Patent Number: 
Section: claims

1. An optical element comprising a substrate and at least a pair of alternate layers provided on the substrate, the alternate layers each having an Mo layer and an Si layer,wherein the Mo layer contains 90% or more of 98Mo and the Si layer contains 99% or more of 23Si. 2. An optical element comprising a substrate and at least a pair of multilayer films of a four-layer structure provided on the substrate, each of the at least the pair of the multilayer films having an Si/B4C/Mo/B4C structure in this order from the substrate,wherein the Mo layer contains 90% or more of 98Mo, the Si layer contains 99% or more of 23Si and the B4C layer contains 99% or more of 11B. 3. An X-ray mirror comprising the optical element according to claim 1. 4. An X-ray mirror comprising the optical element according to claim 2.