Wave-particle duality

The wave-particle duality (or particle-wave duality) refers to the double nature of light and matter at the quantum level.

The debate arguably began in the 17th century with the competing theories of Christiaan Huygens and Isaac Newton. Huygen's observations led him to a wave theory of light while Newton's observations supported a corpuscular or particle theory. Newton's preeminence as the leading mind in related matters led to the dominance of his theory.

Eventually, subsequent research in the 19th and 20th centuries provided compelling support for both theories—light was found to behave as both a particle and a wave, a characteristic shared by the constituents of solid matter (electrons, atoms and molecules).

Wave theory was strongly supported and continued in optics for more than 100 years. However, in chemistry the atomic structure of matter provided insight into molecular laws of definite proportions. In physics, the atomic theories led to the development of interpretation of numerous properties of solids, liquids and gases including the kinetic theory of gases and the atomic constitution of electricity (e.g. the work of J. J. Thomson in 1899) and H. A. Lorentz’s use of electrons in the theory of interaction of matter with electromagnetic radiation.

Thomas Young's double-slit experiment in 1803 strongly indicated that light has wave characteristics. Then in 1924, Prince Louis-Victor de Broglie (1892-1987) presented a hypothesis positing the wave characteristics of particles with small rest mass, such as electrons, and suggested that a wavelength relationship applies to other particles with non-zero rest mass as well. DeBroglie's hypothesis, unequivocally supported by Davisson and Germer's research in 1927 on the diffraction of electron beams by nickel crystals, firmly established the wave-mechanics theory underlying current understanding of the wave nature of matter. In 1926, Erwin Schrödinger's (1887-1961) wave equation described the behaviour of electrons and other particles employing wave concepts.

Work on the theoretical basis of the particle nature of radiation received support when the granular structure of light and other types of radiation was confirmed by the discovery of the photoelectric effect: X-rays or light striking solid matter results in the emission of electrons from that matter, showing a direct relationship between the energy of the electrons and the frequency of the incident radiation (the radiation that strikes the solid matter). The subsequent explanation of this phenomenon led to Albert Einstein's theory of light quanta proposed in 1905. In 1922, Arthur Compton observed and reported what is now called the Compton effect, establishing the particle nature of light - photons. In 1927 Dirac quantized the electromagnetic field, thus putting photons (light quanta) on a firm theoretical foundation.

Example
Wavelengths of various particles with non-zero rest mass according to  $$\lambda = \frac{h}{mv}$$, where &lambda; is the wavelength, h is Planck's constant, m the rest mass, and v the speed of the particle.

From: W. J. Moore, Physical Chemistry, Longmans, London, 2nd edition (1956), p. 271