Refraction occurs only if what two conditions are satisfied?

• A. Oblique incidence; Propagation speeds of the two media are different
• B. Normal incidence; Propagation speeds of the two media are different
• C. Oblique incidence; Propagation speeds are identical
• D. Normal incidence; Propagation speeds are identical

Answer: (A)

Refraction is the bending of a wave as it crosses into another medium because its speed changes. For this bending to occur, two things must happen together: the wave must hit the boundary at an angle (not head-on), and the propagation speeds in the two media must be different. If the incidence is normal, the wave enters perpendicular to the boundary and continues straight, so there’s no bending even if speeds differ. If the speeds are the same in both media, there’s no change in direction regardless of the angle of incidence. Therefore, refraction occurs only when incidence is oblique and the propagation speeds differ across the boundary.