Both in-phase and out-of-phase wave pairs undergo interference.

• A. Interference
• B. Refraction
• C. Diffraction
• D. Scattering

Answer: (A)

Interference is the process that happens when two waves meet and superpose, producing a result that depends on how their phases line up. When the waves are in-phase, their crests align with crests and troughs with troughs, so they reinforce each other and the amplitude grows—constructive interference. When they are out-of-phase, crests meet troughs and partially cancel, lowering the amplitude—destructive interference. In both situations, the key idea is that the waves combine to produce a new resulting wave, which is exactly what interference describes. Refraction involves bending due to a change in speed at a boundary, diffraction is spreading and bending around obstacles (often involving interference as a secondary effect), and scattering is redirection by objects. Hence the phenomenon described is interference.