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Why do photons not pass straight through objects?

  1. Mar 27, 2012 #1
    so i often hear the analogy of an atom being on the same scale as a foot ball in the center of a pitch, with the electrons all in their orbitals at some large distance away.

    in chemistry we have been discussing the difference in energy levels of electrons and using these to describe the colours of certain compounds.

    i am imagining that the spaces between atoms are much larger than the 'gap' a photon can fit through, so what is it that causes the photon to change electrons energy states?

    also, if the transparency of glass is explained by saying it has an amorphous structure, why does this make it transparent? i would have thought density played a part here, but what do i know?

    anyway, if you can clear any of this up for me, i'd be very grateful.

  2. jcsd
  3. Mar 27, 2012 #2


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    Photons primarily interact with electrons in matter. Both photons and electrons behave as waves. Because electrons don't have a specific position, there really aren't any gaps for a photon to fit through.

    As for transparency of glass, it's not because photons somehow manage to fit "between" particles. They move right through. Photon can only be absorbed by electrons in matter if it can take an electron from its current energy level to a higher one. If there are no energy levels available corresponding to energy of the photon, the photon is not absorbed.

    Glass being amorphous has nothing to do with it, though. Quartz is the crystalline form of the same material, and quartz is also transparent. As well as many other crystals.
  4. Mar 27, 2012 #3


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    There are various intuitive pictures you can take to explain this, none correct. You can view the photon as much larger than the atom (with a "size" of the wavelength of the photon), so it's not going to fit in the gaps between electrons in an atom and it's going to hit everything in its way. Or you can view the electron cloud as occupying the whole space around the atom, so there is no gap in the first place.

    There is not a unique concept of "size" in quantum mechanics, and there is no analog to the macroscopic concept of size. For colliding two objects, we use something called the cross section to describe the size, where the cross section is a cross sectional area. The difficulty comes from the fact that the cross section of a particle depends on what it is colliding with, and what kind of collision is taking place, and it can be 0 or finite or infinite.
  5. Mar 27, 2012 #4
    It is now known that photons of visible sunlight pass through and scatter off of individual atoms of air in the atmosphere, producing polarized blue skylight. This scattering (which attenuates the direct sunlight) does not depend on electron energy levels or ionization of the air. Lord Rayleigh worked out the equations of scattering (the famous 1/λ4 law) and the polarization in his paper "On the Light from the Sky, its Polarization and Colour", in Phil. Mag. XLI, pages 107-120 and 274-279 (1871).

    This paper was published about 25 years before the discovery of the electron, 35 years before Rutherford scattering, and 45 years before Bohr's model of the atom. Needless to say, Lord Rayleigh's model did not require any of these.
  6. Mar 27, 2012 #5


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    Yes, to do wave optics, all you need are Maxwell's Equations. They do not, however, completely explain propagation of electromagnetic wave through matter. In particular, none of it explains why some materials are opaque and some are transparent despite very similar electric properties. You do need quantum physics to explain that one.
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