New Theory Revises 180-Year Understanding of Light's Role in Faraday Effect

New theoretical research from the Hebrew University of Jerusalem challenges a fundamental scientific premise regarding light-matter interaction that has stood for 180 years. The study, published on November 19, 2025, in the journal Scientific Reports, provides the first theoretical demonstration that the oscillating magnetic field component inherent in light actively contributes to the Faraday effect.

This finding directly contradicts the long-held assumption that only the electric field component of light dictates the rotation of polarization observed when light passes through a material subjected to a static magnetic field. The investigation was led by Dr. Amir Capua and doctoral student Benjamin Assouline from the Department of Electrical Engineering and Applied Physics at the institution.

The core of the theoretical refinement involves quantifying the magnetic field’s influence, previously deemed negligible. Researchers utilized advanced theoretical modeling, specifically the Landau–Lifshitz–Gilbert (LLG) equation, to demonstrate that light’s optical magnetic field can generate a magnetic torque within a material by interacting with its magnetic spins. This action is analogous to the twisting force exerted by an external static magnetic field.

To substantiate their model, the team applied it to Terbium Gallium Garnet (TGG), a synthetic garnet material, $\text{Tb}_3\text{Ga}_5\text{O}_{12}$, frequently used as a Faraday rotator. Their calculations yielded specific numerical evidence: the magnetic component of light accounts for approximately 17% of the observed polarization rotation within the visible light spectrum. This magnetic contribution escalates to as much as 70% when examining the infrared spectrum for TGG.

The context of this discovery is significant, as it revises the understanding of the Faraday effect, first observed by Michael Faraday in 1845, which describes light polarization rotation in a magnetic field. This theoretical refinement carries substantial weight for advanced technology development, opening avenues in fields such as spintronics and the light-based control of magnetic states.

This work from the Hebrew University of Jerusalem provides a more complete physical model for phenomena that have been cornerstones of optics for nearly two centuries, complementing ongoing experimental efforts to control magnetism using light.