Wood's anomaly

In optics, Wood's anomaly refers to the rapid variation of light intensity at diffracted spectral orders in metallic gratings. It was first observed by American physicist Robert W. Wood in 1902. Initially unexplained by conventional grating theories, the effect was later understood to arise partly from the excitation of surface plasmon polaritons at the grating surface and partly from the coupling of incident light into diffracted orders, one of which becomes evanescent at a grazing angle. The latter effect is also known as Rayleigh anomaly or Rayleigh–Wood anomaly, after Lord Rayleigh's 1907 work on gratings.

Studies on Wood anomalies acted as a progenitor to the fields of plasmonics and metamaterials. Wood anomalies were also observed in acoustic gratings, where they were associated both with diffracted waves at a grazing incidence and surface acoustic waves.