Illustration of Faraday’s 19th-century experiment. Credit: Enrique Sahagún.

In 1845, Michael Faraday showed that light and magnetism are linked. He passed a beam through glass inside a magnetic field and found that its polarization — the direction its waves wiggle — rotated. The results of this elegant experiment are known to this day as the Faraday effect. For nearly two centuries, scientists believed they fully understood it: only the electric part of light mattered.

Not quite so, say physicists at the Hebrew University of Jerusalem. According to their new study, the magnetic component of light — long dismissed as negligible — directly contributes to the Faraday effect. “Light doesn’t just illuminate matter, it magnetically influences it,” said Dr. Amir Capua, who co-led the research with Benjamin Assouline.

The physicists used theoretical modeling to show that light’s oscillating magnetic field can twist the spins of electrons inside materials, producing a measurable change in the way light itself is rotated as it passes through.

A Forgotten Half of Light

Light is an electromagnetic wave. You can think of light as a blend of oscillating electric and magnetic fields. Physicists have long focused on the electric half, which shakes charged particles and drives most familiar optical effects. The magnetic half didn’t seem to matter in the Faraday effect.

“There is a second part of light that we now understand interacts with materials,” Capua told New Scientist. He says researchers overlooked this because magnetic forces in most materials are weaker than electric ones, and because spins — the quantum sources of magnetism — often fall out of sync with light’s oscillations.

But when light is circularly polarized, so its waves twist like a corkscrew, the magnetic component can align more effectively with those spins. Using the Landau–Lifshitz–Gilbert (LLG) equation, which describes how spins behave in a magnetic field, Capua and Assouline showed that this optical magnetic field produces its own magnetic torque — a twisting force inside the material.

When they ran their model using a crystal called terbium gallium garnet (TGG), they found that light’s magnetic field accounts for about 17% of the Faraday effect at visible wavelengths, and as much as 70% in the infrared range.

“Our results show that light ‘talks’ to matter not only through its electric field, but also through its magnetic field,” said Assouline.

Shaking Up Magneto-Optics

The discovery changes how scientists understand the Verdet constant, a number describing how strongly a material rotates light’s polarization under a magnetic field. Traditionally, the Verdet constant has been linked to how the electric component of light interacts with moving charges. But Capua’s team showed that the LLG equation can predict part of that constant using the magnetic component alone.

Their analysis also revealed something subtler: the Faraday effect and its time-reversed twin, the inverse Faraday effect, aren’t perfect mirror images. In the inverse version, intense light pulses can magnetize materials without any external magnetic field — essentially flipping spins with light alone. According to the team, the two effects are not exactly reciprocal at ultrafast timescales, because they depend on different kinds of spin dynamics.

That breakdown of reciprocity could help explain puzzles in ultrafast magnetism — a field that uses femtosecond laser pulses to control spins for next-generation computing and data storage.

‘“What we see is that even when light interacts with matter in incredibly brief bursts, its magnetic component can still play a surprisingly strong role,” Capua said.

The ability of light to magnetically influence matter could open new paths in spintronics, optical data storage, and even quantum computing, where controlling spin states is key.

For Capua, the thrill lies in rewriting a chapter of physics that seemed closed. “The static magnetic field ‘twists’ the light, and the light, in turn, reveals the magnetic properties of the material,” he said in the Hebrew University press release. “What we’ve found is that the magnetic part of light has a first-order effect — it’s surprisingly active in this process.”

Still More Work to Do

For now, Capua and Assouline’s work remains a theoretical breakthrough. But theory alone isn’t proof. No one has yet observed this magnetic influence directly in a lab. The next challenge, says Capua, is designing experiments precise enough to isolate the signal of light’s magnetic torque from the dominant electric one. If confirmed, it would force textbooks to update a law of optics that has stood unchallenged since the 1840s.

One hundred and eighty years after Faraday glimpsed the bond between light and magnetism, scientists have found the missing half of that partnership — and it’s been hiding, oscillating, in plain sight.

The findings appeared in the journal Scientific Reports.