  Relativistic flying mirrors in plasmas are realized as thin dense electron
(or electron-ion) layers accelerated by high-intensity electromagnetic waves to
velocities close to the speed of light in vacuum. The reflection of an
electromagnetic wave from the relativistic mirror results in its energy and
frequency changing. In a counter-propagation configuration, the frequency of
the reflected wave is multiplied by the factor proportional to the Lorentz
factor squared. This scientific area promises the development of sources of
ultrashort X-ray pulses in the attosecond range. The expected intensity will
reach the level at which the effects predicted by nonlinear quantum
electrodynamics start to play a key role.
