He was inspired by the work of Albert Einstein, who had shown that light can behave as both a wave and a particle (the photoelectric effect). de Broglie reasoned that if light, which was thought to be purely wave-like, could exhibit particle-like properties, then matter, which was thought to be purely particle-like, might also exhibit wave-like properties.

His hypothesis, known as the de Broglie hypothesis, states that all matter has wave-like properties and that the wavelength of a particle is inversely proportional to its momentum. This hypothesis was later experimentally confirmed by the observation of electron diffraction by Clinton Davisson and Lester Germer in 1927.