A change in direction of wave propagation when passing from one medium to another is called

• A. Refraction
• B. Reflection
• C. Diffraction
• D. Absorption

Answer: (A)

Refraction is the bending of a wave as it passes from one medium into another because the wave changes speed in the new medium. When part of the wavefront enters the second medium at an angle, it slows down or speeds up relative to the portion that’s still in the first medium, so the overall direction of the wave changes. If the wave enters a denser medium where it travels slower, it bends toward the normal (the line perpendicular to the boundary). If it enters a less dense medium and speeds up, it bends away from the normal. This change in direction is described by Snell’s law, which ties the angles to the speeds or refractive indices of the two media.

A familiar example is a straw appearing bent when it’s partly submerged in water—the light from the submerged part slows down and changes direction as it exits into air. Other phenomena that involve different boundary effects—reflection (bouncing off the boundary), diffraction (bending around edges or through openings), and absorption (loss of energy into the medium)—are distinct from refraction, which specifically describes the change in direction due to a change in wave speed across the boundary.