A photocell is illuminated with monochromatic light of different wavelengths from a filament lamp with interference filters. The maximum energy of the ejected electrons in the photo-cell depends only on the frequency of the incident light, and is independent of its intensity. The stopping voltage Uo at different light frequencies is determined by the U/I caracteristics of the photocell and plotted over the corresponding light frequency f. Planck's quantum of action or Planck's constant (h) is determined from this graph .

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