What is the phenomenon where two or more waves combine to form a new wave?

The phenomenon in which two or more waves combine to form a resultant wave is interference.

Interference follows the principle of superposition: when waves overlap, their displacements add together. If crests and troughs line up in phase, they produce constructive interference and a larger amplitude. If a crest meets a trough, destructive interference reduces the amplitude and can cancel it at particular points.

The effect occurs with water, sound, light, radio, gravitational, and matter waves. Thomas Young’s early-19th-century double-slit experiment made interference a powerful demonstration of light’s wave nature. The bright and dark bands in that experiment arise because different paths give the waves different phase differences.

Interference does not destroy energy. Where waves cancel, energy is redistributed to other locations. Diffraction is related but describes spreading and bending around apertures or obstacles; it often produces interference patterns, but the terms are not interchangeable.