The electrons in an atom are not confined to one region of space, but are spread out. They are acting more like waves than particles. In Louis-de Broglie’s (1892-1987) 1924 doctoral thesis he proposed that electrons behave like waves with a wavelength given by λ = h/mv, where h is Planck’s constant, and m and v the mass and velocity of the electron. The thesis was forwarded to Einstein, who enthusiastically endorsed the idea and recommended that the thesis be approved. De Broglie was awarded the Nobel Prize in 1929 for this work. The de Broglie wavelength is associated with any particle, although for an object the size of a baseball it is much smaller than the diameter of a nucleus!

The de Broglie wavelength of a particle determines its wave-like properties. Just as light photons interfere with themselves in a two-slit experiment, so do particles. The interference of electrons, atoms, and even molecules as large as C₆₀ (so-called Bucky-balls) has been observed, and the measurements fit de Broglie’s wavelength perfectly.

So, both matter and light can act like either a particle or a wave. This phenomenon is given the name “wave-particle duality.”