics phenomena. You out there with broader minds than
>> myself can make a curious student of physics very happy by answering
>> this question: Why does fire have a shadow?
>>
>> In a very dark room, I have seen the flame of a stick candle cast a
>> shadow of itself on the surface immediately below it.
>>
>> I understand that fire contains particles (mass), whether they be
>> plasma or gas. However, these particles themselves emit light if I
>> understand correctly. Why would a light source cast a shadow of itself?
>
> Basically because air has a refractive index, and a flame looks
> something like a concave lens. (remember the flame has a lower
> refractive index than the surrounding air.
Just for fun, check out the shadow of a magnifying glass. OK, this is a
convex lens, rather than effectively a concave lens, but you still get a
shadow through the same mechanism.
However, the flame must also absorb light. Just as the higher the
reflectivity of an object, the lower its emissivity, there is a similar
relationship between transparency and emissivity - the flame emits light,
so it must have appreciable opacity.
--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html
andy everett
I did the same thing, nothing. Try a flashlight.
hm...@yahoo.com
I have arrived---I'm an official dummy!
It is the wick making the shadow.
(I also saw the faint 'shadow patterns' from the flame, like out of a
jet engine)
Because--
I moved the candle side to side in the air, and the shadow made a
mirror image ie:
flame stretches this way <--, but the shadow stretches this way -->. If
it were the flame making the shadow, the shadow would remain under the
flame.
It is hereby confirmed that I am a dummy.
Everyone is a dummy sometimes, like it or not.
But at the end of the day, I learned something, so I don't care.
Thanks everybody, it's been fun!
Chloe H.
Sorcerer
"andy everett" <vze2...@verizon.net> wrote in message news:i3Bkh.2923$oo4.1075@trndny09...
No way. If want to see diffraction the easiest way is a CD or DVD.
http://www.mrfiber.com/images/cddiffract.jpg
http://static.flickr.com/47/147127866_48f2193c2c_m.jpg
My point was his question was presumptuous.
Do you still beat your mother?
a) Yes:
You are an evil tyrant.
b) No:
When did you stop, evil tyrant?
In this newsgroup, such questions are all too common.
Why does fire have a shadow?
That's not a question I can answer because it doesn't.
Why does it take longer for a bus to stop than a car?
Along comes an idiot like Poe and tries to answer it,
but the simple truth is, it doesn't.
*We* know the universe began with Big Bang, but *I*
don't know that at all. To me, the whole concept is
totally ridiculous to begin with, the raving of a lunatic.
Edward Green
> Guess what everybody?
>
> I have arrived---I'm an official dummy!
>
> It is the wick making the shadow.
No! That's impossible! I and the French gentleman have explained your
observation by the inhomogenous and asymmetrical flame, and you are not
allowed to contradict this model by further elucidation of the data!
It's all your fault: you said the flame was casting its own "shadow",
and I took you at your word... damned experimentalist. ;-)
> (I also saw the faint 'shadow patterns' from the flame, like out of a
> jet engine)
>
> Because--
> I moved the candle side to side in the air, and the shadow made a
> mirror image ie:
> flame stretches this way <--, but the shadow stretches this way -->. If
> it were the flame making the shadow, the shadow would remain under the
> flame.
> It is hereby confirmed that I am a dummy.
Not necessarily (I meant that in reply to the penultimate sentence :-).
When you move the flame around in air, you are changing its structure.
Anyway, I am a dummy too, because I ignored the most obvious factor, in
retrospect (the wick). Not that this means the effect is solely wick,
but that we must consider the wick.
> Everyone is a dummy sometimes, like it or not.
We are not alone. A number of dummies in this thread -- some
ordinarily quite smart -- answered the wrong question altogether,
telling you about the effects of refraction and heating air. They
completely ignored what you did ask to give an irrelevant answer they
were fond of.
> But at the end of the day, I learned something, so I don't care.
You are disqualified for Usenet. :-)
jmor...@idirect.com
There;s a neat demo you can do with a Bunsen burner, sodium vapour
lamp, white screen and some table salt. If you put the burner between
the lamp and the screen, and toss some salt into the flame, you'll see
the characteristic yellow glow from the flame. But the <cool> sodium
vapour in the flame will do a perfect job of absorbing the light from
the lamp, causing a shadow of the flame on the screen.
Dirk Van de moortel
"Sorcerer" <Headm...@hogwarts.physics_h> wrote in message news:q_Ekh.164768$bz5....@fe3.news.blueyonder.co.uk...
When Androcles is proven wrong, he starts behaving like a reptile.
"Try a flashlight? - No Way":
http://users.telenet.be/vdmoortel/dirk/Physics/Fumbles/NoWay.html
Dirk Vdm
jmfb...@aol.com
Ben Newsam
That depends.
jmfb...@aol.com
Autymn D. C.
I got the OP's fuddel wriht all along. What's your excus for benan
here?
-Aut