Which description best captures the concept of Snell's law in ultrasonics?

• A. The sum of incidence and refraction angles
• B. The product of velocities is constant
• C. Angular relationships between incident and refracted rays
• D. The difference between angles equals velocity ratio

Answer: (C)

Snell's law in ultrasonics describes how a sound wave bends when it crosses an interface between two media with different sound speeds. The key relationship links the angles of the incident and refracted rays through the speeds in the two media: the sine of the incident angle divided by the sine of the refracted angle equals the velocity in the first medium divided by the velocity in the second. This is all about how the directions on either side of the boundary relate to each other, not about adding or multiplying angles or speeds.

So the best description is that Snell's law expresses the angular relationship between the incident and refracted rays. If the speeds are the same in both media, the angles remain the same and the ray continues straight; if they differ, the ray bends toward or away from the normal depending on the speed ratio. The other ideas—summing the angles, taking a product of speeds, or using the difference of angles—do not capture the actual relationship dictated by Snell's law.