A spectroscopic technique for studying surfaces by reflection of light or ultraviolet radiation. A beam of plane-polarized monochromatic + radiation is directed at right angles onto a single-crystal surface and the reflected radiation along the surface is measured in two mutually perpendicular directions. If a suitable surface plane is chosen, there is a difference in reflectivity in these directions – a reflection anisotropy – and for a cubic crystal this is a property of the surface rather than the bulk. The change in this effect with wavelength gives a reflection anisotropy spectrum, which gives information about electronic surface states. Surface reactions can be investigated by the changes they cause in the spectrum.