How did Michael Faraday discover the effect of magnetism on polarized light?

In 1845, Michael Faraday successfully observed the effect of magnetism on polarized light—now known as the Faraday effect—after receiving a suggestion from William Thomson (later Lord Kelvin) to experiment with magnetic lines of force.1

Following this advice, Faraday passed a beam of plane-polarized light through a piece of high-refractive-index optical glass he had developed years earlier. He then activated an electromagnet so that its magnetic lines of force ran parallel to the path of the light ray.1

Upon doing so, he observed that the plane of polarization was rotated. Faraday further determined that this rotation was not caused by a strain within the glass molecules themselves, but rather by the magnetic lines of force, as the direction of rotation remained consistent even when the light ray's direction was reversed. This discovery provided critical evidence for Faraday’s belief in the unity of nature's forces and helped lay the foundation for modern field theory.1