<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nCan someone tell me the current explanation for how photons travel in\na straight line in air.\n\nPresumably the photons will encounter electrons in their journey (how\nmany encounters on average per meter?) and will be absorbed and\nre-emitted. Is it a property of air (and I guess other transparent\nmedia) that it\'s electrons re-emit the photons in more or less the\nsame direction they came from, or is some other mechanism involved?\n\nPlease include photographs.\n\nMany thanks\n\nCurious\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Can someone tell me the current explanation for how photons travel in
a straight line in air.
Presumably the photons will encounter electrons in their journey (how
many encounters on average per meter?) and will be absorbed and
re-emitted. Is it a property of air (and I guess other transparent
media) that it's electrons re-emit the photons in more or less the
same direction they came from, or is some other mechanism involved?
Please include photographs.
Many thanks
Curious
grelbr
Jul21-04, 05:02 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\njunk@riddell.co.nz (Sylvester) wrote in message news:<205fe6f6.0407191854.4a12c523@posting.google.com>...\n> Can someone tell me the current explanation for how photons travel in\n> a straight line in air.\n[snips]\n\nGet a copy of Feynman\'s book on QED. Be sure it\'s the "non-technical"\nbook. He has a very nice description from a minimal math point of\nview. I have never seen a better explanation pitched at a non-grad\nstudent level. The book is taken from notes for four lectures that\nFeynman gave. Your question, and many other very fun questions, are\nanswered in specific detail. It should be available in paperback for\na modest price.\n\n> Please include photographs.\n\nPhotographs? Try here.\nwww.photosig.com\ngrelbr\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>junk@riddell.co.nz (Sylvester) wrote in message news:<205fe6f6.0407191854.4a12c523@posting.google.com>...
> Can someone tell me the current explanation for how photons travel in
> a straight line in air.
[snips]
Get a copy of Feynman's book on QED. Be sure it's the "non-technical"
book. He has a very nice description from a minimal math point of
view. I have never seen a better explanation pitched at a non-grad
student level. The book is taken from notes for four lectures that
Feynman gave. Your question, and many other very fun questions, are
answered in specific detail. It should be available in paperback for
a modest price.
> Please include photographs.
Photographs? Try here.
www.photosig.com
grelbr
Gjergj Kastrioti
Jul22-04, 05:16 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\njunk@riddell.co.nz (Sylvester) wrote in message news:<205fe6f6.0407191854.4a12c523@posting.google.com>...\n> Can someone tell me the current explanation for how photons travel in\n> a straight line in air.\n>\n> Presumably the photons will encounter electrons in their journey (how\n> many encounters on average per meter?) and will be absorbed and\n> re-emitted. Is it a property of air (and I guess other transparent\n> media) that it\'s electrons re-emit the photons in more or less the\n> same direction they came from, or is some other mechanism involved?\n>\n> Please include photographs.\n>\n> Many thanks\n>\n> Curious\n\ni hope this helps: http://tinyurl.com/5cc9q =\nhttp://www.vega.org.uk/series/lectures/feynman/\n\ntana t\'mirat\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>junk@riddell.co.nz (Sylvester) wrote in message news:<205fe6f6.0407191854.4a12c523@posting.google.com>...
> Can someone tell me the current explanation for how photons travel in
> a straight line in air.
>
> Presumably the photons will encounter electrons in their journey (how
> many encounters on average per meter?) and will be absorbed and
> re-emitted. Is it a property of air (and I guess other transparent
> media) that it's electrons re-emit the photons in more or less the
> same direction they came from, or is some other mechanism involved?
>
> Please include photographs.
>
> Many thanks
>
> Curious
i hope this helps: http://tinyurl.com/5cc9q =
http://www.vega.org.uk/series/lectures/feynman/
tana t'mirat
Rahul Jain
Jul22-04, 05:16 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>junk@riddell.co.nz (Sylvester) writes:\n\n> Can someone tell me the current explanation for how photons travel in\n> a straight line in air.\n>\n> Presumably the photons will encounter electrons in their journey (how\n> many encounters on average per meter?) and will be absorbed and\n> re-emitted. Is it a property of air (and I guess other transparent\n> media) that it\'s electrons re-emit the photons in more or less the\n> same direction they came from, or is some other mechanism involved?\n\nHere\'s my impression (But note from my sig that I\'m an amateur, so I\nmay be wrong. If I am, someone please correct me so that I learn how\nthis stuff actually works):\n\nIt\'s either that or the electron will need to move into another state\ndue to conservation of momentum (and angular momentum, for that matter).\nThe ability of the electrons to do this (and, I suppose, the states\ninvolved) determines whether they can reflect or absorb (depending on\nthe aforementioned states) the photon or not.\n\n> Please include photographs.\n\nI wasn\'t aware that this group was also an online dating service...\n:)\n\n--\nRahul Jain\nrjain@nyct.net\nProfessional Software Developer, Amateur Quantum Mechanicist\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>junk@riddell.co.nz (Sylvester) writes:
> Can someone tell me the current explanation for how photons travel in
> a straight line in air.
