Refraction occurs when crossing an interface under which condition?

Refraction occurs when crossing an interface under which condition?

• A. The beam is perpendicular to the interface.
• B. The speeds of propagation are different and the angle of incidence is not perpendicular.
• C. The speeds of propagation are identical.
• D. The interface is curved.

Answer: (C)

Refraction happens because a wave changes speed as it passes from one medium to another, and that speed change causes the direction to bend only if the entry is not head-on. If the speed is the same in both media, there’s no change in direction. If the ray crosses perpendicularly, even with a speed difference, it travels straight through along the normal with no bend. So the bending—you see refraction—occurs when the speeds are different and the incidence is at an oblique angle.

Refraction happens because a wave changes speed as it passes from one medium to another, and that speed change causes the direction to bend only if the entry is not head-on. If the speed is the same in both media, there’s no change in direction. If the ray crosses perpendicularly, even with a speed difference, it travels straight through along the normal with no bend. So the bending—you see refraction—occurs when the speeds are different and the incidence is at an oblique angle.