The incident angles at which 90 degree refraction of longitudinal and shear waves occur are called:

The incident angles at which 90 degree refraction of longitudinal and shear waves occur are called:

• A. The normal angles of incidence
• B. The critical angles
• C. The angles of maximum reflections
• D. None of the above

Answer: (B)

When a wave hits a boundary and the refracted wave travels exactly along the interface (at 90 degrees to the normal), that incidence is known as the critical angle for that wave mode. In solids, longitudinal (P) and shear (S) waves travel at different speeds, so there is a separate critical angle for each: one for P-waves and one for S-waves. At this angle, the refracted ray is parallel to the boundary; if the incidence angle is larger, that mode cannot propagate into the second medium and energy is reflected (or becomes a surface wave). This concept is a foundational tool in ultrasonic testing because it helps predict how waves will behave at interfaces and how to choose angles to inspect flaws. Therefore, the correct term for these incident angles is the critical angles.

When a wave hits a boundary and the refracted wave travels exactly along the interface (at 90 degrees to the normal), that incidence is known as the critical angle for that wave mode. In solids, longitudinal (P) and shear (S) waves travel at different speeds, so there is a separate critical angle for each: one for P-waves and one for S-waves. At this angle, the refracted ray is parallel to the boundary; if the incidence angle is larger, that mode cannot propagate into the second medium and energy is reflected (or becomes a surface wave). This concept is a foundational tool in ultrasonic testing because it helps predict how waves will behave at interfaces and how to choose angles to inspect flaws. Therefore, the correct term for these incident angles is the critical angles.