>
> Presumably the photons will encounter electrons in their journey (how
> many encounters on average per meter?) and will be absorbed and
> re-emitted. Is it a property of air (and I guess other transparent
> media) that it's electrons re-emit the photons in more or less the
> same direction they came from, or is some other mechanism involved?
Here's my impression (But note from my sig that I'm an amateur, so I
may be wrong. If I am, someone please correct me so that I learn how
this stuff actually works):
It's either that or the electron will need to move into another state
due to conservation of momentum (and angular momentum, for that matter).
The ability of the electrons to do this (and, I suppose, the states
involved) determines whether they can reflect or absorb (depending on
the aforementioned states) the photon or not.
> Please include photographs.
I wasn't aware that this group was also an online dating service...
:)
--
Rahul Jain
rjain@nyct.net
Professional Software Developer, Amateur Quantum Mechanicist
Sylvester
Jul22-04, 05:16 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\ngrelbr@hotmail.com (grelbr) wrote in message news:<1a325379.0407200924.6eaa1aba@posting.google.com>...\n> junk@riddell.co.nz (Sylvester) wrote in message news:<205fe6f6.0407191854.4a12c523@posting.google.com>...\n> > Can someone tell me the current explanation for how photons travel in\n> > a straight line in air.\n> [snips]\n>\n> Get a copy of Feynman\'s book on QED. Be sure it\'s the "non-technical"\n> book. He has a very nice description from a minimal math point of\n> view. I have never seen a better explanation pitched at a non-grad\n> student level. The book is taken from notes for four lectures that\n> Feynman gave. Your question, and many other very fun questions, are\n> answered in specific detail. It should be available in paperback for\n> a modest price.\n>\n> > Please include photographs.\n>\n> Photographs? Try here.\n> www.photosig.com\n> grelbr\n\nThanks, grelbr. I guess my post came out wrong. No, I wasn\'t\nexpecting any photographs, but I am sure the ones you suggested were\nvery nice. And yes, I have already read Feynman, but some time ago,\nand I do not recall the explanation for my question (which I thought\nboiled down to why, when photons encounter electrons in air, as they\nmust, they are not re-emitted in a random direction). And if anyone\ndoes feel inclined to post a substantive reply, please feel free to\nuse any level of math required.\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>grelbr@hotmail.com (grelbr) wrote in message news:<1a325379.0407200924.6eaa1aba@posting.google.com>...
> junk@riddell.co.nz (Sylvester) wrote in message news:<205fe6f6.0407191854.4a12c523@posting.google.com>...
> > Can someone tell me the current explanation for how photons travel in
> > a straight line in air.
> [snips]
>
> Get a copy of Feynman's book on QED. Be sure it's the "non-technical"
> book. He has a very nice description from a minimal math point of
> view. I have never seen a better explanation pitched at a non-grad
> student level. The book is taken from notes for four lectures that
> Feynman gave. Your question, and many other very fun questions, are
> answered in specific detail. It should be available in paperback for
> a modest price.
>
> > Please include photographs.
>
> Photographs? Try here.
> www.photosig.com
> grelbr
Thanks, grelbr. I guess my post came out wrong. No, I wasn't
expecting any photographs, but I am sure the ones you suggested were
very nice. And yes, I have already read Feynman, but some time ago,
and I do not recall the explanation for my question (which I thought
boiled down to why, when photons encounter electrons in air, as they
must, they are not re-emitted in a random direction). And if anyone
does feel inclined to post a substantive reply, please feel free to
use any level of math required.
J.J. Simplicio
Jul22-04, 05:17 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n"grelbr" <grelbr@hotmail.com> wrote in message\nnews:1a325379.0407200924.6eaa1aba@posting.google.com...\n>\n> Get a copy of Feynman\'s book on QED.....\n> The book is taken from notes for four lectures that\n> Feynman gave.\n\nYou can view these lectures on the web at\nhttp://www.vega.org.uk/series/lectures/feynman/\n\nJ.J.\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>"grelbr" <grelbr@hotmail.com> wrote in message
news:1a325379.0407200924.6eaa1aba@posting.google.com...
>
> Get a copy of Feynman's book on QED.....
> The book is taken from notes for four lectures that
> Feynman gave.
You can view these lectures on the web at
http://www.vega.org.uk/series/lectures/feynman/
J.J.
