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Modern Physics

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PHY251 - Photoelectric Effect PHY 251 The Photoelectric Effect Introduction In this experiment you will use the photoelectric effect to measure the Planck constant h . This classical experiment led to the first precise determination of h , and in 1923 R.A. Millikan received the Nobel Prize for it. Earlier, Einstein had provided an explanation for the observations that the energy of ejected electrons was proportional to the wavelength of the incident light, and independent of the light's intensity (leading to his 1921 Nobel Prize). A phototube is illuminated by light of a known wavelength. Electrons are ejected from the photocathode with some kinetic energy K . They are collected as anode current unless a variable retarding potential V is large enough to stop the electrons. For a given potential V all electrons with K < eV will be stopped, and at some value V 0 even the fastest electrons with a kinetic energy K max will be stopped when K max = h ν - W = e V 0 , (1) with ν the frequency of the incident light, and W the workfunction of the cathode material. Derive and
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