What happens to the light once it's left the torch ?

abvidledUK

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Does it just dissipate ?

Does it just peter out ?

Does it get used up ?

Is it converted into something else ?

Is it re-cycled as photons, or something ?

Just wondering.....
 

TedTheLed

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I think it just keeps going till it hits something, then the photon is absorbed by the object while at the same time dislodging another photon if the material is reflective..does this make all reflected light recycled?

if Einstien is right and the photon doesn't hit anything it should come up behind you, eventually, and hit the rear end of where it came from.. or else it could keep going beyond our perceptible event horizon (the 'edge' of 'the universe') and be absorbed or reflected in another invisible universe, possibly bouncing back to where it came from, or more likely elsewhere..

or it could be absorbed into a black hole, where it would be eventually radiated back out into the universe in the form of gamma rays ..that would take a while..what happens to gamma rays, ie if they 'make' (visible) photons at some point in their life, I haven't the foggiest..

this could be erroneous, I just rememebr it from.. somewhere..
 
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carrot

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Oh boy, a smart question I can finally answer.

The law of conservation of energy states that energy cannot be created or lost; it can only be converted.

Thus, photons cannot be used up. A photon is an elementary particle created at the atomic level by an electron dropping from higher orbit (due to an excited state) back to its original orbit. An electron loses energy in the form of a photon, and the higher the orbit, the more energy an electron has. (Electrons like to go back to their original orbits.) This is best represented by phosphorescence in which the material is "charged up" and emits light as electrons drop out of their excited states.

We never see the full output of a light source because as light reflects off surfaces, photons may bounce in a way such that they do not reach our eyes. Brighter lights (with a wide spectrum of wavelengths-- white light) increase the chances that more photons will reach our eyes. Also, light emitted from any source (or any other form of energy) tends to spread out in multiple directions, which is why a beam gets bigger the further away it is from its target. It also explains why lasers have that "grainy" look -- since a laser outputs light of a single wavelength, the light stays in phase and does not spread out much (except in special cases, such as shining through slits where light light act as a wave) -- some spots are darker than others, and some are brighter, due to constructive and destructive interference caused by a surface not being perfectly smooth (a perfectly smooth surface would reflect light of a single wavelength all in one direction, and theoretically a laser shined at it would not appear grainy). Light at the same wavelength but out of phase by λ/2 (half a wavelength) cancel each other out, causing a dark spot in our perception of the laser beam, and light that is in phase causes a brighter spot. You can see this with a graphing calculator or GraphCalc, a free calculator for Windows -- or Grapher, which comes free with OSX Tiger -- assuming you're using radians, try y=sin(x) vs. y=sin(x)+sin(x+pi) (destructive interference) vs. y=sin(x)+sin(x) (constructive interference).

The light emitted from a flashlight, however, is all at different wavelengths (causing us to see it as white light, depending on the ratio of different wavelenghts put out), and also explains why we see things better with white light instead of monochromatic light. With white light, the chances of light reaching us is better, because there is a larger spread of photons moving at different wavelengths, and they end up complimenting each other (not like a pat on the back!). You can see this to an extent by shining two lasers of different colors (wavelengths) at the same point and noticing that the "grainy" look is less apparent. (Try green and red!)

What we see as "colored" materials actually just absorbs light of all the other wavelengths. A green piece of paper, for instance, absorbs the wavelengths that are not associated with "green" and reflects light of the 520-570 nm wavelengths back to your eye. Since energy cannot be lost most of this is converted into heat. (Like a microwave oven, which emits "light" at a frequency we cannot perceive with our eyes.) The light that is reflected that we do not see may get back to us in the form of ambient light. See also black body radiation and a source that Radio mentioned to me, http://galileo.phys.virginia.edu/classes/252/black_body_radiation.html.

We lose light output from the lens of a flashlight, as well. Light reflects off a surface when there is a change in refractive index of the mediums light pass through -- the transition from air to glass, for instance. AR coating works by gradually changing the refractive index between air and glass through the use of multiple (hundreds?) of layers. The higher the difference in refractive index of two mediums, the more reflective a surface is.

There are also lots of particles in the air that diffract light, preventing the photons from reaching us, which is why it's harder to see things in fog (lots of backscatter from light bouncing off water in the air) and partially why a light seems dimmer at a distance. (The other reason is because the eye is an effectively smaller target for the light to reach at a distance. Reference inverse square law on my favorite website, Wikipedia, for more information.)

Hope this helps. This is based on what I've been learning in physics (see, school *is* useful!) and my understanding of the topic. I'd recommend taking a look at the links above, too. If I'm wrong (I really hope I'm not) please correct me (so I can do better on my physics final). Now, back to my paper.
 
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greenLED

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In very general terms (in case you skipped carrot's post): Light can be transmitted (and partially absorbed as it goes through), reflected, or refracted off a surface.
 
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abvidledUK

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carrot said:
Oh boy, a smart question I can finally answer.

Thank you for the compliment.

I try to be though provoking.

A very comprehensive answer.

Now for the silly answers ..................(I like these too )
 

Sturluson

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Carrot,

What a comprehensive answer! I appreciate the effort it took, and award you an "A" for providing the best answer to the best question I've seen this month. You should email a copy to your physics teacher...

:goodjob:
 

AlexGT

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OK so electrons have to shed energy in order to create a photon, what would happen in theory if all photons are used up?, does that atom cease to exist? hmmm...

AlexGT
 

greenLED

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If all photons are "used up", that particular atom would have all its electrons in their corresponding orbits. Emitting photons (or not) per se does not define an atom's existence (unless I'm forgetting something, of course) :nana:
 

carrot

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AlexGT said:
OK so electrons have to shed energy in order to create a photon, what would happen in theory if all photons are used up?, does that atom cease to exist? hmmm...

AlexGT
Take a look at the first comment on this page: http://3quarksdaily.blogs.com/3quarksdaily/2005/05/particle_smashe.html

The relevant bit quoted here for easy reference:
[My father] was happy with me, I believe. Once, though, when I came back from MIT -- I'd been there a few years -- he said to me, 'Now,', he said, 'you've become educated about these things and there's one question I've always had that I've never understood very well and I'd like to ask you, now that you've studied this, to explain it to me,' and I asked him what it was. And he said that he understood that when an atom made a transition from one state to another it emits a particle of light called a photon. I said, 'That's right.' And he says, 'Well, now, is the photon in the atom ahead of time that it comes out, or is there no photon in it to start with?'

I says, 'There's no photon in, it's just that when the electron makes a transition it comes' and he says 'Well , where does it come from then, how does it come out?' So I couldn't just say, 'The view is that photon numbers aren't conserved, they're just created by the motion of the electron.' I couldn't try to explain to him something like: the sound that I'm making now wasn't in me. It's not like my little boy who when he started to talk, suddenly said that he could no longer say a certain word -- the word was 'cat' -- because his word bag was run out of the word cat. So there's no word bag that you have inside so that you use up the words as they come out, you just make them as they go along, and in the same sense there was no photon bag in an atom and when the photons come out they didn't come from somewhere, but I couldn't do much better. He was not satisfied with me in the respect that I never was able to explain any of the things that he didn't understand. So he was unsuccessful, he sent me through all these universities in order to find out these things and he never did find out.
From "The Pleasure of Finding Things Out" edited by Jeffrey Robbins
(A really good story -- I recommend reading it.)
 
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