  For scalar semilinear wave equations, we analyze the interaction of two
(distorted) plane waves at an interface between media of different nonlinear
properties. We show that new waves are generated from the nonlinear
interactions, which might be responsible for the observed nonlinear effects in
applications. Also, we show that the incident waves and the nonlinear responses
determine the location of the interface and some information of the nonlinear
properties of the media. In particular, for the case of a jump discontinuity at
the interface, we can determine the magnitude of the jump.
