The angle at which 90 degrees refraction of a longitudinal 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 wave moves from a denser medium to a less dense one, the refracted wave bends away from the normal as the angle of incidence increases. There is a specific incidence angle at which the refracted ray travels exactly along the boundary, which means its angle to the normal is 90 degrees. This is the limiting case called the critical angle—the onset of grazing transmission. In many ultrasonic contexts this is referred to as the first critical angle, since it marks the first (and only) critical boundary for the given interface and wave mode. The other terms don’t describe this boundary behavior: the angle of incidence is just the incoming angle, not the refracted outcome; the phrase maximum reflection isn’t a standard descriptor for this phenomenon; and a second critical angle isn’t involved here.