  Small corrections to the electromagnetic field in colliding light beams are
evaluated taking into account the interaction of light with the quantum vacuum,
as predicted by quantum electrodynamics. Possible implications for very
energetic light beams and the radiation pressure in interferometers (e.g.,
gravitational waves detector) are considered. It is also shown that a head-on
collision produces secondary waves along conical directions at angles $\arccos
(1/3)$.
