Refraction occurs only if there is which combination?

• A. Normal incidence & different impedances
• B. Oblique incidence & identical speeds
• C. Oblique incidence & different propagation speeds
• D. Oblique incidence & identical impedances

Answer: (C)

Refraction is the bending of a wave as it crosses into a medium where its propagation speed is different. This bending only happens when the wave hits the boundary at an angle, not head-on. If the incidence is normal, the wave passes straight through and the path doesn’t change even if the speeds differ. Likewise, if the speeds are the same in both media, there’s no bending even with an oblique approach. So the condition for refraction to occur is oblique incidence together with different propagation speeds between the two media. In ultrasound terms, a change in speed across the boundary (due to different tissues) at an angle makes the beam bend, as described by Snell’s law.