What Is Interference?

Interference is the phenomenon that occurs when two or more waves of the same physical type overlap in space, producing a resultant wave whose amplitude at each point is determined by the superposition of the individual wave amplitudes at that point. The principle of superposition states that the total displacement of the medium is the algebraic sum of the displacements produced by each wave independently. Interference can increase, decrease, or completely cancel the amplitude of the original waves, depending on the phase relationship between them at each point of overlap. The phenomenon is universal to all wave types, including electromagnetic radiation, acoustic waves, water surface waves, and quantum mechanical probability amplitudes.

Interference was first demonstrated experimentally for light by Thomas Young in 1801, using two narrow slits to split a single light beam and allowing the separated beams to recombine. The alternating bright and dark fringes he observed constituted direct evidence for the wave nature of light. Young's experiment remains the canonical demonstration of interference and is now performed with single photons to illustrate the quantum superposition principle, as documented in research on Young's double-slit interference with single photons.

Constructive and Destructive Interference

Constructive interference occurs when waves arrive at a point in phase, meaning their phase difference is zero or a whole-number multiple of 2π. At such points the amplitudes add, producing a maximum. Destructive interference occurs when waves arrive out of phase by π radians (half a wavelength), causing the amplitudes to cancel. For two equal-amplitude sinusoidal waves, perfect destructive interference results in zero amplitude at those points. Between the extremes, the resultant amplitude varies continuously. In optical systems this produces the familiar pattern of alternating bright and dark fringes whose spacing depends on the wavelength and the geometry of the source configuration. In acoustics, destructive interference between a sound source and a carefully positioned secondary source is the operating principle behind active noise control headphones and architectural acoustic treatment.

Coherence and Fringe Visibility

Interference fringes are observable only when the interfering waves are mutually coherent, meaning they maintain a stable phase relationship over the observation time and across the spatial extent of the overlap region. Temporal coherence quantifies how far apart in time two points on the same wave can be while still interfering; it is related to the coherence length, the path-length difference over which fringe contrast remains above a threshold. Spatial coherence describes the phase correlation between two laterally separated points on the wavefront. Lasers have high temporal and spatial coherence; thermal light sources have low coherence and produce fringes only over short path-length differences. The role of coherence in determining fringe visibility and the boundary between what classifies as coherent versus incoherent interference is analyzed in work on incoherent interference, which extends superposition theory to fields with fluctuating phases. The NIST study on measurement-induced decoherence in double-slit interference explores how gaining which-path information systematically degrades fringe contrast, connecting coherence to quantum information.

Electromagnetic and Radio-Frequency Interference

In electrical engineering and communications, interference refers specifically to the superposition of undesired electromagnetic signals onto a channel of interest. Multipath interference arises when a transmitted signal reaches a receiver by two or more paths of different lengths, so that delayed copies of the transmitted wave add with variable phases. This causes frequency-selective fading in mobile communications and ghost images in analog broadcasting. Intentional use of multipath through antenna arrays, known as beamforming, exploits constructive interference to focus signal energy in a desired direction while using destructive interference to suppress energy toward interferers.

Applications

Interference has applications in a wide range of fields, including:

• Optical metrology and interferometry, where sub-wavelength surface displacements are measured by monitoring fringe shifts
• Gravitational wave detection, where the LIGO observatory uses kilometer-scale Michelson interferometers sensitive to path-length changes smaller than 10^-18 meters
• Thin-film optics, where interference in deposited coatings produces anti-reflection and high-reflectance coatings for cameras, telescopes, and lasers
• Wireless communications, where multipath interference models guide the design of OFDM waveforms and MIMO antenna systems
• Quantum computing, where quantum algorithms exploit interference among probability amplitudes to amplify correct answers and cancel incorrect ones