Out-of-phase waves can produce destructive interference. What happens to the resultant amplitude?

• A. It becomes smaller.
• B. It becomes larger.
• C. It remains the same.
• D. It doubles.

Answer: (A)

Destructive interference happens when two waves are out of phase by 180 degrees, so their peak aligns with the other's trough and they partially cancel each other. The resultant amplitude is the difference of the two amplitudes, |A1 − A2|. This is generally smaller than the original amplitudes, so the net amplitude decreases. If the two waves have equal amplitudes, they cancel completely and the resultant amplitude is zero; if they’re not equal, you’re left with a reduced amplitude equal to the difference. Energy isn’t created or destroyed—it's redistributed, producing patterns of cancellation and reinforcement in space.