The Ewald: Oseen Extinction Theorem
When a beam of light enters a
material medium, it sets in
motion the resident electrons,
whether these electrons are free or bound. The electronic oscillations in turn give rise to electromagnetic radiation which, in the case of linear media, possesses the frequency of the exciting beam. Because Maxwell's equations are linear, one
expects the total field at any point in space to be the sum of the original (exciting) field and the radiation produced by all the oscillating electrons.
In practice, however, the original
beam appears to be absent
within the medium, as though it
had been replaced by a different
beam, one having a shorter wavelength and propagating in a different direction.
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