Wave-particle duality

Electrons and electromagnetic radiation can each show behaviour associated with both waves and particles. This is called wave-particle duality. Which behaviour is observed depends on the experiment: wave-like behaviour can appear in a diffraction pattern, while particle-like behaviour can appear in individual, localised interactions.

Electron diffraction: wave behaviour of particles

When a beam of electrons passes through a thin material, the electrons can form a diffraction pattern. Such a pattern is associated with waves spreading out and combining. Its appearance shows that electrons, although detected as particles, also have wave-like properties.

The photoelectric effect: particle behaviour of radiation

In the photoelectric effect, electromagnetic radiation causes electrons to be emitted from a material. The effect can be understood by treating the radiation as transferring energy in discrete packets called photons.

An electron can absorb energy from a photon. If the photon transfers enough energy, the electron can escape from the material. This transfer in individual packets provides evidence for the particle-like nature of electromagnetic radiation.

de Broglie wavelength and particle diffraction

A moving particle has a de Broglie wavelength: a wavelength associated with its wave-like behaviour. The wavelength depends on the particle’s momentum.

Here, $$\lambda$$ is the de Broglie wavelength, $$h$$ is the Planck constant, $$p$$ is momentum, $$m$$ is mass and $$v$$ is speed. The form $$p = mv$$ applies when the particle’s speed is much less than the speed of light.

Wavelength is inversely proportional to momentum: increasing momentum decreases the wavelength, while decreasing momentum increases it. For example, doubling the momentum halves the wavelength.

de Broglie wavelength and diffraction

Diffraction is the spreading of wave behaviour when a particle passes through a gap or encounters a regularly spaced structure. For a fixed gap or spacing, a longer de Broglie wavelength produces more diffraction; a shorter wavelength produces less.

Diffraction is most evident when the wavelength is comparable to the size of the gap or spacing. If the wavelength is much smaller, the diffraction is less pronounced. Thus, for the same gap or spacing, increasing a particle’s momentum shortens its wavelength and reduces its diffraction.

Development and validation of scientific knowledge

Scientific explanations are based on evidence, but they are not fixed forever. New observations may reveal results that an existing explanation cannot account for. Scientists can then revise or extend the explanation and test whether the changed version accounts for the evidence.

For example, matter was once understood using an exclusively classical particle picture. Evidence such as the diffraction of electrons showed that electrons can also exhibit wave-like behaviour. An explanation based only on classical particles could not account for this behaviour, so understanding of matter had to be extended.

Peer review and wider validation

When scientists propose findings or changes to an explanation, other scientists scrutinise their evidence, methods and reasoning. This process is called peer review. Reviewers consider whether the methods are appropriate and whether the evidence supports the conclusions.

Peer review does not guarantee that a claim is correct. The wider scientific community may examine the work, repeat investigations and gather further evidence. Agreement across independent investigations increases confidence in a finding; conflicting or new evidence may lead to further revision.