Reflection, as mentioned, is caused by a momentum change when the moving air changes direction as a result of striking the surface. The ratio of reflected overpressure to incident pressure is called the reflection factor. The reflection factor is a function of the peak overpressure in the incident wave and the angle at which the wave strikes the surface. Shown in Fig. 4.1 is the effect of angle of incidence on the reflection factor for a wide range of overpressure levels