interference

Interference is the phenomenon that occurs when two or more waves overlap in space, producing a resulting wave whose amplitude is determined by the combination of the individual waves. It follows the principle of superposition: where waves intersect, displacements add together. Interference can be constructive, destructive, or a mixture, depending on the relative phase of the waves. When crests align with crests, or troughs with troughs, the amplitudes reinforce (constructive interference); when crests align with troughs, the waves partially or completely cancel (destructive interference).

Coherence, meaning a stable phase relationship between the sources, is essential for persistent interference patterns. Interference is most striking with light, sound, or other waves that propagate as vibrations. Classic demonstrations include Young’s double-slit experiment, which shows alternating bright and dark fringes from two coherent light sources. Thin-film interference, seen in soap bubbles, oil films, or the iridescence of a CD, arises from light reflecting from multiple interfaces with different optical paths.

Applications of interference include interferometry, metrology, and holography, as well as astronomical techniques that use multiple telescopes to measure small angular separations. Interference can also be a practical concern in communication systems, where signal overlap from other transmitters or reflections degrades quality. Mitigation methods include filtering, shielding, careful antenna design, and phase control.