Photon
One of the fundamental dichotomies in classical physics was that between energy and matter. By the late nineteenth century, most scientists agreed that matter is composed of tiny, discrete particles with measurable mass. At the time, the atom was considered to be the ultimate particle of which all matter consists. Energy, on the other hand, was thought to have no material basis, but corresponded to anything that was able to cause a change of position in some form of matter. Most forms of energy were thought to travel through space as waves. The duality between particles and waves was, therefore, a fundamental tenet of physical theory.
By the turn of the century, however, the wave-particle duality began to come apart fairly rapidly. A critical factor in this change was Albert Einstein's analysis of the photoelectric effect. Einstein showed that the emission of electrons from a metal that has been exposed to light can best be explained by assuming that light consists of tiny "packages" of energy. The energy of each package was found to be a function of the wavelength of the light, , where is a constant of proportionality, Planck's constant.
The concept of an energy "package" was not originally conceived by Einstein. In 1903, Joseph J. Thomson had found it useful to talk about energy " specks" in describing his work on ionization of gases. There is no question, however, that the clearest expression of this concept came from Einstein's work.
In the 1920s, Arthur Holly Compton proposed the name photon (from the Greek photos, for "light") for this "package" of energy. Compton's own work involved the scattering of x-ray s by atoms in a crystalline lattice. Compton adopted Einstein's approach to explain the fact that x-rays reflected from the crystals have longer wavelengths than the incident x-rays.
By assigning x-rays the property of momentum, a property traditionally reserved for particles, Compton was able to calculate exactly the observed properties of the scattered x-rays. He later wrote that his experiments probably "were the first to give, at least to physicists in the United States, a conviction of the fundamental validity of the quantum theory."
The photon concept is important historically because it marks the first time that language normally used for particles was applied to energy phenomena. Consequently, scientist began to study both matter and energy as two manifestations of a common, or at least related, phenomenon. For example, Werner Heisenberg adopted the photon concept in his development of quantum theory. In fact, the photon is usually described today as the quantum of light, a particle with zero charge and zero mass that mediates the transfer of the electromagnetic force. In modern particle physics theory, the photon is described as a boson, a particle with integral spin that acts as carrier of the electromagnetic force.
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