Prior to physically verifying the actual refracted beam angle of a search unit, the ______ must be determined.

• A. Velocity of sound in the material
• B. Beam path length
• C. Beam exit point
• D. Surface finish of the component

Answer: (C)

Understanding how the beam moves across the boundary between the coupling medium and the test material is key. The refracted beam inside the material is determined by the angle at which the wave enters the material from the coupling medium, which hinges on the exact beam exit point on the surface. That exit point defines the local boundary normal and the incidence angle, and through Snell’s law it sets the refracted direction you’ll observe. Without knowing where the beam actually exits into the material, you can’t predict or verify the refracted angle even if you know the speeds of sound in the media. The speeds of sound are needed to calculate the expected angle, but they don’t by themselves tell you where the beam will cross into the material. The beam path length follows from the geometry once the exit point is known, and surface finish affects coupling quality rather than the fundamental refracted direction.