Enlightening photons

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Enlightening photons

#1  Postby Adco » Sep 05, 2012 1:17 pm

I am thinking about photons. They fascinate me daily. Every time I "look" at something I wonder how many photons are all around me at once, bouncing, reflecting, emitting. There must be a mega gazillion all the time. Even in areas where there are shadows. The photons must fly around, reflecting off surfaces to also light up places in the shadows. A shadow is just an area that has less photons hitting it.

What about when photons bump into each other? That must happen all the time. I guess nothing happens, they just carry on flying around until they get absorbed or reflected. OK, there's one answer sorted.

Has anyone calculated the numbers involved? Is it possible to even try? Just concentrate on any image around you. There are never any glitches or breaks in the image under normal viewing. The photons just keep coming, non-stop. It's not like a spray of water coming out a hose pipe that could get slowed down with high speed cameras so that you can see the individual drops, or is it?

Do our eyes, and brains, just interpret it as continuous or do they arrive in bursts or packets (quanta) to fast for us to be visually affected? I've never observed any flickering when you watch very high speed movies of balloons bursting etc. If they do arrive in packets, it must be at an extremely fast rate. And, how many photons in a packet?

Perhaps this is a silly obsession that I have with photons but hopefully there are a few other out there that also regard them as awesome and intriguing particles as I do.
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Re: Enlightening photons

#2  Postby trubble76 » Sep 05, 2012 1:21 pm

Here's something that spun my brain; photons are invisible. If you could see photons, you wouldn't be able to see much else.
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Re: Enlightening photons

#3  Postby Adco » Sep 05, 2012 1:23 pm

Thanks for nothing. Now I have something else to ponder about.
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Re: Enlightening photons

#4  Postby DavidMcC » Sep 05, 2012 1:49 pm

Adco, MOST photons are certainly invisible to us. It's only those with energies corresponding to wavelengths in the region of about 400nm to less than about 800nm that are actually visible to us. That is a rather narrow range, corresponding to about 1.5 to about 3eV. EM radiation covers a hugely greater range than that, such as wavelenths up to Km and photon energies of many MeV. At photon energies of less than about 0.5MeV, they cannot interact with each other, but as soon as that threshold is exceeded, electon-positron pairs can be created by the collision of two photons, so they do interact at these high energies. Obviously, these are gamma rays, and we don't want to interact with them, and certainky don't see them.
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Re: Enlightening photons

#5  Postby epepke » Sep 05, 2012 1:53 pm

You'll enjoy this, or it will make your brain hurt, or both:

http://electron6.phys.utk.edu/phys250/m ... hotons.htm
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Re: Enlightening photons

#6  Postby Adco » Sep 05, 2012 1:59 pm

DavidMcC wrote:Adco, MOST photons are certainly invisible to us. It's only those with energies corresponding to wavelengths in the region of about 400nm to less than about 800nm that are actually visible to us. That is a rather narrow range, corresponding to about 1.5 to about 3eV. EM radiation covers a hugely greater range than that, such as wavelenths up to Km and photon energies of many MeV. At photon energies of less than about 0.5MeV, they cannot interact with each other, but as soon as that threshold is exceeded, electon-positron pairs can be created by the collision of two photons, so they do interact at these high energies. Obviously, these are gamma rays, and we don't want to interact with them, and certainky don't see them.

I know about photons being EM radiation. That's another mind boggler for me. I was thinking more about those in the visible spectrum. Don't know why because they are all the same, just how my thoughts went. We are being bombarded by countless photons every second. There are so many zipping around in the form of radio, TV and other communication waves and yet we are hardly affected by them, luckily. Of course there are dangerous ones that do harm us like X-rays and Gamma rays to name a couple.
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Re: Enlightening photons

#7  Postby DavidMcC » Sep 05, 2012 2:07 pm

Adco wrote:Do our eyes, and brains, just interpret it as continuous or do they arrive in bursts or packets (quanta) to fast for us to be visually affected? I've never observed any flickering when you watch very high speed movies of balloons bursting etc. If they do arrive in packets, it must be at an extremely fast rate. And, how many photons in a packet?

