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#26 Qwickdraw

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Posted 31 July 2013 - 11:09 AM

Also, the re-emitted photons aren't bounced back in the same direction from whence the originals came. If they were, a person could look at a mirror from any angle and still see his or her image. Reflected photon(s) will follow a path that's equal in incidence to the optic normal but opposite in direction to the incoming photon(s).


Sure they do, but only if the angle of incidence equals zero. One of the principles behind a LASER.

#27 Pess

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Posted 31 July 2013 - 02:14 PM


Your light is off toward the outer reaches of the cosmos. Whatever part of it isn't blocked by matter, interstellar dust or other junk will just keep on travelling forever at the speed of c. You can never catch up to it.

/Ira


I am not so sure. What if the Universe is closed and bounded? So that any straight line comes back on itself?

All one needs to do to catch a photon is stand from where it was emitted and catch it as it comes around.

Pesse (Bring a magazine to pass the time) Mist

#28 StarWars

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Posted 01 August 2013 - 02:42 PM



Sun light vs. man made light...


When I go outside I can feel the sun light as a pressure on my skin.

Mankind still does not know how to harvest the power of the sun.... :ohmy:

Maybe if they build a solar collector with Fresnel lens... :grin:

#29 scopethis

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Posted 01 August 2013 - 07:05 PM

since light is absorbed by objects, can one squeeze light out of a sponge that has been exposed to sunlight?

#30 ColoHank

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Posted 01 August 2013 - 09:50 PM

since light is absorbed by objects, can one squeeze light out of a sponge that has been exposed to sunlight?



Why not try that and share your findings with us? First, you'll need a sponge. Then, expose it to light. Finally, take the sponge into a very dark room and squeeze it as hard as you possibly can. Be patient; it might take a long time for the light to come out.

#31 llanitedave

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Posted 01 August 2013 - 11:34 PM

Make sure you have the supplies to clean up the mess afterward.

#32 Pess

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Posted 02 August 2013 - 07:01 AM

since light is absorbed by objects, can one squeeze light out of a sponge that has been exposed to sunlight?



Why not try that and share your findings with us? First, you'll need a sponge. Then, expose it to light. Finally, take the sponge into a very dark room and squeeze it as hard as you possibly can. Be patient; it might take a long time for the light to come out.


The sponge atoms almost immediately re-emit absorbed photons...that's why you can 'see' a sponge by looking at it (your eyes detect the re=emitted photons).

However, if you squeeze hard enough to overcome the Coulomb forces, you'll generate plenty of new photons. However, make sure you hire a janitorial service to keep Dave happy because you might not enjoy the experience.

Pesse (clean up on aisle one) Mist

#33 derangedhermit

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Posted 02 August 2013 - 05:18 PM

Photons from the Sun's core don't make it directly to the surface. They are generaleast tens of thousands of years to dissipate outward, but it's not because any individual packet is traveling slowly. Also, since it's emitted each time in a random direction, as often as not it gets sent back inward again.

The article said the same thing ("photon absorbed, photon later emitted") was happpening in this case, unless my reading skills have failed entirely.

#34 wirenut

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Posted 03 August 2013 - 07:37 PM

since light is absorbed by objects, can one squeeze light out of a sponge that has been exposed to sunlight?



Why not try that and share your findings with us? First, you'll need a sponge. Then, expose it to light. Finally, take the sponge into a very dark room and squeeze it as hard as you possibly can. Be patient; it might take a long time for the light to come out.

you may want to wear eye protection like a wielders mask or dark sunglasses just in case it soaked up too much light.

#35 Ira

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Posted 03 August 2013 - 08:14 PM

Does the sponge actually reemit the photons or just reflect the same photons that hit it?

/Ira

#36 scopethis

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Posted 04 August 2013 - 09:33 PM

I was gonna try what many have suggested but my wife won't let me go into a dark room with a sponge...

#37 Pess

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Posted 05 August 2013 - 07:38 PM

Does the sponge actually reemit the photons or just reflect the same photons that hit it?

/Ira


1) Photons always travel in a straight line

2) Photons always travel at exactly 'c'

3) Photons do not reflect, bounce off or otherwise change direction. They travel in a straight line until they hit something. Their energy is absorbed (usually by the electron shell of an atom) and then can be re-emitted at a different wavelength, the same wavelength and/or any vector all dependent on the energy of the photon and what it hits.