Oz
Jul23-04, 06:34 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nSylvester <junk@riddell.co.nz> writes\n\n>And yes, I have already read Feynman, but some time ago,\n>and I do not recall the explanation for my question (which I thought\n>boiled down to why, when photons encounter electrons in air, as they\n>must, they are not re-emitted in a random direction).\n\n[Caution: I am an amateur]\n\nWhy must they be absorbed and re-emitted?\nIf they were, then air wouldn\'t be transparent.\nThat the em wave must shake the atom must be inevitable,\nand then the atom would be (slightly) excited.\n\nTo see a plausible explanation of why this essentially results in no\neffect (other than a slight delay) consider a laser. Basically the\n(slightly) excited atom should reconstitute the EM wave by what amounts\nto laser action.\n\nWell, that seems a plausible explanation to me.\n\n>And if anyone\n>does feel inclined to post a substantive reply, please feel free to\n>use any level of math required.\n\n<shudder>\n\n--\nOz\nThis post is worth absolutely nothing and is probably fallacious.\n\nBTOPENWORLD address about to cease. DEMON address no longer in use.\n>>Use oz@farmeroz.port995.com<<\nozacoohdb@despammed.com still functions.\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Sylvester <junk@riddell.co.nz> writes
>And yes, I have already read Feynman, but some time ago,
>and I do not recall the explanation for my question (which I thought
>boiled down to why, when photons encounter electrons in air, as they
>must, they are not re-emitted in a random direction).
[Caution: I am an amateur]
Why must they be absorbed and re-emitted?
If they were, then air wouldn't be transparent.
That the em wave must shake the atom must be inevitable,
and then the atom would be (slightly) excited.
To see a plausible explanation of why this essentially results in no
effect (other than a slight delay) consider a laser. Basically the
(slightly) excited atom should reconstitute the EM wave by what amounts
to laser action.
Well, that seems a plausible explanation to me.
>And if anyone
>does feel inclined to post a substantive reply, please feel free to
>use any level of math required.
<shudder>
--
Oz
This post is worth absolutely nothing and is probably fallacious.
BTOPENWORLD address about to cease. DEMON address no longer in use.
>>Use oz@farmeroz.port995.com<<
ozacoohdb@despammed.com still functions.
Rahul Jain
Jul25-04, 09:16 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nOz <oz@farmeroz.port995.com> writes:\n\n> Sylvester <junk@riddell.co.nz> writes\n>\n>>And yes, I have already read Feynman, but some time ago,\n>>and I do not recall the explanation for my question (which I thought\n>>boiled down to why, when photons encounter electrons in air, as they\n>>must, they are not re-emitted in a random direction).\n>\n> [Caution: I am an amateur]\n>\n> Why must they be absorbed and re-emitted?\n> If they were, then air wouldn\'t be transparent.\n\nI don\'t understand. It\'s _because_ of that that air is transparent. If\nair absorbed the photon and didn\'t re-emit it, it would be photoelectric\nor something similar. Only in free space (and not even in that because\nof vacuum fluctuations) can a photon travel unimpeded. Or, I suppose, in\na medium that is composed purely of electrically uncharged particles,\nlike neutrinos... whatever that would be like...\n\n> That the em wave must shake the atom must be inevitable,\n> and then the atom would be (slightly) excited.\n>\n> To see a plausible explanation of why this essentially results in no\n> effect (other than a slight delay) consider a laser.\n\nOK, now I don\'t get you at all. This delay is exactly what is introduced\nin the absorption-re-emission interaction. It determines the index of\nrefraction.\n\n> Basically the\n> (slightly) excited atom should reconstitute the EM wave by what amounts\n> to laser action.\n>\n> Well, that seems a plausible explanation to me.\n\nActually, that\'s probably why the light keeps going forward. The photon\nhas some amplitude to not be absorbed at all, but instead just keep\ngoing without interacting with the electrons. These amplitudes then make\nit more probable for the light to be re-emitted along the same path they\nwere absorbed from instead of being scattered in some random direction.\nOf course, this interaction would be much stronger with multiple\nphotons, as the forward emission could be induced by other photons as\nwell. I suppose this is why coherent light forms a beam much better than\nincoherent light.\n\n--\nRahul Jain\nrjain@nyct.net\nProfessional Software Developer, Amateur Quantum Mechanicist\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Oz <oz@farmeroz.port995.com> writes:
> Sylvester <junk@riddell.co.nz> writes
>
>>And yes, I have already read Feynman, but some time ago,
>>and I do not recall the explanation for my question (which I thought
>>boiled down to why, when photons encounter electrons in air, as they
>>must, they are not re-emitted in a random direction).
>
> [Caution: I am an amateur]
>
> Why must they be absorbed and re-emitted?
> If they were, then air wouldn't be transparent.
I don't understand. It's _because_ of that that air is transparent. If
air absorbed the photon and didn't re-emit it, it would be photoelectric
or something similar. Only in free space (and not even in that because
of vacuum fluctuations) can a photon travel unimpeded. Or, I suppose, in
a medium that is composed purely of electrically uncharged particles,
like neutrinos... whatever that would be like...
> That the em wave must shake the atom must be inevitable,
> and then the atom would be (slightly) excited.
>
> To see a plausible explanation of why this essentially results in no
> effect (other than a slight delay) consider a laser.
OK, now I don't get you at all. This delay is exactly what is introduced
in the absorption-re-emission interaction. It determines the index of
refraction.
> Basically the
> (slightly) excited atom should reconstitute the EM wave by what amounts
> to laser action.
>
> Well, that seems a plausible explanation to me.
