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Huygens's Wave Theory is what you call a first order scalar diffraction theory of light. So what does it describe and what does it fail to describe? First order means that electromagnetic effects like induced currents in surfaces etc. are ignored. These can be described by solving Maxwell's equations for the same system instead of working with the wave ...

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Electric field extends to infinity in the sense that no limit after which the field would vanish was ever found. It is natural assumption that simplifies things. Coulomb's law is consistent with this assumption, but there is no model that would explain Coulomb's law from anything simpler.

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Actually we have models explaining this. The particles that mediates electromagnetic field are massless, so the range of the force predicted by the model is infinite. On the contrary, for massive mediators (see for instance Yukawa force), the range is finite. Notice that real experimental setups have finite precision, so beyond some limit it's pointless to ...

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The atom absorbs the photon that kicks up an electron to an excited state, and it is the atom that will emit a photon when it de-excites. Not the electron. Is the invariant mass of an atom higher when the electron is in an excited state? Take the hydrogen atom. The ground state energy is at -13.6eV. This means that the mass of hydrogen is less than the ...

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Can I simplify this to pick out of the summation only the mode that matches the transition energy $\hbar \omega_{21}$? This would be because photons are emitted/absorbed entirely anyway, so the energies have to match, and the photons which do not have the right energy simply won't interact with the atom That's exactly right. Let's see it in detail. ...

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I'm not a physicist and came to the problem of infinity of the electric field by working about Complex one-dimensional structures in space. In this work I recognized that to build-up dipole fields it needs two different quanta at least and only. Such structures could follow the 1/r²-law, BUT related to the discrete structure the field has a finite range. Why ...

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You shouldn't ignore spin even in the nonrelativistic limit. Schwartz is only ignoring spin for pedagogical reasons. Calculating scattering amplitudes with scalars is easier to do, so he wants you to learn how to do that first before getting into all the complications of spin 1/2 and spin 1 particles (where you need to worry about fermions and gauge ...

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For the theoretical background with an occasional nod to experiments, my bible is "Optical Coherence and Quantum Optics" by Mandel and Wolf. This book is both a text book and a reference for researchers. It covers the basics of random signals, quantum mechanics, the quantum theory of radiation, quantum optics, a bit of nonlinear optics, a bit of laser ...

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