The Faraday effect is the rotation of the polarization state of light when it passes through a magnetic field, B. This field could be induced by an electrical current. This is mechanism that makes up the basic principle of Farday Rotators and Fiber Optic Current Sensors, FOCS. The larger the electric current, the greater the magnetic field and hence the larger the polarization rotation. The proportionality constant which relates magnetic field strength and the angle of rotation is known as the Verdet Constant.
Figure 1: Representation of rotation of light polarisation in a fiber optic current sensor due to the current going through the conductor