Actually, that's probably why the light keeps going forward. The photon
has some amplitude to not be absorbed at all, but instead just keep
going without interacting with the electrons. These amplitudes then make
it more probable for the light to be re-emitted along the same path they
were absorbed from instead of being scattered in some random direction.
Of course, this interaction would be much stronger with multiple
photons, as the forward emission could be induced by other photons as
well. I suppose this is why coherent light forms a beam much better than
incoherent light.
--
Rahul Jain
rjain@nyct.net
Professional Software Developer, Amateur Quantum Mechanicist
BW
Jul25-04, 09:16 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\nOz wrote:\n> Sylvester <junk@riddell.co.nz> writes\n> >And yes, I have already read Feynman, but some time ago,\n> >and I do not recall the explanation for my question (which I thought\n> >boiled down to why, when photons encounter electrons in air, as they\n> >must, they are not re-emitted in a random direction).\n....\n>\n> Why must they be absorbed and re-emitted?\n> If they were, then air wouldn\'t be transparent.\n> That the em wave must shake the atom must be inevitable,\n> and then the atom would be (slightly) excited.\n\nThis is one situation where you can\'t really choose to consider the\nphoton as a classical particle. If you could send in a single photon\ninto a bunch of atoms, obviously it would scatter randomly if it hits\nanything.\n\nIn the "Feynman-derivation" of the phenomena, you should just start by\nconsidering what happens when a photon traverses a vacuum. It does not\ngo in a straight line, in fact it can take any path possible from A to\nB. But the non-classical feature that enters here is of course that it\ntakes all those paths at the same time, adding up the "little arrow\nthat rotates" (the U(1) elements) and squaring this to get the\npropagation probability. So why does it seem to go in a straight line\nin a vacuum ? Because all the other propagations end up with a very\nsmall probability. Paraphrasing Feynman from the recommended book\n(QED), for every crooked path from A to B there is a slightly less\ncrooked path with a distance difference cancelling the "arrow"\ncontribution, except around the almost-straight paths. Thus most of the\nprobability ends up in the straight forward direction.\n\nIf you sprinkle air molecules into this vacuum, the explanation is the\nsame. Much of the probability goes in the forward direction. For every\nencountered air molecule, the photon scatters in all directions at the\nsame time (albeit with a time lag and momentum change, differing the\nscattering from the vacuum propagation). The denser the medium, the\nlesser is the probability cancellation effect and more is scattered,\ngenerally. You can manipulate the probabilities though by adjusting the\nmediums atom density appropriately. This is what happens in a normal\nbog-standard optical lens, or in a mirror / reflector.\n\nMore interestingly, you can manipulate the probabilities like that in\nmore ways - for example, by completely excluding some of the crooked\npaths. This is what happens in a Fresnel zoneplate (a metal plate with\nconcentric rings hollowed out). Counterintuitively, more light is\nfocused in the middle by blocking light through the plate. Similarily,\nyou can make a mirror more effective by removing some of the material -\nthis is what a grating is.\n\nWhen you move up to a thick flow of photons and ignore individual\nphoton detections, you can describe this in a pure classical\nlight-as-a-wave way, as they do in any elementary optics textbook. They\nderive the vacuum and medium propagations by postulating that the wave\nis re-emitted at every point a wavefront traverses, which, of course,\nturns out to give the same propagation, diffraction and interference\neffects that the Feynman particle-calculation gives.\n\n/Bjorn\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Oz wrote:
> Sylvester <junk@riddell.co.nz> writes
> >And yes, I have already read Feynman, but some time ago,
> >and I do not recall the explanation for my question (which I thought
> >boiled down to why, when photons encounter electrons in air, as they
> >must, they are not re-emitted in a random direction).
....
>
> Why must they be absorbed and re-emitted?
> If they were, then air wouldn't be transparent.
> That the em wave must shake the atom must be inevitable,
> and then the atom would be (slightly) excited.
This is one situation where you can't really choose to consider the
photon as a classical particle. If you could send in a single photon
into a bunch of atoms, obviously it would scatter randomly if it hits
anything.
In the "Feynman-derivation" of the phenomena, you should just start by
considering what happens when a photon traverses a vacuum. It does not
go in a straight line, in fact it can take any path possible from A to
B. But the non-classical feature that enters here is of course that it
takes all those paths at the same time, adding up the "little arrow
that rotates" (the U(1) elements) and squaring this to get the
propagation probability. So why does it seem to go in a straight line
in a vacuum ? Because all the other propagations end up with a very
small probability. Paraphrasing Feynman from the recommended book
(QED), for every crooked path from A to B there is a slightly less
crooked path with a distance difference cancelling the "arrow"
contribution, except around the almost-straight paths. Thus most of the
probability ends up in the straight forward direction.
If you sprinkle air molecules into this vacuum, the explanation is the
same. Much of the probability goes in the forward direction. For every
encountered air molecule, the photon scatters in all directions at the
same time (albeit with a time lag and momentum change, differing the
scattering from the vacuum propagation). The denser the medium, the
lesser is the probability cancellation effect and more is scattered,
generally. You can manipulate the probabilities though by adjusting the
mediums atom density appropriately. This is what happens in a normal
bog-standard optical lens, or in a mirror / reflector.
