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Demonstration Description: Light from a mercury arc lamp is pased through a monochromatic filter and is incident on a photocell generating a current. A retarding potential is applied between the anode and the cathode of the photocell until there is zero current-flow. A Keithley Electrometer is used to monitor the current. The kinetic energy of the electrons can be measured from the retarding potential and Planck's constant determined. Four different filters are used in succession in this demonstration and a plot of the retarding voltage versus the wavelength is drawn. The filters used are violet, blue, green, and yellow with wavelengths of 405, 436, 546, and 578 nm, respectively. The yellow filter does not yield good results.
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