The reflective optical element

To increase the reflectivity of a reflective optical element (50) for the extreme ultraviolet wavelength range with a multilayer system (51) on a substrate (52) is proposed, wherein the multilayer system (51) first and second layers (54, 55) of each materials having different real part of the refractive index at a specific wavelength in the extreme ultraviolet wavelength range, which are alternately arranged, whereby between a first (54) and a second layer (55) or a second (55) and a first layer (54), a transition layer ( , 59) 58 is arranged, whose refractive index at the given wavelength over the thickness (D) of the transition layer is variable.