When the frequencies of the waves differ, which interference types can occur?

• A. Only constructive interference
• B. Only destructive interference
• C. Constructive interference and Destructive interference
• D. No interference

Answer: (C)

Interference depends on how the waves line up in phase, not on having the same frequency. When two waves have different frequencies, their phase difference changes over time, so the combined signal periodically strengthens and weakens. At moments when the waves are in phase, you get constructive interference (larger amplitude); when they are out of phase, you get destructive interference (reduced amplitude). This leads to a beating pattern where the overall intensity varies at the difference between the frequencies. So both constructive and destructive interference can occur, even though a stable, fixed fringe pattern isn’t maintained unless the frequencies are equal and the waves are coherent.