Abstract: The author puts forward a formula of velocity with an additional parameter expressing boundary conditions.The experimental study demonstrates that the velocities of particles at free surface are different from those in cratering. Their quantitative relationship is given. Based on the refractive law of waves, the author presents a mathematical model describing the ejective direction of any particle in cratering. The paper also points out the essential differences between the graph of equidistant lines of ejecta in cratering for an explosion in medium bounded by a sloping surface and that by a level surface.