Under which conditions does refraction occur?

Under which conditions does refraction occur?

• A. Normal incidence and same speeds
• B. Oblique incidence and different speeds
• C. Normal incidence and different speeds
• D. Oblique incidence and same speeds

Answer: (B)

Refraction happens when a wave hits a boundary at an angle and the propagation speed changes between the two media. That change in speed makes the path bend, described by Snell’s law. If the wave arrives straight on to the boundary, there’s no bending even if the speeds differ, because there’s no oblique angle to cause a directional change. If the speeds are the same, there’s no bending even for an oblique incidence because the angle of refraction equals the angle of incidence. So both conditions must be met: the incidence is oblique and the speeds differ. In ultrasound, different tissues have different sound speeds, so a beam crossing tissue boundaries at an angle will refract.

Refraction happens when a wave hits a boundary at an angle and the propagation speed changes between the two media. That change in speed makes the path bend, described by Snell’s law. If the wave arrives straight on to the boundary, there’s no bending even if the speeds differ, because there’s no oblique angle to cause a directional change. If the speeds are the same, there’s no bending even for an oblique incidence because the angle of refraction equals the angle of incidence. So both conditions must be met: the incidence is oblique and the speeds differ. In ultrasound, different tissues have different sound speeds, so a beam crossing tissue boundaries at an angle will refract.