Thomas Young's sketch of two-slit diffraction of waves, 1803
A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated.
The interference of two waves. When in phase, the two lower waves create constructive interference (left), resulting in a wave of greater amplitude. When 180° out of phase, they create destructive interference (right).
The photoelectric effect. Incoming photons on the left strike a metal plate (bottom), and eject electrons, depicted as flying off to the right.
The electromagnetic spectrum, with the visible portion highlighted
Interference of right traveling (green) and left traveling (blue) waves in Two-dimensional space, resulting in final (red) wave
Propagation of de Broglie waves in 1d—real part of the complex amplitude is blue, imaginary part is green. The probability (shown as the colour opacity) of finding the particle at a given point x is spread out like a waveform; there is no definite position of the particle. As the amplitude increases above zero the curvature decreases, so the amplitude decreases again, and vice versa—the result is an alternating amplitude: a wave. Top: Plane wave. Bottom: Wave packet.
Interference of waves from two point sources.
Couder experiments, "materializing" the pilot wave model
Beam of sun light inside the cavity of Rocca ill'Abissu at Fondachelli-Fantina, Sicily
A magnified image of a coloured interference pattern in a soap film. The "black holes" are areas of almost total destructive interference (antiphase).
Particle impacts make visible the interference pattern of waves.
Due to refraction, the straw dipped in water appears bent and the ruler scale compressed when viewed from a shallow angle.
Geometrical arrangement for two plane wave interference
A quantum particle is represented by a wave packet.
Hong Kong illuminated by colourful artificial lighting.
Interference fringes in overlapping plane waves
Interference of a quantum particle with itself.
Pierre Gassendi.
Optical interference between two point sources that have different wavelengths and separations of sources.
Christiaan Huygens.
Creation of interference fringes by an optical flat on a reflective surface. Light rays from a monochromatic source pass through the glass and reflect off both the bottom surface of the flat and the supporting surface.  The tiny gap between the surfaces means the two reflected rays have different path lengths. In addition the ray reflected from the bottom plate undergoes a 180° phase reversal.  As a result, at locations (a) where the path difference is an odd multiple of λ/2, the waves reinforce.   At locations (b) where the path difference is an even multiple of λ/2 the waves cancel.  Since the gap between the surfaces varies slightly in width at different points, a series of alternating bright and dark bands, interference fringes, are seen.
Thomas Young's sketch of a double-slit experiment showing diffraction. Young's experiments supported the theory that light consists of waves.
White light interference in a soap bubble. The iridescence is due to thin-film interference.
The Very Large Array, an interferometric array formed from many smaller telescopes, like many larger radio telescopes.

Interference effects can be observed with all types of waves, for example, light, radio, acoustic, surface water waves, gravity waves, or matter waves.

- Wave interference

Like all types of electromagnetic radiation, visible light propagates by massless elementary particles called photons that represents the quanta of electromagnetic field, and can be analyzed as both waves and particles.

- Light

Democritus (5th century BC) argued that all things in the universe, including light, are composed of indivisible sub-components.

- Wave–particle duality

Light can be explained classically by the superposition of waves, however a deeper understanding of light interference requires knowledge of wave-particle duality of light which is due to quantum mechanics.

- Wave interference

The resulting Huygens–Fresnel principle was extremely successful at reproducing light's behaviour and was subsequently supported by Thomas Young's discovery of wave interference of light by his double-slit experiment in 1801.

- Wave–particle duality

Soon after, Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction and interference.

- Light
Thomas Young's sketch of two-slit diffraction of waves, 1803

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