Refraction requires oblique incidence and which condition?

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

Answer: (B)

Refraction occurs when a wave crosses into a new medium and changes direction. This bending only happens if the wave meets the boundary at an angle, not head-on. If the incidence is normal (perpendicular to the boundary), there’s no path bend because there’s no component of the wavefront sliding along the boundary to cause a change in direction.

In addition, the two media must have different propagation speeds. When speeds differ, the transmitted wave travels at a different velocity, and the angle of transmission shifts according to Snell’s law. If the speeds are the same, the path continues straight and the angle of incidence equals the angle of transmission, so no refraction occurs even though some energy might be reflected if there’s impedance mismatch.