CatoptricsCatoptrics (from Greek: κατοπτρικός katoptrikós, from Greek: κάτοπτρον katoptron “mirror”) deals with the phenomena of reflected light using mirrors. Super-strong laser field interacting with matter can create mirrors moving with relativistic velocity. In plasmas, the relativistic flying mirrors are thin and dense electron or electron–ion layers accelerated by the high intensity electromagnetic waves up to velocity close to the speed of light in vacuum. The reflection of the electromagnetic wave at the relativistic mirror results in its energy and frequency change. In the counter-propagation configuration, the up-shifted frequency of the reflected wave is multiplied by the factor proportional to the gamma-factor squared. This scientific area promises the development of the sources of ultrashort x-ray pulses in attosecond range.