Abstract
The directionality of amplifier systems was the subject of the fourth lesson. It plays an important role in the measurement process, since we want to prevent noise from the circuit after the amplifier from being sent back to the input and interfering with the system being measured. In other words, it's important in practice for the amplifier to act as a valve for the signals it processes. Transposed to the field of optics, this property corresponds to the violation of the inverse law of return : in general, optical devices comprising lenses and mirrors operate in a reciprocal manner - " I see the eye that sees me " - but Faraday's rotation violates this law. It applies to the polarization of an electromagnetic wave as it passes through a material medium subjected to a static magnetic field aligned with the direction of propagation.
