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Introduction

For decades, physicists of the nineteenth century knew of the photoelectric effect: the emission of electrons from a metallic surface when light fell on the surface. But an understanding of the phenomenon was lacking until Einstein explained it in 1905 with a radical theory. The basic ingredient of the theory was that light behaved as a collection of particles called photons and not as a wave. Years later, de Broglie made the equally radical suggestion that particles have wave properties.