When light interacts with a mirror which is moving towards it at a speed close to the speed of light, its wavelength is shifted into the extreme ultraviolet region of the spectrum. This effect was first predicted by Albert Einstein. His theory was experimentally confirmed almost 100 years later, following the development of high-intensity laser light sources. Laser physicists at the Laboratory for Attosecond Physics (LAP) at Max Planck Institute for Quantum Optics in Garching (MPQ) and LMU have now characterized the phenomenon in detail under controlled conditions and exploited it to generate high-intensity attosecond light flashes. Moreover, they show that these pulses can be shaped with unprecedented precision for use in attosecond research.
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Source: Phys.org