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What happens when light interacts with the boundary between 2 mediums at a quantum level? Why is it totally reflected back when it is travelling from an optically denser to a less dense medium? How does the quantum of light know that its going through such a boundary?

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"How does the quantum of light know that its going through such a boundary?"

By obeying the quantum mechanical solution of the boundary value problem "photon scattering off boundary"

Here is the light phenomenon:

Total internal reflection is a phenomenon which occurs when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected.

The refractive index is a collective measure of the atoms/molecules of the lattice on which the light is incident and depends on the collective fields built up by them. This is the field on which each individual photon will be scattering and partakes of the boundary conditions of the problem.The fields must be such that the individual photons from that angle on are scattered in only a backward direction .

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  • $\begingroup$ This answer has the flavor of "it happens because it happens." $\endgroup$
    – garyp
    Commented Mar 31, 2016 at 12:17
  • $\begingroup$ You said "The refractive index is a collective measure of the atoms/molecules of the lattice on which the light is incident and depends on the collective fields built up by them." What field are you talking about? The electromagnetic field? I don't understand, aren't most of the atoms/molecules the light encounters essentially neutral because the charge separation is negligible? $\endgroup$ Commented Mar 31, 2016 at 12:23
  • $\begingroup$ @garyp in a sense it is an experimental obervation, that some media are transparent to light and some opaque, and the ones that are transparent have an angular dependence on the back scattering. The refractive index is also an "it happens because it happens" explanation. The quantum mechanical frame description is consistent is all I am saying. $\endgroup$
    – anna v
    Commented Mar 31, 2016 at 12:25
  • $\begingroup$ @Rorschach0007 Atoms and molecules are neutral seen at long distance, but there are spill over fields from the shape of the atomic and molecular orbitals en.wikipedia.org/wiki/File:D_orbitals.svg which make up matter in bulk. These are the fields that hold solids together as the orbital shapes are such that positive ( from the nucleus) and negative ( from the electrons) fields cover different space regions, so there always exist electric field lines. ( true for liquies and gases too in bulk) $\endgroup$
    – anna v
    Commented Mar 31, 2016 at 12:28
  • $\begingroup$ Hmm what I'm trying to ask is, suppose that total internal reflection was never discovered experimentally, would a physicist be able to derive the equations for this phenomena and predict the existence of TIR, purely using mathematics and quantum mechanics? Also can you expound on the boundary value solution to the photon scattering? Where does one even begin? $\endgroup$ Commented Mar 31, 2016 at 12:37

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