Louis de Broglie was a French physicist born on August 15, 1892. He is best known for his groundbreaking theory that matter, like light, exhibits both particle and wave properties. This concept, known as wave-particle duality, was a significant contribution to the field of quantum mechanics. In 1924, de Broglie proposed that particles such as electrons could be described as waves, leading to the development of the de Broglie wavelength. His ideas influenced many scientists, including Albert Einstein and Niels Bohr, and earned him the Nobel Prize in Physics in 1929. De Broglie's work laid the foundation for modern quantum theory.