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True, in the sense one can never set foot in the same stream twice.PeterDonis said:There is no such thing as "the initial EM wavefront"
One can always split the material EM wave solution into two: the new solution is the sum of the orignial wave and the "response". There's no physics here, just subtraction.
So the real question is whether there is utility in thinking of things this way. Well, "sometimes" and "it depends". I can think of only one example (transition radiation) so its probably not helpful all that often.