Which factor determines the angle of incidence in ultrasound inspections?

The angle of incidence during ultrasound inspections is primarily influenced by the relative sound speed in the materials being inspected. The relationship between the angle of incidence and the sound speed is governed by Snell's Law, which states that the angle of incidence is determined by the ratio of the velocities of sound in the two different media at the interface. When sound waves travel from one medium to another, the change in speed alters the angle at which the waves enter the second medium.

While temperature can affect sound speed, it is not a direct determinant of the angle of incidence itself. Probe size and sound frequency can influence various factors in the inspection process, such as resolution and penetration, but they do not dictate the angle of incidence directly. Thus, the correct choice highlights the fundamental physics that underpins the behavior of sound waves at different media interfaces, which is essential knowledge for effective ultrasound inspections.