When two waves meet, they superpose — their amplitudes add together. This creates regions of constructive and destructive interference.

When two wave crests meet, amplitudes add up. The result is a brighter, stronger wave. Path difference = nλ (whole number of wavelengths).

When a crest meets a trough, they cancel out. The result is silence — zero amplitude. Path difference = (n + ½)λ.

The interference pattern in the double-slit experiment is caused by exactly this. Each slit acts as a wave source. Bright fringes = constructive. Dark fringes = destructive.