Refraction driven X-ray caustics at curved interfaces

X-ray refraction related interaction has received rising interest since about two decades in the field of imaging, beam shaping and analysis although being discovered a century ago. Due to refraction at interfaces in inhomogeneous media X-rays undergo natural focusing (or defocusing) of waves, revealing caustics. Such kind of intensity patterns are well-known for visible light, but have been sparsely discussed for X-rays. The variation of irradiation density may be predicted in case of known shapes. Analogously to light optics, the intensity distributions cover several orders of magnitude including complete extinction. The partly convergent (and divergent) caustic stripes originate from narrow zones of typical size of some 10(-6) of the boundary curvature radius. For the deflection of plane wave synchrotron radiation (energy in the range of some keV to some ten keV) at rods and tubes of several mu m diameter, we find good agreement between experiments and modeling by ray tracing according to Snell's law without additional diffraction contributions. Apart from basic research implications, caustics may influence the performance of irradiation technologies such as sterilization or molecular cross-linking.