Electromagnetic reciprocity relates the response obtained after exchanging source and receiver in a linear reciprocal medium. For suitably normalized ports it gives a symmetric scattering matrix. It does not require zero reflection. A magnetically biased Faraday effect provides an important optical example in which reciprocity can fail.
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In electromagnetism, reciprocity refers to a principle that relates the response of a system to an electromagnetic field to the response of the same system when the source of the field and the observation point are interchanged. This principle is grounded in the linearity and time-invariance of many physical systems described by Maxwell's equations, which govern the behavior of electric and magnetic fields.