Which description matches constructive interference?

• A. Destructive interference
• B. Diffraction
• C. Interference
• D. Constructive interference

Answer: (D)

When waves overlap, their displacements combine. If they arrive in the same phase, crests meet crests and troughs meet troughs, so their amplitudes reinforce each other and the resulting wave has a larger amplitude (and, for light, greater intensity). That situation is constructive interference—the outcome where the waves add up rather than cancel. The other ideas are related but describe different things: destructive interference is when waves are out of phase and reduce the overall amplitude; diffraction is the bending of waves around obstacles; interference is the general phenomenon of overlapping waves, with constructive interference being the specific case where they reinforce.