The angle at which the shear wave refracts at 90 degrees is called the:

• A. First critical angle
• B. Second critical angle
• C. Third critical angle
• D. Angle of reflection

Answer: (B)

Critical angles occur when a wave hits a boundary and the refracted wave travels along the interface (that is, at 90 degrees to the normal). In a solid–liquid boundary, there can be multiple such grazing conditions, one for each possible transmitted mode. The angle at which the shear wave would be forced to travel along the boundary is the second critical angle. The other grazing conditions correspond to the other possible modes, and the angle of reflection is simply the wave bouncing back into the first medium, not a refracted grazing condition.