Physicists in the US have shown that light hitting a conductive metal surface at an angle can cause free electrons in the metal to move either in the same direction or in the opposite direction as the photons, depending on its surrounding environment. Jared Strait and a team of researchers at the National Institute for Standards and Technology (NIST) in Maryland observed the direction change through simplified experiments involving a thin gold film. Their observations reveal a significant gap in our knowledge of how light interacts with metals on microscopic scales.

Currently, physical models explain the interactions between light and conductive metals through the framework of classical electrodynamics. In these models of the so-called “photon-drag” effect, the momentum of photons hitting the metal surface at an angle is conserved as it transfers to the material’s free electrons, pushing them forward in the same direction as the incident light. However, many recent experiments have contradicted this idea, observing currents that flow in the opposite direction of the incident light – indicating instead that the photons are pulling electrons backwards.

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