Question regarding photons

Discussion in 'General Science & Technology' started by Equinox, Nov 29, 2017.

  1. Equinox Registered Senior Member

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    I've been trying to find out exactly at what point a photon's energy is 'absorbed' and if there mere observation results in the photon and its energy being absorbed.

    For instance if I look through my telescope and (if it where possible to observe single photons) and see a photon travelling from Alpha Centauri towards Proxima Centauri - does my observation of the photon 'absorb' it? and if not does my observation affect the amount of energy it will have if it's then absorbed by a solar panel?

    I'm having trouble getting my head around how the energy of a photon works - for instance if I took a photograph of a photon does that use 'use up' all it's energy or can it continue on it's way to charge a solar panel for instance.
     
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  3. mathman Valued Senior Member

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    When you "see" a photon, it means it hit your eye, where it is absorbed. Yu would not "see" a photon going from one star to another.
     
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  5. Seattle Valued Senior Member

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    As mathman points out...light radiates. What you see is absorbed.
     
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  7. Equinox Registered Senior Member

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    Thank you both

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    It's kind of hard to get my head around it. For instance if you point a laser pen into the sky - the photon's are travelling upward (you can see the direction of the beam) but they are also travelling towards your eyes - so in fact the only reason you can see the direction of the beam is because of the photon travelling towards your eyes even though the actually beam is pointed elswhere...

    I guess i'll leave that kind of thinking to people cleverer than I.
     
  8. geordief Registered Senior Member

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    When you see the beam traveling up in the air from a laser pen,that is because it(the beam) is being partially reflected back by molecules in the air.

    If you were to try the same thing in space the beam would be completely invisible.

    Am I smart;-)
     
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  9. James R Just this guy, you know? Staff Member

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    Equinox:

    What do you mean by "mere observation"? I would probably replace that with something like "detection".

    You can only see a photon that travels into your eye and is absorbed by your retina. You can't see photons "in transit". If you catch a photon while it is travelling, then you'll absorb it somehow, provided you detect it at all.

    There's no chance of seeing photons travelling from Alpha Centauri towards Proxima Centauri, unless you put yourself in their path. You certainly will never see those photons from Earth.

    With photons, it's all or nothing. Either the photon is absorbed, or it is free to continue travelling. You can't absorb half a photon. That's the whole point of the term "quantum of light". A quantum is a discrete chunk, indivisible.

    It uses up all the energy of that any photon that is absorbed in the photographic process.

    When you consider something like a mirror, there are two ways of looking at what the mirror is doing. One is that it reflects photons without absorbing them (well, most of them; no mirror is a perfect reflector). The other way to think about it is that the mirror absorbs one photon and emits an almost identical one, only with a different direction of travel.

    No. Individual photons only travel upwards or downwards; they can't do both. Your laser pen emits billions upon billions of photons every second, and most of them go up into the sky. A few of them hit dust particles in the air, and those dust particles can then reflect some photons back down into your eye, just like a mirror would.

    Hopefully it is clear to you by now that photons generally travel in straight lines. They never turn in flight.*

    ---
    * I'm ignoring complications like the bending of light by gravity, and more complicated behaviours like diffraction, which require a wave-like description of light as opposed to the particle-like picture we've been discussing above.

    ---

    Yes, you're smart.

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  10. Equinox Registered Senior Member

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    Thank you for the very comprehensive answer

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  11. origin Trump is the best argument against a democracy. Valued Senior Member

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    Yea! A question was asked, several people joined in the discussion and the question was answered to the satisfaction of the thread author. Nobody attacked another person and nobody got angry! Ain't that just dandy, I knew it was possible.

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