Physics of light and magnetism rewritten after almost two centuries

In 1845, physicist Michael Faraday provided the first direct evidence of the connection between electromagnetism and light. It now turns out that this connection is even stronger than Faraday thought.

In his experiment, Faraday passed light through a piece of glass soaked in boric acid and lead oxide and immersed in a magnetic field. He found that this changed the light: when it came out of the glass, its polarization changed.

Light is an electromagnetic wave, and over the last 180 years it has been widely accepted that this “Faraday effect” demonstrates that the combined interaction magnetic fieldThe electrical charges in the glass and the electrical component of light cause the light wave to rotate – to oscillate in a different direction than before it entered the material.

Perhaps surprisingly, it was long believed that the magnetic component of light actually plays no role in the Faraday effect. Amir Kapua And Benjamin Assouline at the Hebrew University of Jerusalem in Israel have now shown that this is not always the case.

“There is a second part of the world that we now understand interacts with materials,” says Capua.

Capua says there are two reasons why researchers have not pursued the idea that the magnetic component of light plays a role in the Faraday effect. First, magnetic forces within materials such as Faraday glass appear relatively weak compared to electrical forces. Second, when materials like Faraday glass are magnetized (meaning the quantum spins of their component parts interact with any magnetic field, as tiny magnets do), those spins are typically out of sync with the magnetic component of the light waves, suggesting they don't interact much.

But Capua and Assouline realized that when the magnetic component of light is circularly polarized—essentially, vortex or corkscrew – it can interact much more intensely with the magnetic spins in the glass. They concluded that this happens even without any special effort to manipulate the light, since its magnetic component always consists of several spiral waves.

Calculations by the two researchers showed that if Faraday's experiment was repeated with a magnetic material called terbium gallium garnet (TGG) instead of glass, this magnetic interaction could actually account for 17 percent of the resulting Faraday effect when visible light passes through the material. If infrared light was instead passed through the TGG material, magnetic interaction would account for up to 70 percent of the resulting Faraday effect.

Igor Rozhansky from the University of Manchester, UK, says the new calculations are compelling and point to plausible experimental tests in the future. The magnetic component of the Faraday effect, which has been neglected until now, could provide researchers with a new way to manipulate spins inside materials, Rojanski said. He adds that it remains an open question whether this effect may actually be stronger than the traditional Faraday effect in some materials.

Future experiments will translate new findings from fundamental physics into applications, and Capua says he can already imagine how the discovery that magnetic spins in some materials can interact with the magnetic component of light could be used to manipulate them. This could ultimately pave the way for new types of rotation-based sensors and hard drives.

Renaissance Science: Italy

Meet the great scientific minds and discoveries of the Renaissance that helped cement Italy's role at the forefront of scientific activity – from Brunelleschi and Botticelli to polymaths such as Leonardo da Vinci and Galileo Galilei.