More interestingly, you can manipulate the probabilities like that in
more ways - for example, by completely excluding some of the crooked
paths. This is what happens in a Fresnel zoneplate (a metal plate with
concentric rings hollowed out). Counterintuitively, more light is
focused in the middle by blocking light through the plate. Similarily,
you can make a mirror more effective by removing some of the material -
this is what a grating is.
When you move up to a thick flow of photons and ignore individual
photon detections, you can describe this in a pure classical
light-as-a-wave way, as they do in any elementary optics textbook. They
derive the vacuum and medium propagations by postulating that the wave
is re-emitted at every point a wavefront traverses, which, of course,
turns out to give the same propagation, diffraction and interference
effects that the Feynman particle-calculation gives.
/Bjorn
Oz
Jul26-04, 11:32 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nBW <bjorn@sparta.lu.se> writes\n\n>This is one situation where you can\'t really choose to consider the\n>photon as a classical particle. If you could send in a single photon\n>into a bunch of atoms, obviously it would scatter randomly if it hits\n>anything.\n\nIt has a wavelength covering thousands of atoms.\nOf course it hits something, depending on your definition of \'hits\'.\n\n>In the "Feynman-derivation" of the phenomena, you should just start by\n>considering what happens when a photon traverses a vacuum. It does not\n>go in a straight line, in fact it can take any path possible from A to\n>B.\n\nIndeed, it can be detected anywhere, that\'s just a trick to make a\nparticle into a wave. A very neat trick it is too.\n\n>But the non-classical feature that enters here is of course that it\n>takes all those paths at the same time, adding up the "little arrow\n>that rotates" (the U(1) elements) and squaring this to get the\n>propagation probability.\n\nAbsolutely, see above.\n\n>So why does it seem to go in a straight line\n>in a vacuum ? Because all the other propagations end up with a very\n>small probability. Paraphrasing Feynman from the recommended book\n>(QED), for every crooked path from A to B there is a slightly less\n>crooked path with a distance difference cancelling the "arrow"\n>contribution, except around the almost-straight paths. Thus most of the\n>probability ends up in the straight forward direction.\n\nAbsolutely.\n\n>If you sprinkle air molecules into this vacuum, the explanation is the\n>same. Much of the probability goes in the forward direction. For every\n>encountered air molecule, the photon scatters in all directions at the\n>same time (albeit with a time lag and momentum change, differing the\n>scattering from the vacuum propagation).\n\nThis explanation is going to have difficulty with a glass of significant\nrefractive index. It will end up being milky, instead of clear.\n\n>More interestingly, you can manipulate the probabilities like that in\n>more ways - for example, by completely excluding some of the crooked\n>paths. This is what happens in a Fresnel zoneplate (a metal plate with\n>concentric rings hollowed out). Counterintuitively, more light is\n>focused in the middle by blocking light through the plate. Similarily,\n>you can make a mirror more effective by removing some of the material -\n>this is what a grating is.\n\nThis is much more easily seen if you consider the photon as just a wave.\nThat\'s true of many other examples, like separate photons from separate\nsources interfering.\n\n>When you move up to a thick flow of photons and ignore individual\n>photon detections, you can describe this in a pure classical\n>light-as-a-wave way, as they do in any elementary optics textbook.\n\nLight as a wave is also better at explaining one-photon-at-a-time\nexperiments.\n\n>They\n>derive the vacuum and medium propagations by postulating that the wave\n>is re-emitted at every point a wavefront traverses, which, of course,\n>turns out to give the same propagation, diffraction and interference\n>effects that the Feynman particle-calculation gives.\n\nYes. That\'s a lovely example.\nWorks even better for light as a wave, mind.\n\n--\nOz\nThis post is worth absolutely nothing and is probably fallacious.\n\nBTOPENWORLD address about to cease. DEMON address no longer in use.\n>>Use oz@farmeroz.port995.com<<\nozacoohdb@despammed.com still functions.\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>BW <bjorn@sparta.lu.se> writes
>This is one situation where you can't really choose to consider the
>photon as a classical particle. If you could send in a single photon
>into a bunch of atoms, obviously it would scatter randomly if it hits
>anything.
It has a wavelength covering thousands of atoms.
Of course it hits something, depending on your definition of 'hits'.
>In the "Feynman-derivation" of the phenomena, you should just start by
>considering what happens when a photon traverses a vacuum. It does not
>go in a straight line, in fact it can take any path possible from A to
>B.
Indeed, it can be detected anywhere, that's just a trick to make a
particle into a wave. A very neat trick it is too.
>But the non-classical feature that enters here is of course that it
>takes all those paths at the same time, adding up the "little arrow
>that rotates" (the U(1) elements) and squaring this to get the
>propagation probability.
Absolutely, see above.
