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Author Topic: Why does light travel in a straight line?  (Read 54421 times)

Offline Geezer

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Why does light travel in a straight line?
« Reply #25 on: 20/08/2009 20:36:20 »
.....and String Theory seems even worse (than QM).

QM is a bit like Thermodynamics - Don't even try to understand it. It works, so just use it!

Thanks for the info!
 

Offline VernonNemitz

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Why does light travel in a straight line?
« Reply #26 on: 21/08/2009 00:57:47 »
Knut Overskeid  asked the Naked Scientists:
When light enters a medium, it slows down. I have heard that this is because it is constantly exiting electrons in the medium and subsequently being re-emitted by those electrons a short time later.
Why does it then travel in a straight path?
I thought that electrons emit photons in a totally random direction, which would disperse a beam quite quickly.
I suspect the very simplest reasonably accurate answer to your question involves the Law of Conservation of Momentum.  Photons do have momentum. An electron that absorbs/re-emits it is part of an atom, and that atom's overall interaction with the photon cannot violate Momentum Conservation.  Net result: the photon very likely gets re-emitted in the same direction it was travelling when it got absorbed.
 

Offline Geezer

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Why does light travel in a straight line?
« Reply #27 on: 21/08/2009 06:17:24 »
Lightarrow:

Apologies, as this post is not directly related to this thread, but would you happen to know of a good reference for the famous double-slit experiment? I have a lot of "how-do-we-know" type questions about the experiment. I'm sure they have been asked by countless people over the years, and they probably should not be asked in this forum.
 

Offline lightarrow

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Why does light travel in a straight line?
« Reply #28 on: 21/08/2009 12:32:45 »
Lightarrow:

Apologies, as this post is not directly related to this thread, but would you happen to know of a good reference for the famous double-slit experiment? I have a lot of "how-do-we-know" type questions about the experiment. I'm sure they have been asked by countless people over the years, and they probably should not be asked in this forum.

http://research.microsoft.com/apps/tools/tuva/#data=4|0||||

You will be asked to install a program; do it and then you will have access to 7 videos of lectures made by R.Feynman in the 60'. Watch lecture n.6
 

Offline Geezer

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Why does light travel in a straight line?
« Reply #29 on: 23/08/2009 04:46:38 »
Lightarrow:

The Fynman lectures are excellent. Thanks!

I have found references to fairly recent double-slit experiments demonstrating that single electrons interfere with themselves. Can you point me at any similar experiments conducted with photons rather than electrons?

One of Fynman's statements is troubling me. While explaining the interference of waves as they reach the screen, he said that the peak amplitudes of the lower amplitude constructive interference distributions were a consequence of interference between the crest of one wave and the crest of the subsequent wave. Of course, this makes sense when there is a continuous supply of wave crests, but I would think it is unlikely in the case of a single photon, speedy as they are.
 

Offline lightarrow

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Offline lightarrow

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Why does light travel in a straight line?
« Reply #31 on: 23/08/2009 10:46:53 »
One of Fynman's statements is troubling me. While explaining the interference of waves as they reach the screen, he said that the peak amplitudes of the lower amplitude constructive interference distributions were a consequence of interference between the crest of one wave and the crest of the subsequent wave. Of course, this makes sense when there is a continuous supply of wave crests, but I would think it is unlikely in the case of a single photon, speedy as they are.
We cannot think to a photon as a localized corpuscle and even if we could, we cannot (at the moment) say that is "made of" em waves or anything else.
« Last Edit: 23/08/2009 10:49:04 by lightarrow »
 

Offline Geezer

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Why does light travel in a straight line?
« Reply #32 on: 23/08/2009 16:49:46 »
Quote
We cannot think to a photon as a localized corpuscle and even if we could, we cannot (at the moment) say that is "made of" em waves or anything else.

Agreed. All we can say is that we detect photons at the screen, and (if I have it right) their distribution has interference "characteristics". That might lead us to believe that waves are somehow involved, but there is no evidence to support that assumption.

Hope that sounds about right.

I'm interested in understanding how we know we only put a single photon into the experiment. I'll study your references. Thanks!
 

Offline labview1958

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Why does light travel in a straight line?
« Reply #33 on: 30/11/2009 09:33:54 »
When water waves pass over a piece of glass at an angle in  a ripple tank, they change  direction. If we find the incidence and refracted angle, can we find the refractive index of the glass?
 

Offline yor_on

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Why does light travel in a straight line?
« Reply #34 on: 03/12/2009 00:54:02 »
Knut Overskeid  asked the Naked Scientists:
   
Hi

When light enters a medium, it slows down. I have heard that this is because it is constantly exiting electrons in the medium and subsequently being re-emitted by those electrons a short time later.

Why does it then travel in a straight path?

I thought that electrons emit photons in a totally random direction, which would disperse a beam quite quickly.

Thanks!

Knut Overskeid

What do you think?

How about the momentum?
Won't that impose a direction?
Particlewise that is.
----------

But in reality I much prefer Laura_Kelly's definition of Feynman's definition 'sum over all paths' or whatever it's called. :)

But it does make our universe mystical.
« Last Edit: 03/12/2009 01:08:00 by yor_on »
 

Offline lightarrow

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Why does light travel in a straight line?
« Reply #35 on: 03/12/2009 20:50:49 »
When water waves pass over a piece of glass at an angle in  a ripple tank, they change  direction. If we find the incidence and refracted angle, can we find the refractive index of the glass?
"Refraction" of water waves in a piece of glass?  ??? ??? ???
 

Offline Mr. Scientist

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Why does light travel in a straight line?
« Reply #36 on: 03/12/2009 20:59:40 »
Photons follow geodesics in spacetime which is itself a bent spacetime pathway http://en.wikipedia.org/wiki/Geodesic . If there is no presence of gravity, then photons tend to move in straght lines.
 

Offline itisus

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Why does light travel in a straight line?
« Reply #37 on: 09/12/2009 03:56:46 »
Light travels on a geodesic in a vacuum.  It's the shortest path, so you can call it a straight line.  Of course "vacuum" is a relative term.
 

Offline johanwarn

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Why does light travel in a straight line?
« Reply #38 on: 16/12/2009 11:10:14 »
Hi, I am Johan

Thanks you all to give us this helpful information..

please post more information about this matter...


Regards

Johan Warn



Mod edit - Spam Removed.  Once more will result in a ban.
« Last Edit: 28/01/2010 11:18:52 by BenV »
 

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Why does light travel in a straight line?
« Reply #38 on: 16/12/2009 11:10:14 »

 

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