Interference is one of the most striking demonstrations of the wave nature of physical phenomena. It occurs when two or more waves meet in space and time, causing the total displacement at any point to be the sum of the individual wave displacements. This simple principle of superposition leads to patterns that can be bright or dark, loud or quiet, enhanced or diminished, depending on how the waves align. Though often explained using light, interference is a universal phenomenon observed in water waves, sound waves, and even quantum mechanical probability waves.
At its core, interference depends on the relative phase of overlapping waves. When the crests of two waves coincide, their amplitudes add to produce a larger crest. This is called constructive interference and results in regions of increased intensity or brightness. Conversely, when the crest of one wave meets the trough of another, they partially or completely cancel each other, producing destructive interference. These cancellations form regions of reduced intensity or darkness. The alternation of constructive and destructive interference creates a characteristic pattern, often visible as bright and dark fringes.
A classic demonstration of interference with light is the double-slit experiment. When coherent light—meaning light with a constant phase relationship, typically produced by lasers—passes through two closely spaced slits, each slit becomes a source of secondary wavefronts. These wavefronts overlap on a screen, generating a pattern of bright and dark bands. The existence of such a fringe pattern cannot be explained by treating light as particles alone; instead, it confirms the wave nature of light. The spacing and intensity of the fringes depend on the wavelength of the light, the separation between the slits, and the distance to the screen.
Interference also appears in everyday experiences. Noise-canceling headphones use destructive interference to reduce unwanted ambient sounds. Small microphones detect incoming noise, and the device generates a sound wave with the same amplitude but opposite phase. The two waves cancel, reducing the sound heard by the listener. Similarly, soap bubbles and oil films show shimmering colors because light reflecting from the top and bottom surfaces of the thin film interferes. Depending on the thickness of the film and the wavelength of the light, certain colors undergo constructive interference while others cancel out, producing vivid iridescence.
In more advanced physics, interference plays a foundational role in quantum mechanics. Particles such as electrons exhibit wave-like behavior, and when sent one at a time through a double-slit apparatus, they still form an interference pattern over time. This outcome suggests that even a single particle interferes with itself, indicating the probabilistic and wave-based nature of quantum states. Interference is also essential in technologies like interferometers, which measure extremely small displacements using the phase difference between light beams. The detection of gravitational waves by LIGO relied on measuring interference patterns to a precision smaller than the width of a proton.
Interference requires coherence, meaning the waves must maintain a stable phase relationship. Light from most everyday sources is incoherent and does not produce stable interference patterns. Only under controlled conditions—such as with lasers or carefully aligned wave sources—can clear, reproducible interference effects be observed. This coherence requirement also explains why interference patterns disappear when the path length difference between waves fluctuates or when disturbances introduce random phase shifts.
Ultimately, interference illustrates the fundamental principle that waves do not collide like solid objects but instead combine in space according to the rules of superposition. The resulting patterns—whether seen as ripples on water, colors on a soap film, or fringes in a laboratory experiment—reveal the rich and sometimes counterintuitive behavior of waves. The study of interference has deepened scientific understanding across classical and quantum physics, demonstrating how waves shape both the observable world and the underlying structure of physical reality.