The invention provides an optical reflector element (1) for a beam of X-rays (R.sub.X) or of gamma-rays or of high-energy particles at grazing incidence, the element being constituted by a stack of superposed silicon plates (10-12). Each plate (10-12) has a reflecting top face (101-121) possibly coated with a metallic film, a multilayer or a dispersive grating and a bottom face carrying ribs (100-120) forming spacers between two successive plates (10-11, 11-12), and defining determined spacing between two successive reflecting faces (101-121). The invention also provides optical instruments comprising several such elements, in particular a type I Wolter telescope comprising two mirrors in tandem having respective paraboloid and hyperboloid surfaces of revolution or a conical approximation thereof or a Kirkpatrick-Beaz system.