>So why does it seem to go in a straight line
>in a vacuum ? Because all the other propagations end up with a very
>small probability. Paraphrasing Feynman from the recommended book
>(QED), for every crooked path from A to B there is a slightly less
>crooked path with a distance difference cancelling the "arrow"
>contribution, except around the almost-straight paths. Thus most of the
>probability ends up in the straight forward direction.
Absolutely.
>If you sprinkle air molecules into this vacuum, the explanation is the
>same. Much of the probability goes in the forward direction. For every
>encountered air molecule, the photon scatters in all directions at the
>same time (albeit with a time lag and momentum change, differing the
>scattering from the vacuum propagation).
This explanation is going to have difficulty with a glass of significant
refractive index. It will end up being milky, instead of clear.
>More interestingly, you can manipulate the probabilities like that in
>more ways - for example, by completely excluding some of the crooked
>paths. This is what happens in a Fresnel zoneplate (a metal plate with
>concentric rings hollowed out). Counterintuitively, more light is
>focused in the middle by blocking light through the plate. Similarily,
>you can make a mirror more effective by removing some of the material -
>this is what a grating is.
This is much more easily seen if you consider the photon as just a wave.
That's true of many other examples, like separate photons from separate
sources interfering.
>When you move up to a thick flow of photons and ignore individual
>photon detections, you can describe this in a pure classical
>light-as-a-wave way, as they do in any elementary optics textbook.
Light as a wave is also better at explaining one-photon-at-a-time
experiments.
>They
>derive the vacuum and medium propagations by postulating that the wave
>is re-emitted at every point a wavefront traverses, which, of course,
>turns out to give the same propagation, diffraction and interference
>effects that the Feynman particle-calculation gives.
Yes. That's a lovely example.
Works even better for light as a wave, mind.
--
Oz
This post is worth absolutely nothing and is probably fallacious.
BTOPENWORLD address about to cease. DEMON address no longer in use.
>>Use oz@farmeroz.port995.com<<
ozacoohdb@despammed.com still functions.
Sylvester
Jul27-04, 01:50 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\n\n"BW" <bjorn@sparta.lu.se> wrote in message news:<ce0b7s\\$kff@odah37.prod.google.com>...\n\nAppreciate the detailed post. Thanks.\n\n> This is one situation where you can\'t really choose to consider the\n> photon as a classical particle. If you could send in a single photon\n> into a bunch of atoms, obviously it would scatter randomly if it hits\n> anything ....\n\nDamn! Someone should post the rules about what you are and are not\nallowed to consider. :) I think I remember some discussions about\nsingle photons in that double slit experiment. They should have known\nbetter.\n\n\n> If you sprinkle air molecules into this vacuum, the explanation is the\n> same. Much of the probability goes in the forward direction.\n\nI\'m still thinking of a random scatter, why is it mostly in the\nforward direction? I guess I have to re-read Feynman.\n\n\n> ... The denser the medium, the\n> lesser is the probability cancellation effect and more is scattered,\n> generally ...\n\nOk this addresses my problem. What do you mean by "density of the\nmedium"? It might relate to number, size, structure and spacing of\nmolecules/nuclei, and the energy of the outer electrons and how\ntightly they are held, to name a few obvious candidates. What is it\nthat makes one medium transparent and another opaque? That sounds\nlike a dumb question but thinking about things at the molecular level,\nit is not obvious to me how photons can pass straight through a medium\ne.g. air or glass, given that the photons are continually being\nabsorbed and re-emitted and crashing into nuclei. Are there less\nphoton-electron interactions in a transparent medium and the nuclei\nare less obtrusive, is it that simple, or are there some properties of\ntransparent media that influence the interaction with photons?\n\nAs you can tell, my head is in a particle cloud.\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>"BW" <bjorn@sparta.lu.se> wrote in message news:<ce0b7s$kff@odah37.prod.google.com>...
Appreciate the detailed post. Thanks.
> This is one situation where you can't really choose to consider the
> photon as a classical particle. If you could send in a single photon
> into a bunch of atoms, obviously it would scatter randomly if it hits
> anything ....
Damn! Someone should post the rules about what you are and are not
allowed to consider. :) I think I remember some discussions about
single photons in that double slit experiment. They should have known
better.
> If you sprinkle air molecules into this vacuum, the explanation is the
> same. Much of the probability goes in the forward direction.
I'm still thinking of a random scatter, why is it mostly in the
forward direction? I guess I have to re-read Feynman.
> ... The denser the medium, the
> lesser is the probability cancellation effect and more is scattered,
> generally ...
Ok this addresses my problem. What do you mean by "density of the
medium"? It might relate to number, size, structure and spacing of
molecules/nuclei, and the energy of the outer electrons and how
tightly they are held, to name a few obvious candidates. What is it
that makes one medium transparent and another opaque? That sounds
like a dumb question but thinking about things at the molecular level,
it is not obvious to me how photons can pass straight through a medium
e.g. air or glass, given that the photons are continually being
absorbed and re-emitted and crashing into nuclei. Are there less
photon-electron interactions in a transparent medium and the nuclei
are less obtrusive, is it that simple, or are there some properties of
transparent media that influence the interaction with photons?
