Detection of extremely low frequency gravitational wave using
  gravitational lens
  The effect of gravitational wave of cosmological wavelength on the
gravitational lensing is investigated. When the source, deflector, and observer
are aligned in a highly symmetric configuration, an Einstein ring will be
observed by the observer. There will be no time delays between different
locations on the Einstein ring in the absence of gravitational wave. Otherwise,
time delays between different locations on the Einstein ring will emerge if
cosmological gravitational wave propagates through the system. Previous studies
demonstrated that the time delay resulting from the aligned
source-deflector-observer configuration in the presence of gravitational wave
of cosmological wavelength could be equivalent to that of a similar lens with a
nonaligned source-deflector-observer configuration in the absence of
gravitational wave. Results in this work show that gravitational lens with the
aligned source-deflector-observer configuration could serve as a potential
gravitational wave detector when the whole Einstein ring observed by the
observer is taken into account.
