The angle that a plane-wave acoustic pulse meeting a surface makes with a normal to the surface. The angle of incidence is important in sonar backscattering. For instance, if the sonar altitude is too low, at long ranges the angle of incidence with the seabed becomes high. Much of the incident energy is not returned to the transducer. Further, if the seabed angles up, out at range, the angle of incidence becomes lower and more energy will be returned, making a notable anomaly in the data.