demo21 - Demonstration for Class 21 - Photoelectric Effect,...

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Demonstration for Class 21 -- Photoelectric Effect, Diffraction Gratings and Photons Quantum physics came into being because classical physics could not explain experimental results on the atomic scale. Max Planck was the first to propose quantized energy levels in 1900, and Einstein explained the photoelectric effect in 1905. (Einstein was awarded the Nobel Prize in 1921 for his explanation of the photoelectric effect, NOT relativity as most people think.) Three conclusions from the photoelectric effect could only be explained using quantum theories: --- the emission of an electron from a clean metal surface is independent of intensity of the light, --- the emission of an electron does depend on frequency and has a minimum "threshold" frequency below which no emission occurs, and --- there is no "time lag" while the electron "soaks up" energy from a weak light source. The electron is emitted immediately, even if the light is very feeble. All you need is a high enough frequency. I showed you a demonstration of the photoelectric effect that might have been how physicists discovered the effect originally. A bright light was shown onto a metal plate. I used zinc because I wanted a material that emitted photoelectrons for ultraviolet light. My light source, therefore, was the carbon arc lamp I used in other demos. This projects white light
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This note was uploaded on 09/17/2008 for the course PHYS 1200 taught by Professor Stoler during the Spring '06 term at Rensselaer Polytechnic Institute.

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demo21 - Demonstration for Class 21 - Photoelectric Effect,...

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