On 30 Sep 1999 10:29:58 -0400, Nieminen Juha <war### [at] cctutfi> wrote:

>  If the photon is absorbed by a molecule, why does the photon leave the
>molecule in the exact same direction as it entered it?

My father tried to explain this to me a lot of times and never
understood it. I only know that this phenomenon is the reason that a
laser beam is virtually non-divergent.


Peter Popov
ICQ: 15002700

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On 30 Sep 1999 10:29:58 -0400, Nieminen Juha <war### [at] cctutfi> wrote:
>Ron Parker <par### [at] fwicom> wrote:
>: The explanation I always got is that the photon is being absorbed and
>: re-emitted each time it hits a water molecule, but it is traveling at
>: c in the interstices between molecules.
>
>  The speed of the photon must slow down at least somewhere so that
>the average speed could be less than c.
>  If the photon is absorbed by a molecule, why does the photon leave the
>molecule in the exact same direction as it entered it?

Okay, new data.  This is from Feynman's _Lectures on Physics_, which 
everyone should own.  He does a non-differential-equation derivation
in chapter 31 of volume 1 for gases (because if the atoms are too 
close together you get interactions that are too hard to account for
without differential equations) which he then summarises in chapter
32 of volume 2 thusly:

  The electric field of the light wave polarizes the molecules
  of the gas, producing oscillating dipole moments.  The acceleration
  of the oscillating charges radiates new waves of the field.  This
  new field, interfering with the old field, produces a changed field
  which is equivalent to a phase shift of the original wave.  Because
  this phase shift is proportional to the thickness of the material,
  the effect is equivalent to having a different phase velocity in
  the material.

This is the wave-based version rather than the particle-based version,
but it makes more sense, I think.

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On 30 Sep 1999 19:04:55 -0400, par### [at] fwicom (Ron Parker) wrote:

>Okay, new data.  This is from Feynman's _Lectures on Physics_, which 
>everyone should own.  

I do, but they are in Bulgarian. Just so that you know :)


Peter Popov
ICQ: 15002700

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> No, it doesn't. If you put it in a vacuum, it won't turn. It turns

Yes, it does work in a vacuum actually.

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Simen Kvaal <sim### [at] studentmatnatuiono> wrote:
: This equation is to be used with bodies travelling at speed v
: *relative* to the observer, but light *always* travels at speed c *relative*
: to the observer.

  There is an effect known as Cherenkov radiation which happens usually in
nuclar reactors when neutrons travel faster than light in water.
  Those neutrons of course travel slower than c but faster than the speed
of the light in water. This causes a radiation which is an equivalent of
the sonic boom for light.

: You can now see that the windmill turns from the heat of the sunlight! The
: black sides absorb much more energy (pressure from the light) and the mill
: starts turning.

  You said it right: from the _heat_ of the sunlight.
  Of course the electromagnetic waves carry energy. I have never said
anything else. You can convert this energy to anything. You can boil water
with sunlight and make a steam engine run. You can use solar panels to
generate electricity and run a car if you want.
  But that's not due to the mass of the light.
  I have read about the solar windmill, and the reason for it to move is
not the mass of the light, but the heat caused by the energy it carries.
Even the theoretical amount of mass of the light would not be enough to
move an object because it's so small.

  The theory says that there's no difference between mass and energy. All
mass is just energy. Everything is just energy.
  This doesn't mean that everything is mass. The fact that everything is
made of atoms doesn't mean that everything is milk just because milk is
made of atoms. Every object has a color, but that doesn't mean that
every object is blue because blue is a color. Every computer program is
made of bits, but that doesn't mean that every computer program is povray
because povray is made of bits.

  (Cool, I succeeded in introducing povray somewhere in this off-topic
thread... :) )

:>main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
:>):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

: What does this code do? I've tried it with Borland, but I get some error
: messages. "Cannot call main from main" et.c.

  You have to compile it as C, not C++.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Lance Birch <lan### [at] usanet> wrote:
: I've got one of these specially made actually.  It's a specially blown glass
: "bubble" in a shape similar to a light bulb... (bulb at the top, a pin goes
: up through the middle point upwards and the mill sits down on top of it on
: the point) it is pressure sealed and thus there is not very much resistance
: inside against the "mills".

: If I leave it in the sun it gets up to about 120 RPM.  I find it quite
: interesting to observe :)

: My opinion, light creates pressure, therefore photons have mass

  Have you ever thought why it has to be a pressure sealed bulb? Why it
doesn't work without it?
  If the movement was caused by massive particles hitting the object, then
it shouldn't matter if it's inside a pressure sealed bulb or not.
  Have you ever thought that light can be used to power electric devices,
boil water, etc? Do you see any connection between these two things?

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Nieminen Juha wrote:

> : You can now see that the windmill turns from the heat of the sunlight! The
> : black sides absorb much more energy (pressure from the light) and the mill
> : starts turning.
> 
>   You said it right: from the _heat_ of the sunlight.
>   Of course the electromagnetic waves carry energy. I have never said
> anything else. You can convert this energy to anything. You can boil water
> with sunlight and make a steam engine run. You can use solar panels to
> generate electricity and run a car if you want.
>   But that's not due to the mass of the light.
>   I have read about the solar windmill, and the reason for it to move is
> not the mass of the light, but the heat caused by the energy it carries.
> Even the theoretical amount of mass of the light would not be enough to
> move an object because it's so small.

See also the off topic group for an continuation of this thread.

-- 
Ken Tyler
1100+ Povray, Graphics, 3D Rendering, and Raytracing Links:
http://home.pacbell.net/tylereng/index.html

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Why it seems to be so hard to understand, that even if we consider the
theoretical mass of the light as real mass, it would never be massive enough
to affect such a tiny object?
  Perhaps if we had a windmill of the size of the earth which weights about
0.1 grams and which doesn't get warm and is put in vacuum near the Sun, it
would theoretically begin to turn in a couple of hundred of years.
  The reason why it turns the tiny windmill is the same as why it is able
to run a car. It carries lots of energy.

-- 
main(i,_){for(_?--i,main(i+2,"FhhQHFIJD|FQTITFN]zRFHhhTBFHhhTBFysdB"[i]
):5;i&&_>1;printf("%s",_-70?_&1?"[]":" ":(_=0,"\n")),_/=2);} /*- Warp -*/

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Yes I do, and especially after reading the other messages you've written :)

I don't know enough about it at the moment, sorry.  But I feel you're right,
it makes more sense.

AAAAAAANNNNNNNYYYYYYYWWWWWWWAAAAAAAAAAYYYYYY, over to p.o-t I go.

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No, it wouldn't work in a total vacuum(the glass bulb still contains
some gas). Then there are no surrounding molecules to get the vanes
moving, and any force from the photons is insufficient to move the vanes
against the resistance of the needle. If you had a frictionless bearing,
a highly intense light source, and a lot of time, the vane might turn
detectably.
Lets move to povray.off-topic. :-)

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