Optical mirrors focus visible light through ordinary reflection at near-normal incidence. X-ray photons carry far more energy per photon: at that same steep angle they simply penetrate into the mirror's atoms and are absorbed instead of bouncing off — so a conventional parabolic dish forms no image at all.
X-rays only reflect efficiently when they strike a polished surface at a grazing angle of a degree or two, the way a flat stone skips off water instead of plunging in. A Wolter Type-I telescope exploits this: photons enter a set of nested cylindrical shells and reflect twice at a shallow angle — first off a shallow paraboloid, then off a shallow hyperboloid — which is enough to steer even the outermost, widest shell's rays down to one common focus.
- Normal incidence (fails) — a single dish mirror facing the beam head-on; every photon is absorbed at the surface, nothing reaches the detector.
- Grazing-incidence Wolter — concentric shells, each photon reflects twice at a shallow angle and converges on the shared focus.
- Nested shells — real telescopes stack many shells of increasing radius to catch more collecting area without increasing the mirror's length.
Real-world relevance: this exact geometry flies on NASA's Chandra X-ray Observatory and ESA's XMM-Newton, the only way humanity images black holes, neutron stars and supernova remnants in X-rays from orbit.