4.5.3 Wave–particle duality

The wave-particle duality is a model used to describe how all matter has both wave and particle properties. De Broglie realised that all particles travel through space as waves and anything with mass that is moving has wave-like properties (these waves are known as matter waves/de Broglie waves).

Usually we would describe electrons as particles (as they have mass and charge) and as a result they can be accelerated and deflected by electric and magnetic fields. But under certain conditions we can also make electrons diffract - they spread out like waves as they pass through a tiny gap and even form diffraction patterns. If an electron gun fires electrons at a thin piece of polycrystalline graphite the electrons pass through the gaps between individual carbon atoms. The gaps are so small the electrons diffract and form a diffraction pattern. The electrons are behaving as particles when they are accelerated by the p.d. and they behave as waves when they diffract. They then behave as particles again when they hit the screen.

De Broglie realised that the wavelength of a particle was inversely proportional to its momentum. Further investigation lead to the de Broglie equation…

λ = h/p

The de Broglie equation can be applied to all particles. For example, protons and neutrons have also been shown to have wave properties and form diffraction patterns. However, as particles become larger their wave properties become harder to observe. The mass of a proton is much greater than the mass of an electron so at the same speed their momentum is significantly greater so their wavelength is much smaller and therefore harder to observe.