Physicists have long assumed that light hitting a metal surface at an angle pushes on the free electrons, moving them forward, yet some recent experiments show the opposite—incident light seems to pull electrons backward. Based on new experiments, a team of researchers now offers partial clarification, at least on the experimental side. Metals in vacuum act one way, while those exposed to air behave differently, possibly because the light first interacts with molecules attached to the surface. The experiments cast doubt on researchers’ basic understanding of how light interacts with solid materials, knowledge that lies at the core of modern photonics technology.

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