How did Michael Faraday’s optical-glass research lead to later discoveries?

In the 1820s, while working on a commission for the Royal Society to improve optical glass for telescopes, Michael Faraday produced a glass with a very high refractive index.1

This specific glass proved instrumental in his later research. In 1845, acting on a suggestion from William Thomson, Faraday passed a beam of plane-polarized light through this high-refractive-index glass while subjecting it to a strong electromagnet. The resulting rotation of the plane of polarization—now known as the Faraday effect—provided evidence that magnetic lines of force could influence light.23

This discovery was a pivotal step in Faraday’s development of field theory, as it led him to conclude that space itself is a medium capable of supporting electric and magnetic strains. These foundational concepts regarding the nature of force and space were later used by James Clerk Maxwell to formulate his electromagnetic field theory.2