The Optics Are Clear As Glass
Trying to prove light had any connection to electricity and magnetism was a tall order. This would have been hard for anyone, especially the elder Faraday. He would do several experiments to prove this but failed many times. That was until he looked to his shelf, and noticed the brick of glass from his greatest failure years prior. It was the key to proving the connection! He used left and right circularly polarized waves, which were placed at slightly different speeds (known as circular birefringence).
Linear polarization can be decomposed into a superposition of two equal-amplitude, circularly polarized components of opposite handedness and a different phase. When this is done, you’ll get a relative phase shift, induced by Faraday’s discovery. This manages to rotate the orientation of a wave’s linear polarization. Today, we know this entire experiment and concept: The Faraday Effect.
It has since been used for measuring instruments. You can see used to measure optical rotatory power & remote sensing of magnetic fields. Think of Fiber Optic Current Sensors there. The Faraday Effect is also used in Spintronics (or spin electronics). On top of that, Faraday Rotators can also be used for amplitude modulation of light and are how we were given optical isolators and optical circulators. The same stuff used in optical telecommunications and various laser applications.