Note the second term proportional to
, which owes its existence to the interference of the two traveling
waves running in opposite directions. An interesting consequence of this is that the radiation pressure does not vanish even if
the two waves have equal powers, i.e.,
, that is, in order to compensate for the radiation pressure force of one field on
the semi-transparent mirror, the other one should not only have the right intensity but also the right phase with respect to the
former one:
