  The photon motion in a Michelson interferometer is re-analyzed in both
geometrical optics and wave optics. The classical paths of the photons in the
background of gravitational wave are derived from Fermat principle, which is
the same as the null geodesics in general relativity. The deformed Maxwell
equations and the wave equations of electric fields in the background of
gravitational wave are presented in flat-space approximation. Both methods show
that the response of an interferometer depends on the frequency of a
gravitational wave, however it is almost independent of the frequency of the
mirror's vibrations. It implies that the vibrating mirror cannot mimic a
gravitational wave very well.
