Louis de Broglie was a French physicist who lived from 1892-1987. He published his theory on wave-particle duality in 1929. It was based on earlier work from Einstein in 1905 when he explained the photoelectric effect by postulating the quantization of light energy via photons. De Broglie took the argument further by postulating that all matter in motion must have both particle and wave-like properties. His equation relating this is:
lambda = h/p
where lambda is the wavelength, h is Planck's constant, and p is the particle's momentum. De Broglie's ideas on the dual nature of matter were later confirmed in independent experiments a few years later. He created the field of wave mechanics in physics and won the Nobel prize in physics in 1929.