Note the second term proportional to √ ℐ1ℐ2-cos Φ0, 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., ℐ1 = ℐ2, 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:
∘ -- cosΦ0 = 1 R-ℐ1√-− ℐ2-. 2 T ℐ1ℐ2