As you can tell, my head is in a particle cloud.
BW
Jul27-04, 01:50 PM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\n\nOz wrote:\n> BW <bjorn@sparta.lu.se> writes\n> >If you sprinkle air molecules into this vacuum, the explanation is\nthe\n> >same. Much of the probability goes in the forward direction. For\nevery\n> >encountered air molecule, the photon scatters in all directions at\nthe\n> >same time (albeit with a time lag and momentum change, differing the\n> >scattering from the vacuum propagation).\n>\n> This explanation is going to have difficulty with a glass of\nsignificant\n> refractive index. It will end up being milky, instead of clear.\n\nIn glass, the atoms are regularly spaced in a way that produces\nconstructive interference of the re-emitted photons/wavefronts in the\nforward direction. The phase differences between the re-emitted waves\nand the incident waves cause the resultant wave to lag in phase, giving\na phase velocity < c (that is, a bigger index of refraction).\n\nSpeaking of which.. wasn\'t there some experiment some years ago where\nsomeone was transmitting Mozart music through a medium with n < 1 ?\nThat is, with a phase lead.\n\n/Bjorn\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Oz wrote:
> BW <bjorn@sparta.lu.se> writes
> >If you sprinkle air molecules into this vacuum, the explanation is
the
> >same. Much of the probability goes in the forward direction. For
every
> >encountered air molecule, the photon scatters in all directions at
the
> >same time (albeit with a time lag and momentum change, differing the
> >scattering from the vacuum propagation).
>
> This explanation is going to have difficulty with a glass of
significant
> refractive index. It will end up being milky, instead of clear.
In glass, the atoms are regularly spaced in a way that produces
constructive interference of the re-emitted photons/wavefronts in the
forward direction. The phase differences between the re-emitted waves
and the incident waves cause the resultant wave to lag in phase, giving
a phase velocity < c (that is, a bigger index of refraction).
Speaking of which.. wasn't there some experiment some years ago where
someone was transmitting Mozart music through a medium with n < 1 ?
That is, with a phase lead.
/Bjorn
Franz Heymann
Jul28-04, 04:58 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\n\n\n"BW" <bjorn@sparta.lu.se> wrote in message\nnews:ce5b5k\\$pnr@odbk17.prod.google.com...\n>\n>\n>\n> Oz wrote:\n> > BW <bjorn@sparta.lu.se> writes\n> > >If you sprinkle air molecules into this vacuum, the explanation\nis\n> the\n> > >same. Much of the probability goes in the forward direction. For\n> every\n> > >encountered air molecule, the photon scatters in all directions\nat\n> the\n> > >same time (albeit with a time lag and momentum change, differing\nthe\n> > >scattering from the vacuum propagation).\n> >\n> > This explanation is going to have difficulty with a glass of\n> significant\n> > refractive index. It will end up being milky, instead of clear.\n>\n> In glass, the atoms are regularly spaced\n\nNo. Glass is not a crystal and neither is water\n\n[snip]\n\nFranz\n\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>"BW" <bjorn@sparta.lu.se> wrote in message
news:ce5b5k$pnr@odbk17.prod.google.com...
>
>
>
> Oz wrote:
> > BW <bjorn@sparta.lu.se> writes
> > >If you sprinkle air molecules into this vacuum, the explanation
is
> the
> > >same. Much of the probability goes in the forward direction. For
> every
> > >encountered air molecule, the photon scatters in all directions
at
> the
> > >same time (albeit with a time lag and momentum change, differing
the
> > >scattering from the vacuum propagation).
> >
> > This explanation is going to have difficulty with a glass of
> significant
> > refractive index. It will end up being milky, instead of clear.
