1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Light, refelection, transparency, direction?

  1. Dec 23, 2008 #1

    I have done a lot of digging on the net for some answers about light interaction with solids and can't find some answers to some questions.

    1. How does light pass STRAIGHT through a transparent object?

    I see a lot of references say that the electrons don't have available energy levels to match the incoming photons, and therefore the photons pass right though. I have some difficulty with this because we know that light travels slower through a solid, so it can't be passing through unimpeded. I've seen other references that say that the photons do interact with the electrons, which absorb and then retransmit the photons. I find this a little easier to believe, however, how do the re-emitted photons know to keep going in the same direction?

    2. Why does light reflect STRAIGHT off of the surface of a substance?

    Again, there must be some photon/electron interaction (I'll buy that much), but how is it that we get a nice clean reflected image? What makes the re-emitted photons travel in such a predictable direction off the surface? Even if the atoms on the surface formed a perfectly flat plane (which would never happen) why wouldn't those surface electrons re-emit the light in random directions?

    3. This is a bit of a side question... I'm just wondering about the scale of atoms in a solid substance. I often hear that an atom is mostly space. I also hear that photons interact with the electrons but not the nucleus. I think the implication is that a solid substance is also, mostly space, and therefore photons can easily pass right through (unless they interact with the electrons). My question is...even though the atoms are mostly space, since there are so many in even the smallest amount of matter, wouldn't the path of the photons always be blocked by the nuclei?

    Last edited: Dec 24, 2008
  2. jcsd
  3. Dec 24, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    You may want to start by reading the FAQ first. It doesn't answer all of your question here, but it will, at least, correct your misunderstanding about the interaction of photons/light with solids, i.e. this isn't influenced by individual atoms.

  4. Dec 24, 2008 #3
    Thanks. In the FAQ I like the part about the "collective behaviour" of the atoms interacting with the photons (makes sense and explains a few things).

    BTW - I know that scientific proof doesn't come from a popularity contest, but...
    It is curious that a Google search doesn't turn up too much info on transparency/phonons/photons, yet there are lots of hits with transparency/electrons/photons. Unfortunately those hits don't seem to fully explain the interaction.

    So if we go with the phonon explanation, what is the interaction between the photon and the lattice? Is the photon hitting the nucleus and making the lattice vibrate? Is it interacting with the electrons? Is it causing movement of the electrons or movement of the entire atom?

    I think the explanation was saying that electrons in individual atoms can only jump into discrete energy states, so there must be something else that absorbs or absorbs/retransmits other energy levels. I assume that as entire atoms move toward/way from each other, there must be a wide continuous spectrum of energy levels that can be satisfied, is that the missing part?

    Anyway, I would still like to know more detail about the interactions and especially why the interaction causes light to go STRAIGHT, whether it it passes through, or is reflected from a substance.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook