  We report on the formation of a dispersive shock wave in a nonlinear optical
medium. We monitor the evolution of the shock by tuning the incoming beam
power. The experimental observations for the position and intensity of the
solitonic edge of the shock, as well as the location of the nonlinear
oscillations are well described by recent developments of Whitham modulation
theory. Our work constitutes a detailed and accurate benchmark for this
approach. It opens exciting possibilities to engineer specific configurations
of optical shock wave for studying wave-mean flow interaction.
