  Laser interferometer detectors are now widely used in an attempt to detect
gravitational waves (gw). The interaction of the gw with the light circulating
in the interferometer is usually explained in terms of the motion of the "free"
mirrors that form the interferometer arms. It is however instructive to show
that the same result can be obtained by simply calculating the propagation of
an electromagnetic plane wave between "free mirrors" in the curved space-time
induced by the gw. One finds that the plane wave acquires frequency modulation
sidebands at the gw frequency, as would be expected from the absorption and
emission of gravitons from and to the gw. Such sidebands are completely
equivalent to the time-dependent phase shift imposed on the plane wave, that
follows from the conventional calculation.
