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AP1200_CH7_ModernPhysics-2007 - Introduction to Quantum...

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Introduction to Quantum Physics The characteristics of blackbody radiation cannot be explained using classical concepts. Planck first introduced the quantum concept when he assumed that the atomic oscillators responsible for this radiation existed only in discrete states. The photoelectric effect is a process whereby electrons are ejected from a metallic surface when light incident on that surface. Einstein provided a successful explanation of this effect by extending Planck’s quantum hypothesis to the electromagnetic field. In this model, light is viewed as a stream of particles called photons, each with energy E = hf, where f is the frequency and h is Planck’s constant. The kinetic energy of the ejected photoelectron is given by φ - = hf K max where φ is the work function of the metal. X-rays from an incident beam are scattered at various angles by electrons in a target such as carbon. In such a scattering event, a shift in wavelength is observed for the scattered x-rays, and the phenomenon is known as the Compton effect . Classical physics does not explain this effect. If the x-ray is treated as a photon, conservation of energy and momentum applied to the photon-electron collisions yields the following expression for the Compton shift: ) cos 1 ( θ λ - = mc h where m is the mass of the electron, c is the speed of light, and θ is the scattering angle.
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Light has a dual nature in that it has both wave and particle characteristics. Every object of mass m and momentum p has wavelike properties . The de Broglie wavelength of an object with momentum p is given by p h = λ By applying this wave theory of matter to electrons in atoms, de Broglie was able to explain the appearance of quantization in the Bohr model of hydrogen as a standing wave phenomenon. The uncertainty principle states that if a measurement of position is made with precision x and a simultaneous measurement of momentum is made with precision p x , then the product of the two uncertainties can never be smaller than a number of the order of h /2.
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