The angle at which 90 degrees refraction of a longitudinal sound wave is reached is called:

The angle at which 90 degrees refraction of a longitudinal sound wave is reached is called:

• A. The angle of incidence
• B. The first critical angle
• C. The angle of maximum reflection
• D. The second critical angle

Answer: (B)

When a ultrasonic longitudinal wave hits a boundary, the speed changes and Snell’s law tells us how the wave bends. The angle at which the refracted wave travels right along the boundary—i.e., at 90 degrees to the normal—is called the critical angle. In ultrasound practice for this case, that specific incidence angle is referred to as the first critical angle. Beyond this angle, no refracted wave enters the second medium and the wave is totally reflected. The other terms aren’t describing this special boundary condition. The angle of incidence is just the initial angle, not the special 90-degree refraction. The angle of maximum reflection isn’t a standard term for this scenario, and the second critical angle would pertain to a different wave mode (such as a shear wave in solids) rather than the primary longitudinal-to-boundary case.

When a ultrasonic longitudinal wave hits a boundary, the speed changes and Snell’s law tells us how the wave bends. The angle at which the refracted wave travels right along the boundary—i.e., at 90 degrees to the normal—is called the critical angle. In ultrasound practice for this case, that specific incidence angle is referred to as the first critical angle. Beyond this angle, no refracted wave enters the second medium and the wave is totally reflected.

The other terms aren’t describing this special boundary condition. The angle of incidence is just the initial angle, not the special 90-degree refraction. The angle of maximum reflection isn’t a standard term for this scenario, and the second critical angle would pertain to a different wave mode (such as a shear wave in solids) rather than the primary longitudinal-to-boundary case.