If two waves of the same frequency travel in the same medium and out of phase by 180 degrees, what happens?

• A. Constructive interference
• B. Refraction
• C. Destructive interference
• D. Reflection

Answer: (C)

When two waves overlap in the same medium, their displacements simply add together (superposition). If they have the same frequency and are exactly 180 degrees out of phase, every crest of one meets a trough of the other. This opposite alignment causes their amplitudes to subtract, so the resulting wave is greatly reduced or can cancel out completely if their amplitudes are equal. This is destructive interference: the waves interfere to reduce the overall amplitude.

If their amplitudes aren’t equal, you still get cancellation to some extent, but not perfect, leaving a smaller resultant wave. This situation is different from refraction (bending at a boundary) or reflection (bouncing off a boundary), and it’s also different from constructive interference, which happens when waves are in phase (0 or multiples of 360 degrees). Destructive interference is the outcome of a 180-degree phase difference.