Reflective optical element and EUV lithography appliance

A reflective optical element for the EUV and soft X-ray wavelength region, comprising a multilayer system and a protective layer system, having at least one layer of a material with a refractive index whose real part at an operating wavelength λB between 12.5 nm and 15 nm is between 0.90 and 1.03, and whose imaginary part at an operating wavelength of 12.5 nm to 15 nm is between 0 and 0.025, so that the reflectivity plotted as a function of the thickness of the protective layer system at first drops, until a thickness d1 is reached, the reflectivity remains essentially constant between thickness d1 and another thickness d2, d2>d1, and the reflectivity further drops for a thickness>d2, wherein a side of the multilayer system facing the protective layer system ends in an absorber layer, wherein the thickness of the protective layer system is smaller than d2, and wherein the thickness d1 is chosen such that a standing wave formed upon reflection at operating wavelength λB has a minimum at a distance from the surface of the reflective optical element of 0.1 λB or less, and the minimum lies in the vacuum.