This is a joint work with M. Palombaro and J. Rauch.
We study the homogenization and singular perturbation of the
wave equation in a periodic media for long times of the order
of the inverse of the period. We consider inital data that are
Bloch wave packets, i.e., that are the product of a fast
oscillating Bloch wave and of a smooth envelope function.
We prove that the solution is approximately equal to two waves
propagating in opposite directions at a high group velocity with
envelope functions which obey a Schr\{o}dinger type equation.