To all intents and purposes, visible photons normally arrive on a semi-continuous basis at a given photoreceptor cell, unless the light level is practically pitch darkness.
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Re: Enlightening photons

#8  Postby Adco » Sep 05, 2012 2:12 pm

I found this interesting.

http://en.wikipedia.org/wiki/Absolute_threshold

Also found a heap of stuff where you can calculate how many photons are released under given conditions.

http://electron6.phys.utk.edu/phys250/m ... hotons.htm

That answers a lot for me.
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Re: Enlightening photons

#9  Postby Adco » Sep 05, 2012 2:19 pm

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Re: Enlightening photons

#10  Postby DavidMcC » Sep 05, 2012 2:30 pm



This merely says that photons are bosons, and therefore can be created (emission) and destroyed (absorption) without breaking any laws of physics.
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Re: Enlightening photons

#11  Postby DavidMcC » Sep 05, 2012 2:36 pm

Adco, bear in mind that the visible threshold described in your Wiki link is about light levels that are orders of magnitude lower than the levels that enable the cone cells in the fovea to fire at a "visible" rate. We do not normally see single photons, because there is usually enough random background neuron firing to disguise such flashes.
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Re: Enlightening photons

#12  Postby Adco » Sep 05, 2012 2:54 pm

DavidMcC wrote:Adco, bear in mind that the visible threshold described in your Wiki link is about light levels that are orders of magnitude lower than the levels that enable the cone cells in the fovea to fire at a "visible" rate. We do not normally see single photons, because there is usually enough random background neuron firing to disguise such flashes.

I realize that. It's just interesting, that's all.
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Re: Enlightening photons

#14  Postby DavidMcC » Sep 14, 2012 11:33 am

Jakov wrote:Frogs eyes can see individual photons.

Presumably, you're referring to this, Jakov:
http://physicsworld.com/cws/article/news/2012/sep/13/frog-photoreceptor-counts-photons
Frog photoreceptor counts photons

Some researchers have claimed the same for human eyes, but that is controversial, because of the background noise in real eyes. I don't know if frogs' rod cells suffer from the same problem when in the context of the frog's eye, rather than this experimental set-up. :dunno:
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Re: Enlightening photons

#15  Postby Jakov » Sep 14, 2012 12:03 pm

I read it in the book The Fabric of Reality by David Deutsch. I guess it says the citation at the back of the book but I don't have it to hand so can't tell you.
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Re: Enlightening photons

#16  Postby Made of Stars » Sep 14, 2012 12:16 pm

Made of Stars, by Neil deGrasse Tyson and zenpencils

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Re: Enlightening photons

#17  Postby Adco » Sep 14, 2012 1:06 pm

Made of Stars wrote:You might like this Adco. :)

http://www.ted.com/talks/ramesh_raskar_ ... econd.html

Yeh, I saw that the other day. Really cool. He uses a special technique so it's not quiet what it says it is but it is fascinating, thanks.
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Re: Enlightening photons

#18  Postby DavidMcC » Sep 14, 2012 2:43 pm

It's funny how studies of the propagation of photons gives such a different view of them than the studies of absorption and inelastic scattering (in the case of gamma rays) of the same. As you will know, it's called wave-particle duality.
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Re: Enlightening photons

#19  Postby Adco » Dec 12, 2013 9:21 am

http://www.wyzant.com/resources/answers ... e_produced

That is a shit load of photons flying around at once.
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Re: Enlightening photons

#20  Postby newolder » Dec 12, 2013 11:26 am

Adco wrote:I am thinking about photons. ...

What about when photons bump into each other? That must happen all the time. I guess nothing happens, they just carry on flying around until they get absorbed or reflected. OK, there's one answer sorted.

...

Nope. http://physics.aps.org/synopsis-for/10. ... 111.080405
Despite what movie lightsabers suggest, light beams pass through each other without effect. However, two photons will, on rare occasion, bounce off each other. This elastic photon-photon scattering, which occurs via intermediate particles, has never been observed directly, but a new analysis in Physical Review Letters shows that the Large Hadron Collider (LHC) at CERN could detect around 20 photon-photon events per year...
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