|MadSci Network: Physics|
According to classical electromagnetic theory, reflection only occurs at the interface of two different media. Because the Maxwell's equations governing the propagation of electromagnetic waves are linear, the existence of one electromagnetic wave does not affect the other and they just simply sum up. The "propagation direction" property is not a general property for electromagnetic fields, it can only be used for description of the spatial dependency of traveling planar waves. For generaly electromagnetic fields, their spatial dependency can not be described by a certain propagation direction, but they can be regarded as superposition of planar waves of different strengths propagating in all directions, i.e. a distribution in propagation direction or angular spectrum. Each electromagnetic field has an angular spectrum. When you sum two electromagnetic wave up, what you actually do is to sum up their electric and magnetic field at different directions respectively. In this way, you can change the final angular spectrum. The simplest example is by summing up a traveling wave in +x direction and a certain standing wave you may get a traveling wave in -x direction, but generally we do not regard this as reflection. Hope this can help. There are previous answers about photon-photon scattering, but they are not interacting directly but through fermions. And here is a previous answer about a very similar question.
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