>
> In glass, the atoms are regularly spaced
No. Glass is not a crystal and neither is water
[snip]
Franz
Oz
Jul28-04, 04:59 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nSylvester <junk@riddell.co.nz> writes\n>Ok this addresses my problem. What do you mean by "density of the\n>medium"? It might relate to number, size, structure and spacing of\n>molecules/nuclei, and the energy of the outer electrons and how\n>tightly they are held, to name a few obvious candidates. What is it\n>that makes one medium transparent and another opaque? That sounds\n>like a dumb question but thinking about things at the molecular level,\n>it is not obvious to me how photons can pass straight through a medium\n>e.g. air or glass, given that the photons are continually being\n>absorbed and re-emitted and crashing into nuclei. Are there less\n>photon-electron interactions in a transparent medium and the nuclei\n>are less obtrusive, is it that simple, or are there some properties of\n>transparent media that influence the interaction with photons?\n\nPhotons are waves. BIG waves compared to tiny atoms.\nMostly they just go straight past.\n\nImagine a sea wave going through an array of vertical bamboo canes,\nwould you expect much effect, even though it \'hits\' every one?\n\nLikely the bamboo will bend one way on the crest (a little bit) and the\nother way on the return, but if the collision is elastic then all the\nenergy lost by the wave to the bamboo in one phase, must be returned in\nanother (less a bit diffracted).\n\nHmm, I rather like that analogy.\n\nOne could go even better. If the resonant frequency of the bamboo stick\nexactly matched that of the incoming wave then one would get lots of\nenergy transferred from the wave to the bamboo sticks (OK *LOTS* of\nbamboo sticks) and the amplitude of the wave would be reduced, which\nwould mimic absorption. Put some nice gloopy high-friction mud to stick\nthe canes in and you could have a nice illustration of an absorbing\nmedium.\n\nGosh, this analogy is going to cause a ruction.\n\n--\nOz\nThis post is worth absolutely nothing and is probably fallacious.\n\nBTOPENWORLD address about to cease. DEMON address no longer in use.\n>>Use oz@farmeroz.port995.com<<\nozacoohdb@despammed.com still functions.\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Sylvester <junk@riddell.co.nz> writes
>Ok this addresses my problem. What do you mean by "density of the
>medium"? It might relate to number, size, structure and spacing of
>molecules/nuclei, and the energy of the outer electrons and how
>tightly they are held, to name a few obvious candidates. What is it
>that makes one medium transparent and another opaque? That sounds
>like a dumb question but thinking about things at the molecular level,
>it is not obvious to me how photons can pass straight through a medium
>e.g. air or glass, given that the photons are continually being
>absorbed and re-emitted and crashing into nuclei. Are there less
>photon-electron interactions in a transparent medium and the nuclei
>are less obtrusive, is it that simple, or are there some properties of
>transparent media that influence the interaction with photons?
Photons are waves. BIG waves compared to tiny atoms.
Mostly they just go straight past.
Imagine a sea wave going through an array of vertical bamboo canes,
would you expect much effect, even though it 'hits' every one?
Likely the bamboo will bend one way on the crest (a little bit) and the
other way on the return, but if the collision is elastic then all the
energy lost by the wave to the bamboo in one phase, must be returned in
another (less a bit diffracted).
Hmm, I rather like that analogy.
One could go even better. If the resonant frequency of the bamboo stick
exactly matched that of the incoming wave then one would get lots of
energy transferred from the wave to the bamboo sticks (OK *LOTS* of
bamboo sticks) and the amplitude of the wave would be reduced, which
would mimic absorption. Put some nice gloopy high-friction mud to stick
the canes in and you could have a nice illustration of an absorbing
medium.
Gosh, this analogy is going to cause a ruction.
--
Oz
This post is worth absolutely nothing and is probably fallacious.
BTOPENWORLD address about to cease. DEMON address no longer in use.
>>Use oz@farmeroz.port995.com<<
ozacoohdb@despammed.com still functions.
BW
Jul28-04, 06:52 AM
<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no,location=no,scrollbars=yes,resizable=yes,status=no,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>\n\nFranz Heymann wrote:\n> > In glass, the atoms are regularly spaced\n>\n> No. Glass is not a crystal and neither is water\n\nRight, so much I learned too when I actually looked up the structure\nafter writing that :)\n\nThe key properties that make glass transparent seemed to be that its\natoms are placed in a liquid (random) way as glass cools down to form a\nsolid, in combination with that the atoms are not very interested in\nabsorbing visible lights energies (turning them to heat) or having\nfree-floating electrons that can do the same (like in a metal). They\ncan absorb and retransmit though I think, otherwise the medium would\nnot have n <> 1. The interface to the surrounding medium (and\nmicroscopic domains inside the material) need to not reflect too much\neither of course.\n\nAlso, clarifying/correcting my last post with regards to the wave\ninterference effects inside the material - it seems that the\nconstructive interference in the forward direction does not really\ndepend on the arrangement of the atoms, but the destructive\ninterference that cancels scattering in the other directions does (and\nincreases by order). This much my old optics book implied at least :)\n\nSo transparency depends on a lot of factors. There *are* transparent\ncrystals, right ?\n\n/Bjorn\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form"> View this Usenet post in original ASCII form </a></div><P></jabberwocky>Franz Heymann wrote:
> > In glass, the atoms are regularly spaced
>
> No. Glass is not a crystal and neither is water
Right, so much I learned too when I actually looked up the structure
after writing that :)
The key properties that make glass transparent seemed to be that its
atoms are placed in a liquid (random) way as glass cools down to form a
solid, in combination with that the atoms are not very interested in
absorbing visible lights energies (turning them to heat) or having
free-floating electrons that can do the same (like in a metal). They
can absorb and retransmit though I think, otherwise the medium would
not have n <> 1. The interface to the surrounding medium (and
microscopic domains inside the material) need to not reflect too much
either of course.
Also, clarifying/correcting my last post with regards to the wave
interference effects inside the material - it seems that the
constructive interference in the forward direction does not really
depend on the arrangement of the atoms, but the destructive
interference that cancels scattering in the other directions does (and
increases by order). This much my old optics book implied at least :)
So transparency depends on a lot of factors. There *are* transparent
crystals, right ?