The interaction of two optical waves in a Kerr medium is considered. One of the waves is initially unmodulated and the other wave is amplitude modulated. Due to the Kerr effect, the amplitude modulated wave produces a refractive index grating and the first wave gets diffracted because of this grating. It is shown that there are two Bragg angles for a single modulating frequency at which the incident wave can be diffracted. The Bragg angle for the second order diffracted wave is approximately twice that for the first order wave and the interaction length required for complete power transfer into the second order diffracted wave is more than that for the first order wave. Some numerical calculations are made to find the interaction length for the first and second order diffracted waves and the optical power requirements.