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Thread: Photon Question

  1. #1 Photon Question 
    Forum Professor leohopkins's Avatar
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    Now one about photons.

    If something is green in colour, it is green because it "absorbs" all other wavelengths of visible light and "reflects" the photons who carry the green wavelengths.

    My question is, what is meant by "absorb" - i.e what happens to the photon ?

    and what is meant by "reflect" ? - do the green photons merely "bounce" off or are repelled by the electron cloud ? and if so - how ?


    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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    Absorption: A photon is merely a quantum of energy. When it interacts with matter the photon is captured (absorbed) by the matter and the matter acquires the energy. The photon ceases to exist.

    Reflection: Again the photon is captured by an atom and it is destroyed. But the energy is just the right amount to excite an electron into a new orbital. After a very short delay the atom returns to its ground state, emitting a new photon of the same frequency as the original photon, but not necessarily the same phase.

    Note that photons do not behave as classical particles. They do not "bounce off" when they interact with matter. They are quantum particles and behave according to their own laws of physics.


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    Quote Originally Posted by SteveF
    Absorption: A photon is merely a quantum of energy. When it interacts with matter the photon is captured (absorbed) by the matter and the matter acquires the energy. The photon ceases to exist.

    Reflection: Again the photon is captured by an atom and it is destroyed. But the energy is just the right amount to excite an electron into a new orbital. After a very short delay the atom returns to its ground state, emitting a new photon of the same frequency as the original photon, but not necessarily the same phase.

    Note that photons do not behave as classical particles. They do not "bounce off" when they interact with matter. They are quantum particles and behave according to their own laws of physics.
    Hi, okay thanks. How does it work with a reflective surface such as a mirror ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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    I can tell you many interesting things about reflection, such as about phase reversal, polarization and Brewster's angle, non-reflective films and such, but you'd first have to start a new topic. None of this information belongs under the misleading "photon question."

    All of the good stuff is readily explained using classical theory, that light is a wave, an electromagnetic wave. It can't be done using photons. I mean, not with the beginner's notion of photons as little ball bearings with classical properties. Photons have their own weird quantum properties.

    For those who always insist on treating light as a stream of little marbles behaving classically, all we can really do for them concerning reflection from a mirror is to say that "the photons just bounce off. The angle of incidence equals the angle of reflection."

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    Quote Originally Posted by SteveF
    Reflection: Again the photon is captured by an atom and it is destroyed. But the energy is just the right amount to excite an electron into a new orbital. After a very short delay the atom returns to its ground state, emitting a new photon of the same frequency as the original photon, but not necessarily the same phase.
    Steve, I was getting ready to disagree with your explanation, considering that a mirror reflects light in a certain direction, but then I looked up the Wikipedia article on reflection and it says "Specular reflection (following Hero's equi-angular reflection law) is a quantum mechanical effect explained as the sum of the most likely paths the photons can take." I guess this is just more wave-particle duality weirdness.
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    ... is a quantum mechanical effect explained as the sum of the most likely paths the photons can take."
    Harold, that is true of course, but what does it tell you? Nothing. Absolutely nothing. One is no better placed than before.

    To deal with reflection by quantum means, one must realize that photons emerge in all possible directions and with all possible phases. Then you set up the probability function as vectors and integrate it through all of space. The result, after much mathematical labor, is that photons not in the reflected path will cancel each other out due to phase differences. You get the same incidence/reflected result as is apparent just by observation.

    It's nice to know the problem can be solved in quantum terms, but it is so much more effective to deal with such properties of light in classical terms.

    Polarization, anyone?

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