During the last three hundred years scientists have spent a great deal of time discussing and investigating the nature of light. In the seventeenth century Sir Isaac Newton believed that light rays consisted of streams of very small particles. This Corpuscular Theory persisted for many years through Christian Huygens, a contemporary of Newton, had the notion that light might be transmitted by vibrations (i.e. waves) in the aether.

However, at the beginning of the nineteenth century Thomas Young carried out his famous interference experiments. His observations could be explained only by assuming that light is transmitted as waves and not as a stream of particles. Furthermore the Wave Theory seemed to account for all experimental observations made at that time, and it appeared that this theory had replaced the Corpuscular theory for all time.

Then at the end of the nineteenth century it was found that under certain conditions electrons were liberated when light feel on a surface. The Wave theory could not explain this photoelectric effect. This startling new discovery left physicists with a serious dilemma. The photoelectric effect could best be explained by reverting to Corpuscular Theory, although almost all other evidence pointed to light’s being a form of wave motion.

These were some of the theoretical problems which faced physicists when Louis De Broglie, a young Frenchman of noble birth, came on the scene. In a thesis published in 1922, when he was only thirty years old, he suggested that light could behave either as a wave or as a steam of particles, but not both at the same time.

He argued that if light which was normally a form of wave motion could take on a corpuscular (particle) form, then small particles such as electrons could also have wave-like characteristics associated with them. However, he had to wait five years for the evidence. Then in 1927 two Americans, Clinton J. Davisson and L.H. Germer working at the Bell Telephone Laboratory succeeded in diffracting (bending into shadows) a beam of electrons using a crystal as the diffraction grating.

Louis De Broglie’s dual theory can be applied to any moving particle whatever its nature. The wavelength of the ‘de Broglie’ wave (the wave associated with the particle) is found by dividing the momentum of the particle into Planck’s constant.

Louis Victor De Broglie was born as Dieppe in France in 1892. His elder brother Maurice the sixth Duc de Broglie, was also a physicist of some note. Louis was at first interested in history and literature, but after serving in the French army during World War I he took up physics.

In recognition of his contribution to the advance of theoretical physics, Louis de Broglie was awarded the Noble Prize in 1929. Since 1928 he has been Professor of theoretical physics at the University of Paris where he had previously received his training.

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