When you look at a reflection in a mirror, you are not detecting a single photon that 'hit' the mirro5r. You are detecting the re-emitted photons the atoms of the mirror re-emitted after the original photons hit them.

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#38 Ira

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Posted 05 August 2013 - 08:33 PM

Really? I find this staggering. I never thought of it that way. So, how does gravitational lensing work if photons always travel in a straight line?And if photons are reemitted by whatever they strike why does <angle>i=<angle>r? Why aren't photons reemitted every which way?

/Ira

#39 llanitedave

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Posted 05 August 2013 - 09:14 PM

Straight lines can curve in gravitational fields.

(The second and third questions have always made me wonder too)

#40 Pess

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Posted 06 August 2013 - 11:36 AM

Really? I find this staggering. I never thought of it that way. So, how does gravitational lensing work if photons always travel in a straight line?And if photons are reemitted by whatever they strike why does <angle>i=<angle>r? Why aren't photons reemitted every which way?

/Ira


What Dave said about gravity & such.

Take a piece of paper.

Draw a razor straight line on it.

Now crumple the paper.

Is the line still straight? What changed?

Pesse (Fun with a 2D Universe) Mist

#41 scopethis

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Posted 06 August 2013 - 11:38 AM

if that's the case; when I turn off the light in a room, the area should remain well illuminated....

#42 ColoHank

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Posted 06 August 2013 - 12:01 PM

if that's the case; when I turn off the light in a room, the area should remain well illuminated....



Who knows? The room may remain illuminated for another trillionth of a second or so (it depends on the size of room and the texture and color of the walls, etc.) until all of those light photons bouncing around are absorbed and converted to heat energy. I'd attempt to measure it, but my Timex's stopwatch only registers down to 1/100 second.

#43 Pess

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Posted 06 August 2013 - 12:39 PM

if that's the case; when I turn off the light in a room, the area should remain well illuminated....



Who knows? The room may remain illuminated for another trillionth of a second or so (it depends on the size of room and the texture and color of the walls, etc.) until all of those light photons bouncing around are absorbed and converted to heat energy. I'd attempt to measure it, but my Timex's stopwatch only registers down to 1/100 second.


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#44 scopethis

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Posted 06 August 2013 - 10:33 PM

shape the room like a pyramid and line the walls with highly reflective mirrors....ah, the Indian tee pees were so close..

#45 dickbill

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Posted 09 August 2013 - 12:34 PM

hmmm, when you say a photon is absorbed and reemitted, it cannot be reemitted with the same energy, right?
Otherwise it seems to me it'd be a violation of thermodynamic's 2nd law: entropy must increase, some energy must have been lost in the process or it's like there had been no event at all. So i would assume the photons are slightly 'redened' after the event. So a reflected image, for example, should be slighty redened.
That's one thing.

The other thing now is about quantum uncertainty principle. If the absorbtion/reemission is a quantum event, i'd assume that based on this principle, the energy of the reemitted photon should fluctuate a bit, with some reemission events quite far energetically from the absorbtion event in both redened and blued effects. Here the colors of the reflected image should be blurred.

Am i all wrong here?

#46 llanitedave

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Posted 09 August 2013 - 02:07 PM

Entropy doesn't apply to single photons as far as I know, only an aggregate of them. As I understand it, and could be mistaken, reflection efficiency does not affect the wavelength or energy of the photons, but how many are simply not reemitted at all. Those few that are absorbed would go into very slightly heating the mirror.

#47 dickbill

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Posted 09 August 2013 - 02:56 PM

You make the case that thermodynmic doesn't apply to single event. I am still annoyed by the concept of any event that would occur without an energy difference, but ok.

But what about the quantum uncertainty then? why should the energy of the reemitted photon be exactly the same as the absorbed photon?

#48 dickbill

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Posted 09 August 2013 - 03:03 PM

Oh, i got it, it's dictated by the electronic levels of energy, which are set. So if one electron absorbes one photon and jump one level, it can only go back to the initial level by releasing the same amount of energy.
But why the uncertainty principle doesn't apply, no matter what?

#49 Carl Coker

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Posted 09 August 2013 - 04:22 PM

The uncertainty principle does apply, and the energy will almost certainly not be exactly the same. But it will be roughly the same, and which way it changes (and how much) is completely random. Over many events, the re-emitted energy distribution is approximately a Gaussian with a width inversely proportional to the lifetime of the excited state.






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