A polarizer is an optical filter that passes light of a specific polarization and blocks other waves based on the angle of the rotation. For instance, when the polarized light has the same polarization angle as the filter (equivalently, think of two linear filters parallel to each other), all of the light passes through. However, when the relative polarization angle between the light and the polarizer is at \(\frac{\pi}{2}\) (two linear filters perpendicular to each other), none of the light passes through.

Using the basic knowledge of linear filters, we can now apply that to circular rotation of light through a magnetic field. In our experimental set up, see Fig \ref{fig:Apparatus}, the intensity of light that passes through the polarizer is measured by a photodiode.

In our experiment, the light from the output of the laser is at least partially polarized, and the laser has a small polarizer attached to the output so that the light is fully linearly polarized in a particular direction.