The bending of a wave as it enters a new medium at an angle. This occurs because one side of the wave moves more slowly than the other side.

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

Answer: (A)

Refraction is the bending of a wave as it passes from one medium into another at an angle. When the wave hits the boundary, part of it enters the new medium first and travels at a different speed than the rest of the wave still in the original medium. This difference in speed on the two sides of the boundary causes the wavefront to tilt, so the direction of the wave changes as it continues. Since the frequency stays the same across the boundary, the change in speed means the wavelength also changes in the new medium.

This is what you see when light enters water from air and appears to bend, or why sound travels at different angles in air versus air with different temperatures. The other options aren’t describing the boundary behavior: reflection is bouncing off the surface, diffraction is bending around edges, and wavelength is a property that relates to speed and frequency but doesn’t by itself explain bending at a boundary.