If a sound beam encounters an interface at a 90-degree angle with tissue sound speeds of 1540 m/s and 1450 m/s, what is the angle of transmission?

• A. Less than 90 degrees
• B. 90 degrees
• C. Greater than 90 degrees
• D. Undefined

Answer: (B)

When a sound beam strikes an interface at a 90-degree angle, it is referred to as normal incidence. In this specific scenario, the angle of transmission is determined by the principle of refraction. According to Snell's Law, the angle of incidence and angle of transmission are related to the sound speeds in the two different media.

Since the sound beam is entering the second medium at exactly 90 degrees, it means that the direction of the sound wave does not change upon transmission. When the wave propagates into the medium with a different sound speed (from 1540 m/s in the first tissue to 1450 m/s in the second), it maintains the same angle of 90 degrees. The change in speed does not influence the angle; the sound wave will continue to travel straight into the second tissue at that same angle.

This clarity in understanding the behavior of sound waves at a boundary ensures that one recognizes that there is no refractive effect altering the direction, and therefore, the angle of transmission remains at 90 degrees. This is why the solid answer is that the angle of transmission is indeed 90 degrees.