The duality concept states that all particles, even molecules, can behave as waves. This also holds for light. In the nineteenth century, Maxwell showed that light consists of electromagnetic waves; and in 1905 Einstein described its particle nature as 'photons' — quanta of energy. The link with matter was made by de Broglie in 1924, when he showed by analogy that a particle has a wavelength associated with it by virtue of its motion.

The complementarity of these two descriptions is interwoven with the concept of coherence. Two or more waves can produce interference patterns when their amplitudes add up coherently. Interference phenomena, observed for both light and matter, embody many abstract concepts, including superposition of waves and the uncertainty principle.

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