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INTRODUCTION: In 1905 Albert Einstein suggested an experiment that provided one of the first convincing experimental verifications of the theory of quantum mechanics. The actual phenomenon was observed by Hertz in 1887 and proved to be impossible to explain by the wave theory of light. Einstein postulated that not only is light emitted and absorbed in discrete but tiny bundles of energy, as proposed by Max Planck, but it propagates that way as well: it travels through space at the velocity of light. This conjecture nicely explained the photoelectric effect’s experimental results in which the energy of the electrons leaving the surface of a metal, when irradiated by monochromatic light, depends upon the wavelength and not upon the intensity of the radiation. See figure 1. Figure 1 When Einstein made his suggestions, there was not sufficient quantitative evidence to confirm or deny his equations. Experimental work followed and subsequently the theory was completely verified. Here you will be able to repeat the essential part of the experiment that established the quantum theory of radiation. In the experiment, a photocathode is irradiated by a source of monochromatic radiation and a potential is applied to the tube so that it opposes the energy of the emitted photoelectrons. The voltage required to just stop the current flow is proportional to the energy of the photoelectron. Plotting the stopping potential as a function of the reciprocal of the wavelength of the radiation gives a straight line plot. The slope of this line can be used to calculate Planck`s constant. APPARATUS : Photoelectric Effect unit, 4 filters, Digital Multimeter, Mercury arc lamp, He-Ne laser with diffuser, black cardboard square, Computer with Excel software, and one wooden block and lab jack for elevation. PART I: MERCURY ARC LAMP
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This note was uploaded on 02/29/2012 for the course PHYS 227 taught by Professor Rabe during the Fall '08 term at Rutgers.

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