Which condition correctly describes refraction at a boundary?

Which condition correctly describes refraction at a boundary?

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

Answer: (D)

Refraction happens when a light ray crosses a boundary between two media with different speeds and the incidence is not perpendicular. The change in speed causes the wavefront to tilt, so the ray bends. If the ray hits the boundary straight on (normal incidence), it doesn’t bend even if the speeds differ. If the speeds are the same on both sides, there’s no bending regardless of the angle. So the condition that produces refraction is oblique incidence combined with different speeds.

Refraction happens when a light ray crosses a boundary between two media with different speeds and the incidence is not perpendicular. The change in speed causes the wavefront to tilt, so the ray bends. If the ray hits the boundary straight on (normal incidence), it doesn’t bend even if the speeds differ. If the speeds are the same on both sides, there’s no bending regardless of the angle. So the condition that produces refraction is oblique incidence combined with different speeds.