Wave-Particle Duality - Definition

Light Acts As Both a Wave and a Particle

Wave Particle Duality means every elementary particle exhibits properties of both particles and waves.
Wave Particle Duality means every elementary particle exhibits properties of both particles and waves. Duncan Walker, Getty Images

Wave-Particle Duality Definition

Wave-particle duality describes the properties of photons and subatomic particles to exhibit properties of both waves and particles. Wave-particle duality is an important part of quantum mechanics as it offers a way to explain why concepts of "wave" and "particle", which work in classical mechanics, don't cover the behavior of quantum objects. The dual nature of light gained acceptance after 1905, when Albert Einstein described light in terms of photons, which exhibited properties of particles, and then presented his famous paper on special relativity, in which light acted as a field of waves.

Particles That Exhibit Wave-Particle Duality

Wave-particle duality has been demonstrated for photons (light), elementary particles, atoms, and molecules. However, the wave properties of larger particles, such as molecules, have extremely short wavelengths and are difficult to detect and measure. Classical mechanics is generally sufficient for describing behavior of macroscopic entities.

Evidence for Wave-Particle Duality

Numerous experiments have validated wave-particle duality, but there are a few specific early experiments that ended the debate about whether light consists of either waves or particles:

Photoelectric Effect - Light Behaves as Particles

The photoelectric effect is the phenomenon where metals emit electrons when exposed to light. The behavior of the photoelectrons could not be explained by classical electromagnetic theory. Heinrich Hertz noted that shining ultraviolet light on electrodes enhanced their ability to make electric sparks (1887).

Einstein (1905) explained the photoelectric effect as resulting from light carried in discrete quantized packets. Robert Millikan's experiment (1921) confirmed Einstein's description and led to Einstein winning the Nobel Prize in 1921 for "his discovery of the law of the photoelectric effect" and Millikan winning the Nobel Prize in 1923 for "his work on the elementary charge of electricity and on the photoelectric effect".

Davisson-Germer Experiment - Light Behaves as Waves

The Davisson-Germer experiment confirmed the deBroglie hypothesis and served as a foundation for the formulation of quantum mechanics. The experiment essentially applied the Bragg law of diffraction to particles. The experimental vacuum apparatus measured the electron energies scattered from the surface of a heated wire filament and allowed to strike a nickel metal surface. The electron beam could be rotated to measure the effect of changing the angle on the scattered electrons. The researchers found that the intensity of the scattered beam peaked at certain angles. This indicated wave behavior and could be explained by applying the Bragg law to the nickel crystal lattice spacing.

Thomas Young's Double-Slit Experiment

Young's double slit experiment can be explained using wave-particle duality. Emitted light moves away from its source as an electromagnetic wave. Upon encountering a slit, the wave passes through the slit and divides into two wave fronts, which overlap. At the moment of impact onto the screen, the wave field "collapses" into a single point and becomes a photon.